batch 1 final report draft 6

R.V. COLLEGE OF ENGINEERING, BENGALURU - 560059 (Autonomous Institution Affiliated to VTU, Belagavi)

CFD SIMULATION OF VERTICAL THERMOSYPHON

REBOILER AND ESTIMATION OF HEAT TRANSFER

COEFFICIENT

PROJECT REPORT

Submitted by

ASHWANTH SUBRAMANIAN 1RV12CH007

NIKHIL S 1RV12CH022

RISHABH KHURANA 1RV12CH024

SUDDHADEEP SARKAR 1RV12CH032

Under the guidance of

DR. VINOD KALLUR

Associate Professor

Department of Chemical Engineering

In partial fulfilment for the award of degree

of

Bachelor of Engineering

in

DEPARTMENT OF CHEMICAL ENGINEERING

2016

R.V. COLLEGE OF ENGINEERING, BENGALURU - 560059

(Autonomous Institution Affiliