transient heat transfer analysis of a solid with a protective fabric system under hot air jet...

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TRANSIENT HEAT TRANSFER ANALYSIS OF A

SOLID WITH A PROTECTIVE FABRIC UNDER

HOT AIR JET IMPINGEMENT

by

Aashique Alam Rezwan (Std. No. 06 10 012)

Sarzina Hossain (Std. No. 06 10 063)

Supervised by

Dr. Md. Ashraful Islam

Professor, Dept. of Mechanical Engineering, BUET

Department of Mechanical Engineering

Bangladesh University of Engineering & Technology,

Dhaka – 1000, Bangladesh

March 2012

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TRANSIENT HEAT TRANSFER ANALYSIS OF A

SOLID WITH A PROTECTIVE FABRIC UNDER

HOT AIR JET IMPINGEMENT

by

Aashique Alam Rezwan (Std. No. 06 10 012)

Sarzina Hossain (Std. No. 06 10 063)

Supervised by

Dr. Md. Ashraful Islam

Professor, Dept. of MechE, BUET

Submitted to the

Department of Mechanical Engineering

Bangladesh University of Engineering & Technology,

Dhaka – 1000, Bangladesh

March 2012

The authors hereby grants to BUET permission to reproduce and to

distribute publicly paper and electronic copies of this thesis document in whole or in part in any

medium now known or hereafter created.

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DISCLAIMER

This is to certify that the works on Transient Heat Transfer Analysis of a Solid with a Protective

Fabric under Hot Air Jet Impingement presented in this thesis carried out by the authors Aashique

Alam Rezwan (Std. No. 06 10 012) & Sarzina Hossain (Std. No. 06 10 063) under the supervision of

Dr. Md. Ashraful Islam, Professor, Dept. of MechE, BUET.

It is hereby declared that this thesis or any part of it has not been submitted elsewhere for the award of

any degree or diploma.

Authors

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ABSTRACT

The present study focuses on the performance of fire protective clothing under the flame blast

condition. This investigation has been conducted using a hot air jet that is impinged normally on a base

plate with/without protective fabric to mimic the flame blast condition. The air jet temperature is 125°C

and the jet velocity is 15 m/s and 19 m/s. The temperatures at various radial positions of the solid are

measured by thermocouples and are used to calculate the surface heat flux transferred to the solid base

plate. Subsequently, the local heat transfer coefficient and the local Nusselt Number for different radial

positions of the base plate have been estimated, exhibited and analyzed for different longitudinal distances

between nozzle and base plate with/without protective fabric. A numerical simulation model has been

developed and validated for the same purpose. Both the experimental and simulation results show a

significant decrease in heat transfer rate using the protective fabric.

Keywords: Transient Heat Transfer, Jet Impingement, Hot Air Jet, Fire Protective Fabric.

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ACKNOWLEDGEMENT

We would like to express our gratitude to all those who gave us the possibility to complete this thesis

work. We express our gratitude to the Department of Mechanical Engineering, BUET for providing both

financial and laboratory facilities. We would also like to thank our respectable teacher and thesis

supervisor Dr. Md. Ashraful Islam, Professor, Dept. of MechE, BUET. We would like to gratefully

acknowledge his enthusiastic supervision, contribution, valuable advice, right direction and criticism

which have been sources of tremendous inspiration to us. Without his constant encouragement and proper

guidance in performing the research work, it would be quite impossible for us to complete the thesis. He

was extraordinarily patient and spent his valuable time in listening to us and reading our drafts and

making the correction.

We have furthermore to thank, Md. Ali Noor, Technician, Grade-II, Md. Abdur Razzak, Lab. Atten-

dant, and Indrojeet Roy, Lab. Attendant of the Fluid Mechanics and Turbulence Laboratory for their

support in the experimental works. We also want to thank Md. Masudur Rahman, Instrument Engineer

and Mr. Raja, Electrician of BUET for their help in installing the setup in the laboratory.

We wish to thank AKM Nazrul Islam, Masters Student and all the staffs of Machine Shop, Brazing

and Sheet Metal Shop and Carpentry Shop for conducting the construction of the setup of the experi-

mental work. Without those help it would be difficult for us to construct the experimental setup.

We are also very much grateful to BAFFESCO and its‟ CEO Md. Zahir Uddin Babar for letting us to

use the protective suit for the experimental purpose which they import for commercial purpose.

We also like to forward our thanks and gratefulness to all our friends for their help, support,

encouragement, interest and valuable hints. We are deeply devoted and grateful to our parents and family

for their sacrifice to complete the thesis work. Finally, all thanks and glory to The Almighty, to whom

everything‟s belongs.

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Table of Contents

1. Introduction ................................................................................................................................. 14

1.1 Objectives ................................................................................................................................ 15

2. Literature Review ........................................................................................................................ 16

2.1 Study of Skin ........................................................................................................................... 16

2.1.1 Epidermis ......................................................................................................................... 16

2.1.2 Dermis ............................................................................................................................. 18

2.1.3 Subcutaneous Layer ......................................................................................................... 18

2.2 Thermal Injury: The Burn ....................................................................................................... 19

2.3 Classification of Thermal Injuries ........................................................................................... 19

2.4 Stoll Second Degree Burn Criterion ........................................................................................ 20

3. Methodology & Theoratical Backgraound .................................................................................. 21

3.1 Methodology............................................................................................................................ 21

3.2 Transient Heat Flow in a Semi-Infinite Solid .......................................................................... 22

3.2.1 Constant Heat Flux on Semi-infinite Solid ...................................................................... 23

3.3 Jet Impingement Heat Transfer ............................................................................................... 24

3.3.1 Impinging Jet Regions ..................................................................................................... 24

3.3.2 Nondimensional Heat and Mass Transfer Coefficients ................................................... 26

3.3.3 Correlation for Single Round Nozzle, Pipe and Tapered Nozzle .................................... 27

4. Experimental Setup ..................................................................................................................... 28

4.1 Wind Tunnel ............................................................................................................................ 28

4.1.1 Testing of Wind Tunnel ................................................................................................... 30

4.2 Heater Section ......................................................................................................................... 31

4.2.1 Testing of Heater Section ................................................................................................ 35

4.3 Fabric Holder & Base Plate Material ...................................................................................... 37

4.4 Data Logger ............................................................................................................................. 39

4.4.1 Principle of Operation ..................................................................................................... 39

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4.5 Fabric ....................................................................................................................................... 40

4.6 Experimental Procedure .......................................................................................................... 41

4.7 Calculation / Data Reduction Procedure ................................................................................. 42

5. Numerical Simulation Methodology & Procedure ...................................................................... 43

5.1 Numerical Simulation Methodology ....................................................................................... 43

5.2 Governing Equations ............................................................................................................... 44

5.2.1 Transport Equations ......................................................................................................... 44

5.2.2 The Continuity Equation ................................................................................................. 44

5.2.3 The Momentum Equations .............................................................................................. 44

5.2.4 The Total Energy Equation.............................................................................................. 44

5.2.5 The Thermal Energy Equation ........................................................................................ 45

5.3 Turbulence Models .................................................................................................................. 46

5.3.1 Two Equation Turbulence Models .................................................................................. 47

5.3.2 The k-epsilon Model in ANSYS CFX ............................................................................. 47

5.4 Code Validation ....................................................................................................................... 49

5.4.1 Comparison...................................................................................................................... 49

5.4.2 Simulation Settings for Validation .................................................................................. 50

5.5 Computer Numerical Simulation Procedure ............................................................................ 51

5.5.1 Geometry for Simulation ................................................................................................. 51

5.5.2 Meshing ........................................................................................................................... 51

5.5.3 Simulation Setup ............................................................................................................. 52

6. Results & Discussions ................................................................................................................. 55

6.1 Experimental Result ................................................................................................................ 55

6.2 Calculated Result ..................................................................................................................... 68

6.2.1 Heat Flux – Time ............................................................................................................. 68

6.3 Transient Effect on Nusselt Number ....................................................................................... 75

6.4 Effect of Space between Plate and Nozzle (l/d) ...................................................................... 79

6.5 Effect of Velocity on Surface Heat Flux ................................................................................. 84

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6.6 Numerical Simulation Results ................................................................................................. 88

6.7 The Safety Comparison ........................................................................................................... 91

7. Conclusion ................................................................................................................................... 93

7.1 Future Work............................................................................................................................. 93

8. References ................................................................................................................................... 94

Appendix A ........................................................................................................................................... 97

Heater Design ................................................................................................................................... 97

Appendix B ......................................................................................................................................... 100

Radial Nusselt Number Distribution ........................................................................................... 100

Appendix C ......................................................................................................................................... 107

Design Files .................................................................................................................................... 107

Appendix D ......................................................................................................................................... 111

PicoLog Specification ..................................................................................................................... 111

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List of Figures

Figure 2-1: Structure of Human Skin (13) ............................................................................................ 17

Figure 2-2: Human Tissue Tolerance to Pain Sensation (20) ............................................................... 20

Figure 2-3: Human Tissue Tolerance to Second Degree Burn (20) ...................................................... 20

Figure 3-1: Experimental Methodology (Left), Thermocouple Position (Right).................................. 21

Figure 3-2: Nomenclature for Transient Heat Flow in a Semi-infinite Solid ....................................... 22

Figure 3-3: Temperature Distribution in the Semi-infinite Solid (21) .................................................. 23

Figure 3-4: The Flow Region of an Impinging Jet (23) ........................................................................ 24

Figure 3-5: The Flow Field of Free Submerged Jet (23) ...................................................................... 25

Figure 4-1: Schematic of Wind Tunnel along with Heater Section ...................................................... 28

Figure 4-2: Wind Tunnel (Downstream) .............................................................................................. 29

Figure 4-3: Wind Tunnel (Upstream) ................................................................................................... 29

Figure 4-4: Velocity Profile for Single Fan .......................................................................................... 30

Figure 4-5: Velocity Profile for Double Fan ......................................................................................... 30

Figure 4-6: Velocity Profile for Single Fan (Concise Form) ................................................................ 30

Figure 4-7: Design of Heater Section ................................................................................................... 32

Figure 4-8: Photograph of Heater Section (Inside) ............................................................................... 32

Figure 4-9: Temperature of Air through Heater Section ....................................................................... 33

Figure 4-10: Heater Surface Temperature ............................................................................................ 33

Figure 4-11: Sectional View of Heater Section .................................................................................... 34

Figure 4-12: Temperature of Air (mid line) along the Heater Section ................................................. 34

Figure 4-13: Temperature of Air Jet with Electrical Energy Input ....................................................... 35

Figure 4-14: Temperature Ratio with Electrical Energy Input ............................................................. 35

Figure 4-15: Temperature of Heater Surface with Electrical Energy Input .......................................... 36

Figure 4-16: Comparison between Simulation Result and Experimental Result .................................. 36

Figure 4-17: Fabric Holder Stand ......................................................................................................... 38

Figure 4-18: Base Plate ......................................................................................................................... 38

Figure 4-19: Chemical Structure of Kevlar Monomer (35) .................................................................. 40

Figure 4-20: Kevlar Monomers Bonding (36) ...................................................................................... 40

Figure 4-21: Fire Protective Fabric Suit Sample .................................................................................. 40

Figure 4-22: Experimental Setup in The Lab ........................................................................................ 41

Figure 5-1: Schematic of Fabric System ............................................................................................... 43

Figure 5-2: Temperature before the Fabric Phase ................................................................................. 49

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Figure 5-3: Temperature after the Fabric Phase .................................................................................... 50

Figure 5-4: Geometry for Modeling...................................................................................................... 51

Figure 5-5: Mesh Generation for the Geometry .................................................................................... 51

Figure 6-1: Temperature Variation with Time without Fabric for l/d = 2 and velocity 15 m/s ............ 56

Figure 6-2: Temperature Variation with Fabric for l/d = 2 and velocity 15 m/s................................... 57


Figure 6-4: Temperature Variation with Time with Fabric for l/d = 2 and velocity 19 m/s ................. 59






Figure 6-10: Temperature Variation with Time with Fabric for l/d = 6 and velocity 15 m/s ............... 65

Figure 6-11: Temperature Variation with Time without Fabric for l/d = 6 and velocity 19 m/s .......... 66

Figure 6-12: Temperature Variation with Time with Fabric for l/d = 6 and velocity 19 m/s ............... 67

Figure 6-13: Surface Heat Flux Variation with Time for l/d = 2 and velocity 15 m/s .......................... 69

Figure 6-14: Surface Heat Flux Variation for l/d = 4 and velocity 15 m/s ........................................... 70

Figure 6-15: Surface Heat Flux Variation for l/d = 6 and velocity 15 m/s ........................................... 71

Figure 6-16: Surface Heat Flux Variation with Time for l/d = 2 and velocity 19 m/s ......................... 72



Figure 6-19: Radial Nusselt Number Distribution for l/d = 2 and velocity 15 m/s .............................. 76






Figure 6-25: Effect of l/d on Surface Heat Flux for velocity 15 m/s .................................................... 80

Figure 6-26: Effect of l/d on Surface Heat Flux for velocity 19 m/s .................................................... 81

Figure 6-27: Radial Nusselt Number Distribution for Different l/d Position for velocity 15 m/s; time

interval t = 30s ............................................................................................................................................. 82


interval t = 65s ............................................................................................................................................. 82


interval t = 180s ........................................................................................................................................... 82

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interval t = 30s ............................................................................................................................................. 83


interval t = 65s ............................................................................................................................................. 83


interval t = 180s ........................................................................................................................................... 83

Figure 6-33: Effect of Velocity on Surface Heat Flux for l/d = 2 ......................................................... 85



Figure 6-36: Temperature at the Fabric Front ....................................................................................... 89

Figure 6-37: Temperature after the Fabric Phase .................................................................................. 89

Figure 6-38: Comparison between Simulation and Experimental Result ............................................. 90

Figure 6-39: Human Tissue Tolerance to Pain Sensation ..................................................................... 92

Figure 6-40: Human Tissue Tolerance to Second Degree Burn ........................................................... 92

Figure A-1: Configuration 1 ................................................................................................................. 97

Figure A-2: Temperature Profile along the Configuration 1 Setup ...................................................... 97


Figure A-4: Wall Temperature Requirement with Heating Lenght to attain Target Jet Temperature

above 100°C ................................................................................................................................................ 97


Figure A-6: Results for Configuration 3 ............................................................................................... 98



Figure A-9: Result of Configuration 5 .................................................................................................. 99

Figure B-1: Nu - r/d for t = 30s ........................................................................................................... 101


Figure B-3: Nu - r/d for t = 180s ......................................................................................................... 101









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Figure B-12: Nu - r/d for t = 180s ....................................................................................................... 104







List of Tables

Table 2-1: Classification of Burn Injury based on Depth (17).............................................................. 19

Table 4-1: Summary of Previous & Designed Air Jet Heater Section .................................................. 31

Table 4-2: Thermal Properties of Different Material (32) (21)............................................................. 37

Table 4-3: Thermal Properties of Kevlar 49 (37) (38) .......................................................................... 40

Table 4-4: Experimental Conditions ..................................................................................................... 41

Table 4-5: Fire Retardant Fabric Characteristics .................................................................................. 41

Table 4-6: Thermal Sensor .................................................................................................................... 41

Table 5-1: Domain Modeling for Validation ........................................................................................ 50

Table 5-2: Interface Modeling for Validation ....................................................................................... 50

Table 5-3: Mesh Report ........................................................................................................................ 51

Table 5-4: Boundary Details ................................................................................................................. 53

Table 5-5: Constant for Kevlar 49 ........................................................................................................ 54

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Nomenclature

Symbol Meaning Unit

A Area (m2)

C Coefficient -

Cp Heat Capacity (kJ/kg.°C)

D Diameter/Hydraulic Diameter (m)

d Nozzle Diameter (mm)

E/ε Ergun Coefficient -

h Heat Transfer Coefficient (W/m.°C)

k Thermal Conductivity (W/m.°C)

K Permeability (m2)

L Length (m)

l Nozzle to Plate Distance (mm)

Nu Nusselt Numeber -

P Pressure (Pa)

Pr Prandtl Number -

q Heat Flux (W/m)

r Radial Distance (mm)

Re Reynolds Number -

T Temperature (°C)

t Time (s)

v Velocity (m/s)

W Width (m)

α Thermal Diffusivity (m2/s)

ρ Density (kg/m3)

Superscripts and Subscripts

avg Average

fab fabric

g gas

i Surface/Initial

jet Impinging Jet

n Interval Number

s solid

sg solid-gas interface

x Axial Position

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1. INTRODUCTION

The thermal performance of protective clothing has been a point of interest for several decades. The

protection of firefighters and other employees working in many hazardous environments against high heat

flux exposure is very crucial for their safe guard and for others‟ safety as well. So, the protective clothing

is essential requirement in daily business for the staff of Fire Fighting Department, Policemen and Navy

Military Staff and also for the workers in many chemical and bio-hazardous industries. Fire Retardant

clothing has steadily improved over the years as new fabric materials and improved designs have reached

the market. But their performance has not been studied in much detail for the safe keeping of their users

yet. Many of these studies are based on fire services field experience.

Most of this work dates back in 1960s‟ and 1970s‟ (1) when computers were significantly less ad-

vanced. Torvi (2) provides a review of work done on heat and mass transfer models applicable to fabrics

in the high heat flux range that a firefighter may experience. The Government Industry Research Commi-

ttee on Fabric Flammability considered mainly flammable fabrics used by ordinary consumer (3) (4).

Morse et al. (5) studied heat transfer and burn injury risk from exposure to jet fuel fires. Only three pro-

tective clothing materials were examined for use in US Air Force flight Fabrics. Also, some model pro-

perties were determined by fitting the model results to experimental data. Stoll et al. (6) (7) (8) used a

combination of analytical and experimental techniques to measure the thermal response of single fabric

layers over skin. They developed diagnostics to rate the protection offered by a fabric with known pro-

perties. Their work eventually led to the thermal protective performance test. Many recent studies for the

permeable protective fabric system have been conducted using air jet impingement. Many of these studies

formulated a numerical model for comparing the thermal performance under various conditions (9).

Bamford and Boydell (10) developed a finite-difference based burn injury evaluation code and Torvi (2)

developed a finite element code to simulate the test. Anguiano (11) performed a study by using skin

simulant material to understand the extent of fire burn injury. Very recently, Lee et al. (9) published an

important study for protective clothing using both computer simulation and experiments.

In the present study, jet impingement heat transfer has been experimented to mimic the flame impact

to a solid surface covered with Fire Retardant fabric. Jet impingement has many practical applications due

to its high heat and mass transfer capabilities. There are many factors affecting the heat transfer of jet

impingement. These include the nozzle diameter (d), nozzle-to-plate distance (l), nozzle-exit velocity

(vjet), thermophysical properties of the jet fluids. These factors are normally lumped into dimensionless

groups: Nusselt Number (Nu), Reynolds Number (Re), Prandtl Number (Pr) and nozzle-to-plate

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separation (l/d), which are then correlated. Later, a computer simulation has been performed to further

visualize the thermal response of the fire retardant fabric and compare the two results.

1.1 Objectives

1. To setup a test structure according to the international standard (12) for testing the thermal

response of any fibrous system made for both protective and non-protective purposes under

hot air jet impingement

2. To calibrate and test the constructed apparatus for testing the thermal response of fibrous

system

3. To test a locally available protective clothing made for fire retardant purpose

4. To analyze the thermal response of the skin and/or skin simulant material under the fire

retardant clothing

5. To determine and analyze the effect of heat transfer into the skin under the fire retardant

clothing

6. To develop a numerical computer simulation model for the above mentioned purposes and

compare the experimental and simulated results

Firefighters face many risks when doing their jobs; the most frequent injuries are burns. Burn injuries

are costly to treat and, depending on the magnitude, greatly affect the livelihood of the firefighter. Fire-

fighters are trained to be aware of these threats and manage their risks. But the danger of being burned and

the loss of financial security is a constant worry for firefighter‟s every day. The data presents in the

present work can be used to predict the burn conditions with and without using their protective clothing.

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2. LITERATURE REVIEW

2.1 Study of Skin

The skin is one of the largest organs in the body, having a surface area of 1.8m2 and making up about

16% of body weight. It has many functions, the most important of which is as a barrier to protect the body

from noxious external factors and to keep the internal systems intact. Skin is composed of three layers

(Figure 2-1)

1. The Epidermis

2. The Dermis

3. The Subcutis

2.1.1 Epidermis

The epidermis is defined as a stratified squarmous epithelium which is about 0.1mm thick, although

the thickness is greater (0.8 – 1.4 mm) on the palm and sole. Its prime function is to act as a protective

barrier. The main cell of the epidermis is the keratinocyte, which produces the protein keratin. The four

layers (Figure 2-1) of the epidermis represent the stages of maturation of keratin by keratinocytes.

Basal Cell Layer (Stratum Basle)

The basal cell layer of the epidermis is comprised mostly of keratinocytes which are either dividing or

nondividing. The cells contain keratin tonofibrils and are secured to the basement membrane by hemi-

desmosomes. Melammocytes make up 5 – 10% of the basal cell population. These cells synthesize mela-

nin and transfer it via dendritic processes to neighboring keratinocytes. Melanocytes are most numerous

on the face and other exposed sites and are of neural crest origin. Merkel cells are also found, albeit in-

frequently, in the basal cell layer. These cells are closely associated with terminal filaments of cutaneous

nerves and seem to have a role in sensation. Their cytoplasm contains neuropeptide granules, as well as

neurofilaments and keratin.

Prickle Cell Layer (Stratum Spinosum)

Daughter basal cells migrate upwards to form this layer of polyhedral cells which are interconnected

by desmosomes (the prickles seen at light microscope level). Keratin tononfibrils form a supportive mesh

in the cytoplasm of these cells. Langerhans cells are found mostly in this layer.

Granular Cell Layer (Stratum Granulosum)

Cells become flattened and lose their nuclei in the granular cell layer. Keratohyalin granules are seen

in the cytoplasm together with membrane-coating granules (which expel their lipid contents into the inter-

cellular spaces).

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Figure 2-1: Structure of Human Skin (13)

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Horny Layer (Stratum Corneum)

The end result of keratinocyte maturation can be found in the horny layer which is comprised of

sheets of overlapping polyhedral cornified cells with no nuclei (corneocytes). The layer is several cells

thick on the palms and soles, but less thick elsewhere. The corneocyte cell envelope is broadened, and the

cytoplasm is replaced by keratin tonofibrils in a matrix formed from the keratohyalin granules. Cells are

stuck together by lipid glue which is partly derived from membrane coating granules.

2.1.2 Dermis

The dermis is defined as a tough supportive connective tissue matrix containing specialized structures,

found immediately below and intimately connected with the epidermis. It varies in thickness, being thin

(0.6mm) on the eyelids and thicker (3mm or more) on the back, palms and soles. The papillary dermis –

the thin upper layer of the dermis – lies below and interdigitates with the epidermal rete ridges. It is com-

posed of loosely interwoven collagen. Coarser and horizontally running bundles of collagen are found in

the deeper and thicker reticular dermis.

Collagen fibres make up 70% of the dermis and impart a toughness and strength to the structure.

Elastin fibres are loosely arranged in all direction in the dermis and provide elasticity to the skin. They are

numerous near hair follicles and sweat glands, and less so in the papillary dermis. The ground substance

of the dermis is a semi-solid matrix of glycosaminoglycans (GAG) which allows dermal structures some

movement.

The dermis contains fibroblasts (which synthesisze collagen, elastin, other connective tissue and

GAG), dermal dendrocytes (dendritic cells with a probable immune function), mast cells macrophages

and lymphocytes.

2.1.3 Subcutaneous Layer

The subcutis consists of loose connective tissue and fat (up to 3cm thick on the abdomen).

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2.2 Thermal Injury: The Burn

The most common thermal injuries to human are the burn. The burning of human skin occurs as a

complex heat transfer between a hot medium (by conduction, convection, radiation or a combination of)

and the surface of the skin. The rate of heating depends upon the temperature and the heat capacity of the

sources as well as the heat capacity and thermal conductivity of the skin layers (14).

A burn is a type of injury to flesh caused by heat, electricity, chemicals, light, radiation or friction

(15). Most burns only affect the skin (epidermal tissue and dermis). Rarely, deeper tissue such as muscle,

bone and blood vessels can also be injured. Burns may be treated with first aid, in an out-of-hospital

setting, or may require more specialized treatment such as those available at specialized burn centers.

Managing burns is important because they are common, painful and can result in disfiguring and

disabling scarring, amputation of affected parts or death in severe cases. Complication such as shock,

infection, multiple organ dysfunction syndrome, electrolyte imbalance and respiratory distress may occur.

The treatment of burns may include the removal of dead tissue (debridement), applying dressing to the

wound, administering large volumes of intravenous fluids, administering anibiotics and skin grafting.

2.3 Classification of Thermal Injuries

Burns can be classified by mechanism of injury, depth, extent and associated injuries and comor-

bidities. Currently burns are described according to the depth of injury to the dermis and are loosely

classified into first, second, third and fourth degrees. This system was devised by the French barber-

surgeon Ambroise Pare and remains in use today (16).

Table 2-1: Classification of Burn Injury based on Depth (17)

Burn

Injuries Layer Involved Appearance Texture Sensation

Time to

Healing

First

Degree Epidermis Redness Dry Painful

1 week or

less

Second

Degree Dermis

Red with clear

blister Moist Painful 2-3 weeks

Third

Degree

Extend through entire

Dermis

Stiff and white /

brown

Dry,

Leathery Painless

Requires

excision

Fourth

Degree

Extends through skin,

subcutaneous tissue

Charred with

eschar Dry Painless

Requires

excision

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2.4 Stoll Second Degree Burn Criterion

Stoll (8) (18) has carried out many investigations in the field of skin burns and skin damage as related

to fire retardant fabrics. Some of her early work included the experimental determination of the time it

takes to produce second-degree burns in human skin. In those experiments, blackened human forearms

were exposed to various thermal irradiances from a 1000W projection lamp. The lamp was attached to a

variable resistor so as to produce a wide range of heat fluxes. The subjects were exposed to one heat flux

at a time until they expressed unbearable pain and/or until blistering occurred.

The time and skin temperature – minimal blistering within 24 hours – at which intolerable pain occur-

ed were recorded (19). Her results can be translated into a relationship that calculates the amount of

energy needed to produce second degree burns. American Standards for Testing and Measurement

(ASTM) (20) use that relationship based on the energy absorbed by test sensors. Figures 2-2 and 2-3 show

the time required to produce second-degree burns for various heat fluxes based on following equation.

q” = 50.123 t

-0.7087 Equation 2-1

where incident heat flux, q”, is in kW/m2 and time, t, in seconds. The lower limit of injurious temp-

erature is approximately 44°C (at the basal interface). Injury will occur anytime and all the time this

interface remains above that limit. This means that injury happens during cooling as well as heating. At

this temperature, irreversible damage may be caused to sweat glands and hair follicles.

Figure 2-2: Human Tissue Tolerance to Pain Sensation

(20)

Figure 2-3: Human Tissue Tolerance to Second Degree

Burn (20)

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3. METHODOLOGY & THEORATICAL BACKGROUND

3.1 Methodology

Figure 3-1 depicts the experimental methodology of the present work. Two different types of

experiment have been conducted in the present work to study and analyze the behavior of heat transfer

of a system of fabric usually made for fire retardant purpose. Firstly, heated jet of air has been im-

pinged upon a base plate to determine the temperature distribution on the flat base plate. Thermo-

couple has been installed in a radial fashion starting at the stagnation point and radially outward in up

and down direction from the stagnation point. After that, the temperature distribution on the flat base

plate has been again measured with the fire retardant fabric system placed in the surface of the base

plate. These temperatures have been used to calculate the heat flux and Nusselt Number to study and

analyze the heat transfer as a time dependent variable. A wind tunnel has been used to generate the

required air flow with a certain velocity. The oncoming air from the wind tunnel is then passes

through a heater section for increasing the air temperature to the required jet temperature. The heated

air then passes through a nozzle to create the jet of air. This heated air jet then used to impinge upon

the experimental surface placed in the stand at a certain distance from the nozzle.

Figure 3-1: Experimental Methodology (Left), Thermocouple Position (Right)

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3.2 Transient Heat Flow in a Semi-Infinite Solid

The base plate under jet impingement can be modeled as semi-infinite solid having a boundary of

constant heat flux. Consider the semi-infinite solid shown in Figure 3-2 maintained at some initial

temperature Ti. The surface temperature is suddenly lowered and maintained at a temperature T0, and

we seek an expression for the temperature distribution in the solid as a function of time. This temp-

erature distribution may subsequently be used to calculate heat flow at any x position in the solid as a

function of time. For constant properties, the differential equation for the temperature distribution T(x,

τ) is, (21)

Equation 3-1

Figure 3-2: Nomenclature for Transient Heat Flow in a Semi-infinite Solid

The boundary and initial conditions are

T(x, 0) = Ti

T(0, τ) = T0 for τ > 0

This is a problem which may be solved by the Laplace-transform technique. The solution is given

in (22) as,

( )

√ Equation 3-2

where the Gauss error function is defined as,

√

√ ∫

√ Equation 3-3

It will be noted that in this definition η is a dummy variable and the integral is a function of its

upper limit. When the definition of the error function is inserted in equation 2, the expression for the

temperature distribution becomes

( )

√ ∫

√ Equation 3-4

The heat flow at any x position may be obtained from

Equation 3-5

23

Performing the partial differential of equation 4 gives

( )

√

(

√ )

√

Equation 3-6

At the surface (x = 0) the heat flow is

( )

√ Equation 3-7

The surface heat flux is determined by evaluating the temperature gradient at x = 0 from Equation

3-6. A plot of the temperature distribution for the semi-infinite solid is given in Figure 3-3.

Figure 3-3: Temperature Distribution in the Semi-infinite Solid (21)

3.2.1 Constant Heat Flux on Semi-infinite Solid

For the same uniform initial temperature distribution, we could suddenly expose the surface to a

constant surface heat flux q0/A. The initial and boundary conditions on Equation 3-1 would then

become

T(x,0) = Ti

The solution for this case is

( ) √ ⁄

(

)

( (

√ )) Equation 3-8

On the surface (x = 0) this equation reduces to,

( ) √ ⁄

Equation 3-9

24

3.3 Jet Impingement Heat Transfer

3.3.1 Impinging Jet Regions

The flow of a submerged impinging jet passes through several distinct regions, as shown in Fig 3-

4. The jet emerges from a nozzle or opening with a velocity and temperature profile and turbulence

characteristics dependent upon the upstream flow. For a pipe-shaped nozzle, also called a tube nozzle

or cylindrical nozzle, the flow develops into the parabolic velocity profile common to pipe flow plus a

moderate amount of turbulence developed upstream. In contrast, a flow delivered by application of

differential pressure across a thin, flat orifice will create an initial flow with a fairly flat velocity

profiles, less turbulence, and a downstream flow contraction (vena contracta). Typical jet nozzle

designs use either a round jet with an axisymmetric flow profile or a slot jet, a long, thin jet with a

two-dimensional flow profile.

Figure 3-4: The Flow Region of an Impinging Jet (23)

After it exits the nozzle, the emerging jet may pass through a region where it is sufficiently far

from the impingement surface to behave as a free submerged jet. Here, the velocity gradients in the jet

create a shearing at the edge of the jet which transfers momentum laterally outward, pulling additional

fluid along with the jet and raising the jet mass flow, as shown in Fig. In the process, the jet loses

energy and the velocity profile is widened in spatial extent and decreased in magnitude along the sides

of the jet. Flow interior to the progressively widening shearing layer remains unaffected by this

25

momentum transfer and forms a core region with a higher total pressure, though it may experience a

drop in velocity and pressure decay resulting from velocity gradients present at the nozzle exit. A free

jet region may not exist if the nozzle lies within a distance of two diameters (2D) from the target. In

such cases, the nozzle is close enough to the elevated static pressure in the stagnation region for this

pressure to influence the flow immediately at the nozzle exit.

If the shearing layer expands inward to the center of the jet prior to reaching the target, a region of

core decay forms. For purposes of distinct identification, the end of the core region may be defined as

the axial position where the centerline flow dynamic pressure (proportional to speed squared) reaches

95% of its original value. This decaying jet begins four to eight nozzle diameters or slot-widths down-

stream of the nozzle exit. In the decaying jet, the axial velocity component in the central part

decreases, with the radial velocity profile resembling a Gaussian curve that becomes wider and shorter

with distance from the nozzle outlet. In this region, the axial velocity and jet width vary linearly with

axial position. Martin (24) provided a collection of equations for predicting the velocity in the free jet

and decaying jet regions based on low Reynolds number flow. Viskanta (25) further subdivided this

region into two zones, the initial “developing zone”, and the “fully developed zone” in which the

decaying free jet reaches a Gaussian velocity profile.

Figure 3-5: The Flow Field of Free Submerged Jet (23)

As the flow approaches the wall, it loses axial velocity and turns. This region is labeled the stag-

nation region or deceleration region. The flow builds up a higher static pressure on and above the

wall, transmitting the effect of the wall upstream. The nonuniform turning flow experiences high

normal and shear stresses in the deceleration region, which greatly influence local transport pro-

perties. The resulting flow pattern stretches vortices in the flow and increases the turbulence. The

26

stagnation region typically extends 1.2 nozzle diameters above the wall for round jet (24). Experi-

mental work by Maurel and Solliec (26) found that this impinging zone was characterized or deli-

neated by a negative normal-parallel velocity correlation ( ). For their slot jet this region ex-

tended to 13% of the nozzle height H (distance L for horizontal jet), and did not vary with Re or H/D.

After turning, the flow enters a wall jet region where the flow moves laterally outward parallel to

the wall. The wall jet has a minimum thickness within 0.75-3 diameters from the jet axis, and then

continually thickens moving farther away from the nozzle. This thickness may be evaluated by meas-

uring the height at which wall-parallel flow speed drops to some fraction of the maximum speed in the

wall jet at that radial position. The boundary layer within the wall jet begins in the stagnation region,

where it has a typical thickness of no more than 1% of the jet diameter (24). The wall jet has a

shearing layer influenced by both the velocity gradient with respect to the stationary fluid at the wall

(no-slip condition) and the velocity gradient with respect to the fluid outside the wall jet. As the wall

jet progresses, it entrains flow and grows in thickness, and its average flow speed decreases as the

location of highest flow speed shifts progressively farther from the wall. Due to conservation of mom-

entum the core of the wall jet may accelerate after the flow turns and as the wall boundary layer de-

velops. For a round jet, mass conservation results in additional deceleration as the jet spreads radially

outward.

3.3.2 Nondimensional Heat and Mass Transfer Coefficients

A major parameter for evaluating heat transfer coefficients is the Nusselt number,

⁄ Equation 3-10

where h is the convective heat transfer coefficient defined as

Equation 3-11

The selection of Nusselt number to measure the heat transfer describes the physics in terms of

fluid properties, making it independent of the target characteristics. The jet temperature used, Tjet, is

the adiabatic wall temperature of the decelerated jet flow, a factor of greater importance at increasing

Mach numbers. The non-dimensional recovery factor describes how much kinetic energy is

transferred into and retained in thermal form as the jet slows down

⁄

Equation 3-12

This definition may introduce some complications in laboratory work, as a test surface is rarely

held at a constant temperature, and more frequently held at a constant heat flux. Experimental work by

Goldstein et al. (27) showed that the temperature recovery factor varies from 70% to 110% of the full

27

theoretical recovery, with lowered recoveries in the stagnation region of a low H/D jet (H/D = 2) and

100% elevated stagnation region recoveries for jets with H/D = 6 and higher. The recovery comes

closest to uniformity for intermediate spacing‟s around H/D = 5. Entrainment of surrounding flow into

the jet may also influence jet performance, changing the fluid temperature as it approaches the target.

The fluid properties are conventionally evaluated using the flow at the nozzle exit as a reference

location. Characteristics at the position provide the average flow speed, fluid temperature, viscosity,

and length scale D. In the case of a slot jet the diameter D is replaced in some studies by slot width B,

or slot hydraulic diameter 2B in others.

A complete description of the problem also requires knowledge of the velocity profile at the

nozzle exit, or equivalent information about the flow upstream of the nozzle, as well as boundary

conditions at the exit of the impingement region. Part of the effort of comparing information about jet

impingement is to thoroughly know the nature and magnitude of the turbulence in the flow field.

3.3.3 Correlation for Single Round Nozzle, Pipe and Tapered Nozzle

Source: Mohanty and Tawfek (28)

Range of validity: H/D from 6 to 41, Re from 4,860 to 34,500

Equations:

for H/D from 10 to 16.7, Re from 4,860 to 15,300:

Nu0 = 0.15 Re0.701

(H/D)-0.25

for H/D from 20 to 25, Re from 4,860 to 15,300:

Nu0 = 0.17 Re0.701

(H/D)-0.182

for H/D from 6 to 58, Re from 6,900 to 24,900:

Nu0 = 0.388 Re0.696

(H/D)-0.345

for H/D from 9 to 41.4, Re from 7,240 to 34,500:

Nu0 = 0.615 Re0.67

(H/D)-0.38

Please refer to the reference (23) for other correlation regarding jet impingement heat transfer.

28

4. EXPERIMENTAL SETUP

The experimental study has been carried out by using a circular air jet facility as shown in the

figure 4-1 located in the Turbulent Lab in Mechanical Engineering Department. The overall length of

the flow facility is 9.0 m. It has axial flow fan unit, two settling chambers, two diffusers, a silencer

and a flow nozzle. The detail of the experimental setup has been described below.

4.1 Wind Tunnel

Figure 4-1 shows the detail section of the wind tunnel along with the heater section. The wind

tunnel used for the present study has been located in the turbulent laboratory under the circular air jet

facility. The detail explanation of different segment of the wind tunnel has been given in the previous

studies (29) (30). The fan unit consists of three Woods Aerofoil fans of the same series. The fan unit

receives air through the butterfly valve and discharges it into the silencer of the flow duct. Flow from

the silencer passes on to the settling chamber through a diffuser. At the discharge, side of this cham-

ber there is a flow straighter and wire screen of 12 meshes to straighten the flow and to breakdown

large eddies present in the air stream. Air from this chamber then flows to the second settling chamber

through a nozzle and second diffuser. The flow straighter and wire screens are used here to ensure a

uniform axial flow free of large eddies which may be present in the upstream side of the flow. The

flow from the second settling chamber then enters the 100 mm long and 80 mm diameter circular

nozzle. At the farthest end the diameter of the flow facility is reduced from 475 mm to 88.9 mm

where the heating section is placed.

The air flow through the nozzle is controlled by regulating the speed of the fan units. The whole

setup is mounted on rigid frames of M.S. Pipes and plates and these frames are securely fixed with the

ground so that any possible unwanted vibration of the system is reduced to a minimum. To avoid the

effect of ground shear, the setup is installed at an elevation of 1.4 m from the ground.

Figure 4-1: Schematic of Wind Tunnel along with Heater Section

29

Figure 4-2: Wind Tunnel (Downstream)

Figure 4-3: Wind Tunnel (Upstream)

30

4.1.1 Testing of Wind Tunnel

The velocity profile of the wind tunnel has been measured for the purpose of calculation and

computer modeling for further investigation. The velocity profile of air at different l/d position has

been measured using pitot static tube. Figures show the velocity profile for single fan and double fan.

Figure 4-4: Velocity Profile for Single Fan

Figure 4-5: Velocity Profile for Double Fan

Figure 4-6: Velocity Profile for Single Fan (Concise Form)

31

4.2 Heater Section

The heater section has been designed in the present work to heat up the oncoming air from wind

tunnel to the required jet temperature. The previous heater presents in the department has been in-

adequate to perform the high temperature flow experiment due to the inefficient air heating capability

at high Raynolds number. The heater section has been designed after studying different types of

configuration. The required temperature of the air in this work is about 110°C - 130°C. To heat the air

at high Raynolds number to that temperature with consuming minimum electrical energy we have

been conduct many calculation and computer simulation for different heater configuration. The detail

study of different heater configuration has been described in the Appendix A of this manuscript. The

final design of the heater section is describe below.

Figure 4-7 shows the different parts of the heater section. The overall length of the heater section

is 0.9 m. It consists of 4 cartridge heaters of total power consumption of 3 kW. Five half circular

baffles are placed in series in the heating section to enhance the heating capacity of the air. Air from

the wind tunnel is passed through the entrance section, heating section and settling chamber and then

finally through the nozzle to produce the desired jet of air.

Swirling effect of the air due to the baffles ensures the uniform heating of air from the heater

surface. The heated air then passes through the long settling chamber to the nozzle having a diameter

of 25.4 mm. This is to note that the settling chamber is insulated with asbestos cloth and heat tape to

prevent the heat loss to the surrounding from the hot air.

The temperature of the air is regulated by controlling the supply voltage of the heater.

Table 4-1: Summary of Previous & Designed Air Jet Heater Section

Comparison Criteria Previous Heater

Designed & Constructed Heater

Simulation

Value Experimental Value

Heater Type Electric Resistance

Wire and Mica Sheet

Cartridge Heater

1200W (each)

Number of Section 4 3 3

Number of Heaters 6 4 4

Maximum Design Capacity

3 kW 3 kW 3 kW

Maximum Capacity - 4.8 kW 4.8 kW

(minimum) 0.925 0.74 0.72

Maximum Jet Temperature 54°C 136°C 147°C

Red 3.72x104

3.17 x104 3.17 x10

4

Intermittent Mixing

Capability Present Present Present

Temperature Control Varying the Supply

Voltage Given Input

Variable Voltage

Supply

32

Figure 4-7: Design of Heater Section

Figure 4-8: Photograph of Heater Section (Inside)

33

Figure 4-9: Temperature of Air through Heater Section

Figure 4-10: Heater Surface Temperature

34

For designing the heater computer simulation has been used. In the present works, ANSYS CFX

has been used to simulate the air flow and heating process in the heater section. The detail simulation

methodology has been discussed in section 5-1. Figure 4-9 shows the contour plot of air temperature

through the heater section. It can be seen that the swirling effect of the baffle greatly enhance the

heating process. In the fig. 4-10 the surface temperature of individual cartridge heater has been

showed. This measurement is required for the safe heating of air. Due to the lack of heat transfer

capability of air, the confined air in the section does not heat up properly. If the air near the surface of

the heater can‟t transfer heat to the surrounding air in the section, the surface of the cartridge heater

will be increase due to the lack of heat transfer. If the temperature increases in excess of the safe

temperature for the heater, the heater could be break. Fig. 4-12 shows the mid line air temperature

along the heater section.

Figure 4-11: Sectional View of Heater Section

Figure 4-12: Temperature of Air (mid line) along the Heater Section

35

4.2.1 Testing of Heater Section

To ensure that the designed heater has been performing up to the requirement the heater section

has been tested to its limit and compare with the computer simulation. Figures 4-13, 14 depict the air

jet temperature and temperature ratio of the jet of air for the increasing electrical energy input. With

the increasing input energy the temperature of the jet of air increases exponentially. The surface

temperature of the heater section (Fig. 4-15) has been determined for the safety of the cartridge heater

as described in the previous section. Then the experimental result has been compared with the

computer simulation result. Fig. 4-16 shows the comparison between experimental result and the

computer simulation result.

Figure 4-13: Temperature of Air Jet with Electrical Energy Input

Figure 4-14: Temperature Ratio with Electrical Energy Input

36

Figure 4-15: Temperature of Heater Surface with Electrical Energy Input

Figure 4-16: Comparison between Simulation Result and Experimental Result

From the figure 4-16 it can be seen that at low power input, the temperature of air for the

computer simulation is quite greater than the experimental result. As the power input increases, the

difference between the two values also decreases and become equal at the rated condition for safe

limit of the heater. After that, the experimental value of the temperature is increases more than the

computer simulation result. This may due to the heat loss at low power, as there is cooler air in the

surrounding. At high power input, as the time of heating increases, the temperature of the baffles and

the pipe also increases. This restricts the air to loose heat to the surrounding environment.

37

4.3 Fabric Holder & Base Plate Material

The fabric holder has been used to hold the base plate and the fabric itself. It is consisted with two

parts, the base plate and the stand. The base plate material selected for the present experiment is

Masonite (hardboard) which mimic as a skin. The base plate has been selected as per regulation in

ASTM F1060-01 (20) and ASTM F2703-08 (12). The thermal experience depends upon the property

of skin. When skin is subjected to different environmental condition, the coldness felt upon the skin is

governed by these properties. On the other hand, when skin comes in contact with different materials,

the thermal experience of coldness is depended upon both the properties of skin and the material in

contact. Cornwell (31) developed a theoretical relation that provides a useful index to compare the

coldness felt on touching different materials. This relation is based on the thermal properties of each

material. It can be referred to as contact coefficient, b (J/m2.K.s

1/2), or thermal inertia, b

2 (J

2/m

4.K

2.s)

Equation 4-1

Equation shows this relationship, where c, k and ρ are specific heat, thermal conductivity and

density, respectively. The comparison between different materials for simulating the skin with the

skin properties are shown in the table below.

Table 4-2: Thermal Properties of Different Material (32) (21)

Property Human Skin

Masonite Glass Epidermis Dermis

k (W/m.K) 0.255 0.523 0.18 0.78

c (J/kg.K) 3598 3222 1340 840

ρ (kg/m3) 1200 1200 1050 2700

b2×10

6

(J2/m

4.K

2.s)

1.10 2.02 0.25 1.77

b (J/m2.K.s

1/2) 1050 1414 503.25 1330

The property for Glass is more closely matches to the human skin properties. But the machine

shop work for glass is more complicated. Thus preparing the glass for present experimental condition

can‟t be done. So, the easy to fabricate hardboard material Masonite (Fig. 4-18) has been chosen for

present experiment.

For measuring the temperature on the surface of the base plate, K type thermocouple has been

used. The thermocouple has been installed on the surface of the base plate. One thermocouple is

placed at the stagnation point, and 3 others in both top and bottom side from the centered thermo-

couples are installed on the base plate at a distance of 1.5 nozzle diameter from each other.

The remaining portion of the fabric holder is the stand. The stand is made with wooden frame

(Fig. 4-17). The detail dimension of each of the components has been shown in Appendix C.

38

Figure 4-17: Fabric Holder Stand

Figure 4-18: Base Plate

39

4.4 Data Logger

Data logger used for the present work is PICO TC-08. The PICO TC-08 is a complete thermo-

couple input device for use with IBM compatible computers. It is used with the supplied PicoLog data

logging program. The TC-08 software provides all of the calculations necessary for cold junction

compensation and for thermocouple curve normalization. The TC-08 is a highly accurate without

calibration; the software does contain facilities to make minor adjustments to the gain and offset.

Picolog and the drivers support up to four TC-08 units under DOC, and nine TC-08 units under

Windows. Specification of PICO TC-08 is given in the appendix D.

4.4.1 Principle of Operation

In 1822, an Estonian physician named Thomas Seebeck discovered that the junction between two

metals generates a voltage which is a function of temperature. With a circuit made of two metals and

two junctions, if the two junctions are at different temperatures, it is possible to measure the diff-

erence between the voltages generated by the two junctions. For practical temperature measurement,

we place the HOT junction where we wish to measure the temperature, and then measure the voltage.

In order to calculate the HOT temperature, we must either place the COLD junction at a known temp-

erature. The TC-08 contains a circuit to measure the temperature of the cold junction and the software

driver carries out the calculation necessary for the cold junction compensation.

The voltage between two points depends on the type of metal. A type K thermocouple produces a

voltage which changes by about 40μV per degree Celsius. The TC-08 first amplifies this signal, and

then feeds it into a 16-bit analog to digital converter. The amplifier and ADC are set up to give an

input sensitivity of approx. 1μV per LSB, the resolution for a type K thermocouple is therefore

1/40°C. The ADC can take a measurement for a channel every 800ms.

The software continuously takes readings from the selected channels and from the cold junction

compensation circuit. For each reading, it updates a low pass filter. Filtered value is much less prone

to electrical noise, but it tends to lag behind if the measured value changes quickly. The filter works

by adjusting the filtered value by a proportion of the difference between the measured and filtered

values. This proportion is controlled by the filter factor. A high filter factor means that only a small

proportion of the difference is added each time, so the filtered value changes very slowly. The filter

factor is fixed at 10 for PicoLog, and with the drivers it can be adjusted between 1 (no filter) and 100

(very slow filter). The filter time constant is also affected by the number of channels that are in use.

The more channels selected, the slower the filter (33).

40

4.5 Fabric

Fabric used for the present experiment is collected from a NAFFCO brand fire protective suit.

The suit is made of aluminized glass fiber with vapor absorbent stitched with Kevlar fiber.

Kevlar is an aramid, a term invented as an abbreviation for aromatic polyamide. The chemical

composition of Kevlar is poly para phenylengeterephithalamide, and it is more properly known as a

para-aramid. Aramids belong to the family of nylons, such as nylon 6,6, do not have very good

structural properties, so, the para-aramid distintion is important. The aramid ring gives Kevlar thermal

stability, while the para structure gives it high strength and modulus (34).

Figure 4-19: Chemical Structure of Kevlar

Monomer (35)

Figure 4-20: Kevlar Monomers Bonding (36)

There are three grades of Kevlar available, Kevlar 29, Kevlar 49 and Kevlar 149. The table below

shows the different thermal properties of Kevlar 49 fibers.

Table 4-3: Thermal Properties of Kevlar 49 (37) (38)

Properties Value

Coefficient of Expansion, ×10-6

(cm/cm.°C)

Longitudinal (0 - 100°C)

Radial (0 - 100°C) -2, -6

+59

Specific Heat at 23°C (J/kg.°C) 1420

Thermal Conductivity at 23°C (W.m-1

.K-1

) 0.04

Heat of Combustion (kJ/g) 34.8

Density (kg/m3) 424

Figure 4-21: Fire Protective Fabric Suit Sample

41

4.6 Experimental Procedure

The fan motors of the wind tunnel were first started for a particular air flow with the help of

butterfly valve and run for about 15 minutes isolating the base plate by an isolator made of MS plate

with circulating water inside. The power to the heating section has been supplied for the desired air

temperature keeping the base plate isolated and waited until the desired steady temperature of the air

jet has been reached. At the same time PicoLog was made ready for data recording. Then the hot air

jet was impinged on the bas plate by removing the isolator and the thermocouple reading were

recorded for about three minutes. The same experiments were repeated for different conditions as

given in Table. The collected data were used to estimate surface heat flux, heat transfer coefficient

and local Nusselt numbers following the mathematical protocol given in Section with a view to

getting heat transfer performance of the base plate under jet impingement.

Table 4-4: Experimental Conditions

Parameters Value

Jet Velocity (m/s) 15, 19

Reynolds Number 3.17×104

Temperature (°C) 125

Nozzle Dia (mm) 25.4

l/d 2, 4, 6

r/d -4.5 ~ 4.5

Environmental Temperature (°C) 25 ~ 32

Table 4-5: Fire Retardant Fabric Characteristics

Brand NAFFCO

Model KA – 800 (PO 300)

Material Aluminized glass fiber with vapor absorbent

stitched with Kevlar fiber

Table 4-6: Thermal Sensor

Base Plate Masonite (Hardboard)

Thermocouple Type K (Ni-Cr/Ni-Al)

Calorimeter Copper Slag Calorimeter* *Copper Slag Calorimeter has been set up as described in ASTM F2703-08 Standard (12)

Figure 4-22: Experimental Setup in The Lab

42

4.7 Calculation / Data Reduction Procedure

The base plate under jet impingement can be modeled as semi-infinite solid having a boundary of

constant heat flux. Considering this, one can easily find the temperature distribution within the base

plate as given in Equation 3-8.

( ) √ ⁄

(

)

( (

√ ))

On the surface (x=0) this equation reduces to Equation 3-9 where T(t) are recorded during experi-

ments and can be used to calculate the surface heat flux.

( ) √ ⁄

Given a heat flux at a particular location and time, Newton‟s law of cooling can be used to cal-

culate the local heat transfer coefficient, h using Equation 4-2.

( ( )) Equation 4-2

The local Nusselt number as defined by Equation 10 at any radial location, r/d, was derived from

the surface heat flux history.

⁄

43

5. NUMERICAL SIMULATION METHODOLOGY & PROCEDURE

5.1 Numerical Simulation Methodology

Computer simulation of the experimental system has been solved using a commercially available

CFD code ANSYS CFX. The simulation is based on three dimensional; two medium treatments

assuming thermal non-equilibrium between fabric and air phases was used to predict the thermal

responses of the fabric system. Figure 5-1 shows the schematic diagram of the fabric system used for

the numerical modeling. The air jet impingement from a nozzle is applied to the front surface of the

fabric. The air flows through the permeable fabric into the base plate.

Figure 5-1: Schematic of Fabric System

The simulation procedure is mainly based on the porous model concept. The fabric system can be

considered as a porous medium with the permeability defined by the Modified Forchheimer equation

(9)

(

)

√

Equation 5-1

where, Pin is the upstream pressure, Pout is the downstream pressure of the fabric layer, L is the

fabric thickness and Ac is the cross-sectional area of the fabric for air flow.

The heat transfer by the gas flow in the permeable fabric system (considered as a porous medium)

can be calculated using the semi-empirical correlation developed by Whitaker (39)

(

)

Equation 5-2

( ) Equation 5-3

here, Dy is the hydraulic diameter of the fabric which can be determined by the Equation

( ) Equation 5-4

Equation 5-5

44

The velocity in a porous medium is given by Darcy‟s Law of Velocity in Porous Medium

Equation 5-6

5.2 Governing Equations

The set of equation solved by ANSYS CFX are the unsteady Navier-Stokes equation in their

conservation form.

5.2.1 Transport Equations

In this section, the instantaneous equation of mass, momentum, and energy conservation are

presented. For turbulent flows, the instantaneous equations are averaged leading to additional terms.

These terms, together with models for them, are discussed in Turbulence and Wall Function Theory.

The instantaneous equations of mass, momentum and energy conservation can be written as

follows in a stationary frame:

5.2.2 The Continuity Equation

Equation 5-7

5.2.3 The Momentum Equations

Equation 5-8

where the stress tensor, τ, is related to the strain rate by

Equation 5-9

5.2.4 The Total Energy Equation

Equation 5-10

45

where htot is the total enthalpy, related to the static enthalpy, h(T, p) by,

Equation 5-11

The term ( ) represents the work due to viscous stresses and is called the viscous work term.

This models the internal heating by viscosity in the fluid, and is negligible in most flows. The term

U.SM represents the work due to external momentum sources and is currently neglected.

5.2.5 The Thermal Energy Equation

An alternative form of the energy equation, which is suitable for low-speed flows, is also avai-

lable. To derive it, an equation is required for the mechanical energy K.

Equation 5-12

The mechanical energy equation is derived by taking the dot product of U with the momentum

equation

Equation 5-13

Subtracting this equation from the total energy equation yields the thermal energy equation

Equation 5-14

The term is always positive and is called the viscous dissipation. This models the internal

heating by viscosity in the fluid, and is negligible in most flows.

With further assumptions, we obtain the thermal energy equation

Equation 5-15

The assumptions are:

1. If h is actually interpreted as internal energy,

46

then the equation can be written as

This interpretation is appropriate for liquids, where variable-density effects are negligible. Note

that the principal variable is still called „Static Enthalpy‟ in CFD-Post, although it actually represents

internal energy. Note also that, for liquids that have variable specific heats the solver includes the P/ρ

contribution in the enthalpy tables. This is inconsistent, because the variable is actually internal

energy. For this reason, the thermal energy equation should not be used in this situation, particularly

for subcooled liquids.

2. On the other hand if

and are neglected then the Equations 26 follows directly. This

interpretation is appropriate for low Mach number flows of compressible gases.

The thermal energy equation, despite being a simplification, can be useful for both liquids and

gases in avoiding potential stability issues with the total energy formulation. For example, the thermal

energy equation is often preferred for transient liquid simulations. On the other hand, if proper

acoustic behavior is required, or for high speed flow, then the total energy equation is required.

5.3 Turbulence Models

Turbulence consists of fluctuations in the flow field in time and space. It is a complex process,

mainly because it is three dimensional, unsteady and consists of many scales. It can have a significant

effect on the characteristics of the flow. Turbulence occurs when the inertia forces in the fluid become

significant compared to viscous forces, and is characterized by a high Reynolds number.

In principle, the Navier-Stokes equations describe both laminar and turbulent flows without the

need for additional information. However, turbulent flows at realistic Reynolds number span a large

range of turbulent length and time scales, and would generally involve length scales much smaller

than the smallest finite volume mesh, which can be practically used in a numerical analysis. The

Direct Numerical Simulation (DNS) of these flows would require computing power which is many

orders of magnitude higher than available in the foreseeable future.

To enable the effects of turbulence to be predicted, a large amount of CFD research has

concentrated on methods which make use of turbulence models. Turbulence models have been

specifically developed to account for the effects of turbulence without resource to a prohibitively fine

mesh and direct numerical simulation. Most turbulence models are statistical turbulence model, as

47

described below. The two exceptions to this in ANSYS CFX are the Large Eddy Simulation Model

and the Detached Eddy Simulation Model.

5.3.1 Two Equation Turbulence Models

Two-equation turbulence models are very widely used, as they offer a good compromise between

numerical effort and computational accuracy. Two-equation models are much more sophisticated than

the zero equation models. Both the velocity and length scale are solved using separate transport

equations.

The k-ε and k-ω two-equation models use the gradient diffusion hypothesis to relate the Reynolds

stresses to the mean velocity gradients and the turbulent viscosity. The turbulent viscosity is modeled

as the product of turbulent velocity and turbulent length scale.

In two-equation models, the turbulence velocity scale is computed from the turbulent kinetic

energy, which is provided from the solution of its transport equation. The turbulent length scale is

estimated from two properties of the turbulence field, usually the turbulent kinetic energy and its

dissipation rate. The dissipation rate of the turbulent kinetic energy is provided from the solution of its

transport equation.

5.3.2 The k-epsilon Model in ANSYS CFX

k is the turbulence kinetic energy and is defined as the variance of the fluctuations in velocity. It

has dimensions of (L2T

-2); for example, m

2/s

2. ε is the turbulence eddy dissipation, and has dimensions

of k per unit time (L2T

-3); for example, m

2/s

3.

The k-ε model introduces two new variables into the system of equations. The continuity equation

is then

Equation 5-16

and the momentum equation becomes

Equation 5-17

where SM is the sum of body forces, μeff is the effective viscosity accounting for turbulence, and

p’ is the modified pressure.

The k-ε model, like the zero equation model, is based on the eddy viscosity concept, so that:

48

Equation 5-18

where μt is the turbulence viscosity. The k-ε model assumes that the turbulence viscosity is linked

to the turbulence kinetic energy and dissipation via the relation:

Equation 5-19

where Cμ is a constant.

The values of k and ε come directly from the differential transport equations for the turbulence

kinetic energy and turbulence dissipation rate

Equation 5-20

where Cε1, Cε2, σk and σε are constants.

Pkb and Pεb represent the influence of the buoyancy forces, which are described below. Pk is the

turbulence production due to viscous forces, which is modeled using

Equation 5-21

For incompressible flow, ( ⁄ ) is small and the second term second term on the right side

of Equation does not contribute significantly to the production. For compressible flow, ( ⁄ ) is

only large in regions with high velocity divergence, such as at shocks.

The term 3μt in equation is based on the “frozen stress” assumption. This prevents the values of k

and ε becoming too large through shocks, a situation that becomes progressively worse as the mesh is

refined at shocks. The parameter Compressible Production can be used to set the value of the factor in

front of μt, the default value is 3. A value of 1 will be providing the same treatment as CFX-4.

49

5.4 Code Validation

We have been using the NAFFCO brand Fire Protective Suit for our experimental purpose. The

NAFFCO brand fire retardant suit is made with aluminized glass fiber with vapor absorbent stitched

with Kevlar fiber. The systems of fiber are very much complicated for modeling in simulation. So, we

have decided for the simplified model using only the Kevlar fiber and visualize the effect of thermal

response if only Kevlar fibers are used instead of the systems of fibers. For this reason we can‟t

directly compare the simulation result with our own experiment. So, we have validated the CFD code

and the process by comparing the result obtained by Lee et al (9). The conditions and simulation

properties are discussed in brief here.

Schematic Diagram

Conditions

1. Jet Velocity = 32 m/s

2. Jet Temperature = 100°C

3. Nozzle Diameter = 20.6 mm

4. Nozzle to plate distance, l/d = 3.7

5.4.1 Comparison

Figure 5-2: Temperature before the Fabric Phase

50

Figure 5-3: Temperature after the Fabric Phase

5.4.2 Simulation Settings for Validation

Table 5-1: Domain Modeling for Validation

Domain Type Fluid

Material

Fluid Definition Air at 25°C

Morphology Continuous Fluid

Settings

Buoyancy Model Non Buoyant

Domain Motion Stationary

Reference Pressure 1 atm

Heat Transfer Model Total Energy

Turbulence Model k epsilon

Turbulent Wall Functions Scalable

High Speed Model Off

Table 5-2: Interface Modeling for Validation

Domain Interface Fluid-Porous

Interface Models General Connection

Heat Transfer Conservative Interface Flux

Mass and Momentum Side Dependent

Mesh Connection GGI

51

5.5 Computer Numerical Simulation Procedure

5.5.1 Geometry for Simulation

The geometry for computer simulation has been drawn using the SolidWorks. Instead of modeling

the node for the whole geometry, one part among the four equal division parts of the geometry has

been modeled for computation. Thus the computational accuracy is increased by modeling only a

similar part with higher volume nodes. The dimension of the part modeled has been shown in the

Figure 5-4.

Figure 5-4: Geometry for Modeling

5.5.2 Meshing

The mesh generation has been done using the CFX-Mesh. Figure 5-5 shows the mesh generation

for the geometry.

Figure 5-5: Mesh Generation for the Geometry

Table 5-3: Mesh Report

Domain Nodes Elements Element Shape

Fluid Domain 25817 130776 Tetrahedral

Porous 8464 6075 Quadrilateral

All Domains 34281 136851

52

5.5.3 Simulation Setup

For simplicity we have only modeled the Kevlar fiber for visualization of the thermal response. In

experiment we could not collect a single layer of Kevlar fiber, so the experiments are done on the

systems of fabrics. So, the simulation also helps us to understand the total effect of the system.

Domain - Fluid Domain

Type Fluid

Location B237

Materials

Air at 25 C

Fluid Definition Material Library


Settings



Reference Pressure 1.0000e+00 [atm]





Domain - Porous

Type Porous

Location B270

Materials

Air at 25 C

Fluid Definition Material Library


Settings



Reference Pressure 1.0000e+00 [atm]





Domain Interface - Domain Interface 1

Boundary List1 Domain Interface 1 Side 1

Boundary List2 Domain Interface 1 Side 2

Interface Type Fluid Porous

53

Settings

Interface Models General Connection

Mass And Momentum Conservative Interface Flux

Mesh Connection GGI

Table 5-4: Boundary Details

Domain Boundaries

Fluid Domain

Boundary - Inlet

Type INLET

Location Inlet

Settings

Flow Regime Subsonic

Heat Transfer Static Temperature

Static Temperature 3.9800e+02 [K]

Mass And Momentum Normal Speed

Normal Speed 1.9000e+01 [m s^-1]

Turbulence Medium Intensity and Eddy Viscosity Ratio

Boundary - Domain Interface 1 Side 1

Type INTERFACE

Location Interface_Air_Side

Settings



Turbulence Conservative Interface Flux

Boundary - Surrounding

Type OPENING

Location Surrounding

Settings

Flow Regime Subsonic

Heat Transfer Opening Temperature

Opening Temperature 2.9800e+02 [K]

Mass And Momentum Entrainment

Relative Pressure 1.0000e+00 [atm]

Turbulence Zero Gradient

Boundary - Symmetry

Type SYMMETRY

Location Symmetry

Settings

Boundary - Pipe

54

Type WALL

Location Pipe

Settings

Heat Transfer Adiabatic

Mass And Momentum No Slip Wall

Wall Roughness Smooth Wall

Porous

Boundary - Domain Interface 1 Side 2

Type INTERFACE

Location Interface_Fabric_Side

Settings



Turbulence Conservative Interface Flux

Boundary - Outlet

Type WALL

Location Outer_Side

Settings

Heat Transfer Heat Transfer Coefficient

Heat Transfer Coefficient 7.0940e+01 [W m^-2 K^-1]

Outside Temperature 2.9800e+02 [K]



Boundary - Surrounding_Fabric

Type WALL

Location Surrounding_Fabric

Settings

Heat Transfer Heat Transfer Coefficient

Heat Transfer Coefficient 7.0940e+01 [W m^-2 K^-1]

Outside Temperature 2.9800e+02 [K]



Table 5-5: Constant for Kevlar 49

Ergun coefficient (ε) 0.626

Permeability 0.2×10-10

m2

Hydraulic Diameter of the Yarn 0.22m

Correction Factor Csg 1

55

6. RESULTS & DISCUSSIONS

6.1 Experimental Result

A total of 12 experiments were performed yielding over 16,000 data points for different

conditions. In this section the experimental result and the calculated surface heat flux and Nusselt

number variation for different experimental condition are displayed and analyzed in this section.

Figures 6-1 to 12 show the PicoLog data attained for different experimental conditions. The data

is presented for the entire 7 channel which represents the 7 thermocouple temperature readings. All

readings are directly fed to the computer attached with the PicoLogger Data Acquisition System. The

computer saves all the data in a table. Those are then used for calculation.

The PicoLogger starts recording the thermocouple reading sometimes before the isolation shutter

has been removed. So, first few data (about for 3 – 8 sec) are the initial reading of the base plate of the

thermocouple. The PicoLogger has been stopped recording data automatically after 200 seconds.

Results in this section are presented in a Temperature – Time plot. The Temperature increases

rapidly at the beginning of the experiments and become steady after about 65 seconds of experiments.

It has been noted that temperature data with the fire retardant fabric system has been increasing very

steadily from the very beginning. With the increase of velocity the temperature reading also increases.

This is due to the effect of increasing heat transfer at higher Reynolds number. The temperature

reading for the central thermocouple (Channel 4) has been noted to be the maximum in any time

interval period. The temperature reading drops in the adjacent thermocouple as the radial distance

increases. The temperature reading also decreases as the space between nozzle to base plate increases.

56

Conditions:

l/d = 2;

v = 15 m/s;

Tjet = 126°C

Base Plate: Massonite (Hardboard)

Fabric Used: No

Figure 6-1: Temperature Variation with Time without Fabric for l/d = 2 and velocity 15 m/s

57

Conditions:

l/d = 2;

v = 15 m/s;

Tjet = 124°C


Fabric Used: Yes

Figure 6-2: Temperature Variation with Fabric for l/d = 2 and velocity 15 m/s

58

Conditions:

l/d = 2;

v = 19 m/s;

Tjet = 134°C


Fabric Used: No


59

Conditions:

l/d = 2;

v = 19 m/s;

Tjet = 118°C


Fabric Used: Yes

Figure 6-4: Temperature Variation with Time with Fabric for l/d = 2 and velocity 19 m/s

60

Conditions:

l/d = 4;

v = 15 m/s;

Tjet = 132°C


Fabric Used: No


61

Conditions:

l/d = 4;

v = 15 m/s;

Tjet = 124°C


Fabric Used: Yes


62

Conditions:

l/d = 4;

v = 19 m/s;

Tjet = 134°C


Fabric Used: No


63

Conditions:

l/d = 4;

v = 19 m/s;

Tjet = 124°C


Fabric Used: Yes


64

Conditions:

l/d = 6;

v = 15 m/s;

Tjet = 126°C


Fabric Used: No


65

Conditions:

l/d = 6;

v = 15 m/s;

Tjet = 130°C


Fabric Used: Yes


66

Conditions:

l/d = 6;

v = 19 m/s;

Tjet = 133°C


Fabric Used: No


67

Conditions:

l/d = 6;

v = 19 m/s;

Tjet = 125°C


Fabric Used: Yes


68

6.2 Calculated Result

6.2.1 Heat Flux – Time

In this section the surface heat flux variation at the stagnation point of the base plate has been

displayed and analyzed for the both condition of base plate, with and without the fire retardant fabric.

The results are presented in terms of Surface Heat Flux (kW/m2) and Interval Time (sec).

Figures 6-13 to 18 show the surface heat flux variation of the base plate for both the conditions. In

all of these figures the upper solid line represents the condition for base plate without fire retardant

fabric system and the lower dashed line represents the condition with fire retardant fabric attached

adjacent to the base plate. As shown in the Figure 6-13, the heat flux of the base plate without the fire

retardant fabric rapidly increases and then slowly decreases with the increasing time. The maximum

heat flux occurs within 20 second from the commencement of the jet impingement. The heat flux of

the base plate with the fire retardant fabric system follows a different trend as shown. The heat flux in

this case increases suddenly at the beginning and then decreases slowly having the values much lower

than those without fabric conditions.

As the space between nozzle to base plate increases the maximum heat flux for the condition

decreases. This may due to the effect of lower heat carrying capability of air and also for the entrain-

ment occurs as the distance increases.

It has also been noted that with the increase of velocity (Fig. 6-16 to 18) the maximum heat flux

occurs for each condition also increases. At higher velocity – higher Reynolds number – the heat

transfer coefficient increases, so the heat flux also increases with increasing velocity. This has been

illustrated in details in the later part of this section.

69

Figure 6-13: Surface Heat Flux Variation with Time for l/d = 2 and velocity 15 m/s

70

Figure 6-14: Surface Heat Flux Variation for l/d = 4 and velocity 15 m/s

71

Figure 6-15: Surface Heat Flux Variation for l/d = 6 and velocity 15 m/s

72


73


74


75

6.3 Transient Effect on Nusselt Number

In this section the transient effect on Nusselt number has been discussed. The results presented in

this section are in terms of nondimentional local Nusselt Number (Nu) and radial distance from the

stagnation point (r/d).

Figures 6-19 to 24 show the Nusselt number distribution for later intervals. At interval time t=65s

the Nusselt number is slightly increased from the previous interval for both cases. This is the maxi-

mum heat transfer coefficient. After this time interval the heat transfer converges to steady state and

the heat transfer coefficient decreases. This is also the assumption for the semi-infinite solid theory.

The distribution of Nusselt number for various radial directions are much more uniform then the

previous time interval. For a further later interval of t=180s (Fig. 6-19), the heat transfer coefficient

decreases more than the previous time interval.

The maximum Nusselt number occurs at the stagnation point. At the stagnation point all the

kinetic energy transformed into pressure energy. As a result the velocity air at the stagnation point is

zero. But at stagnation point the temperature of the air is very high, thus the heat transfer coefficient is

higher at stagnation point than the other radial position. So, the maximum pick of Nusselt number

occurs at the stagnation point.

After the stagnation point the velocity of air start to increase due to the pressure different between

the surroundings and the stagnation point. But still the velocity of air is lower and the temperature of

the jet decreases as the heat transferred to the base plate and the surroundings. For lower velocity and

decreasing air temperature, the heat transfer coefficient also decreases as well as the Nusselt number.

As the radial distance increases, the velocity of air at the surface of the base plate increases. With

the increasing velocity the heat transfer coefficient of the air also increases. Although the temperature

of the air is now very low then at the stagnation point, but due to the increase in velocity the Nusselt

number is again increasing.

In the Figures 6-19 to 24, there have been seen a second pick after the stagnation point, for the

upper side. This may occur due to the improper surface condition of the base plate. As a result the

velocity and temperature of air may increases at the point where the second pick has been noticed.

Thus the heat transfer coefficient as well as the Nusselt number increases at those point. The smooth

surface may reduce the effect of second pick.

76

Figure 6-19: Radial Nusselt Number Distribution for l/d = 2 and velocity 15 m/s


77



78



79

6.4 Effect of Space between Plate and Nozzle (l/d)

In this section the effect of space between nozzle to plate separation has been discussed in terms

of both the Surface Heat Flux – Time and local Nusselt number (Nu) – radial position (r/d).

Figures 6-25, 26 show the surface heat flux of the base plate for both condition – without and with

fire retardant fabric – for different nozzle to plate separations. For the base plate only the heat flux to

time curve follow a similar trend for the entire nozzle to plate separation. For lower nozzle to plate

separation the heat flux is higher and decreases with increasing the distance.

For the condition of the base plate with fabric (Fig. 6-25), the curve follows a similar trend for l/d

of 4 and 6. But for l/d of 2 the curve follows a different trend. It suddenly increases with the start of

impingement and then deceases to the steady state.

With higher velocity, as shown in figure 6-26, the surface heat flux follows a similar trend as for

the lower velocity. But for higher velocity the maximum heat flux become higher than for the lower

velocity.

Figures 6-27 to 32 show the radial local Nusselt number distribution for different l/d position at

different interval time. For larger nozzle to plate separation (Fig. 6-32), the radial Nusselt number

distribution depicts a much more complete distribution, with a pick at the stagnation point and

decreases with the increasing the increasing radial position. But for l/d of 2 with fire retardant fabric,

the result shows a deviation from other two l/d separation. Here a secondary pick has been noticed

with the pick at stagnation point. Unlike for the condition of base plate without fabric, the second pick

for this condition may occur due to the leakage in the fabric support system.

For lower velocity (Fig. 6-27 to 29) the maximum Nusselt number occurs at some distance offset

from the stagnation point for l/d = 6. These may due to the effect of jet center offset as experimented

earlier with the wind tunnel.

80

Figure 6-25: Effect of l/d on Surface Heat Flux for velocity 15 m/s

81

Figure 6-26: Effect of l/d on Surface Heat Flux for velocity 19 m/s

82

Figure 6-27: Radial Nusselt Number Distribution for Different l/d Position for velocity 15 m/s; time interval t = 30s



83




84

6.5 Effect of Velocity on Surface Heat Flux

In this section the effect of changing velocity on surface heat flux has been discussed. The results

are presented in terms of surface heat flux – Interval Time.

The lower velocity of the impinging jet (Fig. 6-33) depicts a lower maximum heat flux for the

stagnation point and for the subsequent radial position than the higher velocity. But like the higher

velocity air jet impingement, similar trends for the heat flux are observed for the lower velocity.

In Figures 6-33 to 35 the effects of velocity on surface heat flux have been shown. The surface

heat flux of the base plate for velocity of 19 m/s has been found higher than for 15 m/s for the

condition of base plate without fire retardant fabrics. But with the fire retardant fabric, the heat flux

for velocity 15 m/s increase to a higher value suddenly after the start of jet impingement than for

velocity of 19 m/s for the position of l/d = 2 only. For other l/d position the heat flux for the earlier

case has been found to be higher than the later cases.

The surface heat flux increases with the increasing velocity due to the higher heat transfer occurs

at higher Reynolds number. With the fire retardant fabric in adjacent to the base plate, the heat

transfer rate suddenly increase after the sudden start of the impingement. But steadily decrease as the

time interval increases.

85

Figure 6-33: Effect of Velocity on Surface Heat Flux for l/d = 2

86


87


88

6.6 Numerical Simulation Results

In this section the computer simulation results have been discussed and compared with the

experimental results. The computer simulation result (Fig. 6-36, 37) shows the similar trend in both

conditions – with and without fabric before the base plate. But in simulation the maximum heat flux is

much higher than the experimental case. For simplicity we have been conducted the simulation model

using only the Kevlar fiber. This allows us to visualize the effect of total system of fabrics using

which we conduct our experiments.

Figures 6-36, 37 show the result of the computer simulation. The condition is for l/d of 2 and the

velocity of 19 m/s. The temperature at the fabric front is about 120°C where in case of experiment, the

temperature increase only up to 93°C. The lower temperature attained in experiment may due to the

heat loss to surrounding, which is not modeled in the simulation. Though a lower percentage of

entrainment have been modeled using computer simulation, the effect is only 5°C decrease in temp-

erature than the jet temperature. Again the numerical simulation does not perform to 100% accuracy.

There are certain errors in numerical calculation. All these contribute to the difference in temperature

between computer simulation and experimental results.

For the temperature after the fabric phase, the numerical model shows a higher temperature than

the experiments. This may due to several reasons. Firstly, the only Kevlar fiber has been modeled for

computer simulation, where in experiment we have been using a system of fabrics which includes the

Kevlar fiber. The system of fabric may contribute to the higher heat resistance than the Kevlar fiber

only. Secondly, again, the surrounding heat transfer effect may cause the experimental value to be

lower. While a small percentage of entrainment is modeled in computer simulation but the effect may

not be in such that it decreases the temperature to near the experimental value. And lastly, there are

some error involve in numerical computation.

Figure 6-38 shows the comparison of surface heat flux between the numerical solution and the

experimental solution for the condition of base plate with fabric. Here again the two curve follows the

similar trends. But in numerical solutions the heat flux is much higher than the experimental solution.

As previously explained the cause may due to the above mentioned three reasons.

89

Figure 6-36: Temperature at the Fabric Front

Figure 6-37: Temperature after the Fabric Phase

Conditions: l/d = 2; v = 19 m/s; Base Plate = Massonite (Hardboard)

90

Figure 6-38: Comparison between Simulation and Experimental Result

Conditions:

l/d = 2;

v = 19 m/s;


91

6.7 The Safety Comparison

The Stoll criterion (8) (20) is a good approximation to compare the protective clothing / fire

retardant clothing for their thermal response to the thermal injury. For this reason, we compared our

findings with the Stoll criterion to visualize the effect.

Figure 6-39 shows the comparison for human tissue tolerance for pain sensation. In this figure, it

can be seen that for both experimental and simulation result, the surface heat flux is well below the

Stoll criterion, while the result for the condition of the base plate without fabric shows that it crosses

the Stoll criterion curve. Thus it can be said that the Kevlar fabric, both alone and with other fabric,

can resist the pain sensation.

Figure 6-40 shows the comparison for human tissue tolerance for the second degree burn. Here

again the Kevlar fiber, both alone and with a system of fibers, can save the person who wear the

protective suit made with Kevlar fiber.

92

Figure 6-39: Human Tissue Tolerance to Pain Sensation

Figure 6-40: Human Tissue Tolerance to Second Degree Burn

Conditions: l/d = 2; v = 19 m/s; Base Plate = Massonite (Hardboard)

93

7. CONCLUSION

Experiments are conducted using a hot air jet impinged on a base plate with/without the protective

fabric system to mimic the flame blast condition for different experimental conditions. The temp-

eratures measured are used to calculate the surface heat flux transferred to the solid base plate. More-

over, the local heat transfer coefficient and the local Nusselt number for different radial positions of

the base plate have been exhibited and analyzed in this study. A computer model is also developed for

measuring the thermal response of the protective clothing. The results – both in simulation and experi-

ments – show a significant decrease in heat transfer rate using the protective fabric. The setup can be

used for testing the heat transfer capability for any kind of fabric layer used for different purposes.

7.1 Future Work

Future research in this area is very broad, in particular for the evaluation of protective clothing.

The experiments and numerical modeling we have been done is only compared the thermal response

of Protective Suit or Kevlar Fiber. Use of Kevlar fiber, as a fire retardant fabric, has been significantly

decreases as new fibers like NOMEX comes to the market. Also, Kevlar fiber is now very much

suitable for fabricating the bullet proof vest. So, in future NOMEX fabric could be tested and com-

pared for thermal response. Again, non-protecting fibers, like cotton, spandex could also be tested to

visualize the thermal response.

New test structure could also be constructed for visualizing direct flame effect on fabric. Thermal

stability and thermal resistivity could be measure using direct flame. Also the present test structure

can produce a very high temperature air jet. This could be used for other high temperature jet

impingement testing.

94

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96

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http://composite.about.com/od/aboutcompositesplastics/l/aa050597.htm.

35. Downing, J. W., Newell, J.A. Characterization of Structural Changes in Thermally

Enhanced Kevlar-29 Fiber. New Jersey : Department of Chemical Engineering, Rowan University,

2002.

36. Yip, N., Wieand, D. Kevlar (poly-phenyleneterephtalamide). chempolymerproject. [Online]

Tangient LLC. https://chempolymerproject.wikispaces.com/Kevlar-E-nydw.

37. Lee, S. M. Handbook of Composite Reinforcements. California : Wiley VCH, 1992. pp. 19,

Table 11.

38. Kevlar, Aramid Fiber. Technical Guide. s.l. : DuPont.

39. Whitaker, S. Fundamental Principles of Heat Transfer. s.l. : Krieger Publishing Company,

1983.

97

APPENDIX A

Heater Design

For the purpose of design the heater many different configurations has been mathematically and

numerically analyzed. The final design has been described in Section 4.3. Here other configuration

has been shown.

Configuration 1:

Figure A-1: Configuration 1

Result:

Figure A-2: Temperature Profile along the Configuration 1 Setup

Configuration 2:


Result:

Figure A-4: Wall Temperature Requirement with Heating Lenght to attain Target Jet Temperature above 100°C

98

Configuration 3:


Initial Condition:

Inlet Air Temperature = 30°C

Heater Wall Temperature = 205°C

Correlation:

Nu=0.37 Re0.8 for 17 < Re < 70000

Results:

Figure A-6: Results for Configuration 3

Configuration 4:


Initial Condition:

Inlet Air Temperature = 30°C

Heater Wall Temperature = 205°C

Correlations: Knudsen and Katz suggested,

Nu=C Ren Pr1/3

For Tube Banks of 4 rows high and 6 rows

deep,

C = 0.27

n = 0.63

Result:

Temperature Increased per Stage: 20.2°C

99

Configuration 5:


Figure A-9: Result of Configuration 5

100

APPENDIX B

Radial Nusselt Number Distribution

101

Nu – r/d for l/d = 2 and velocity 15 m/s

Figure B-1: Nu - r/d for t = 30s



102





103





104





105





106





107

APPENDIX C

Design Files

111

APPENDIX D

PicoLog Specification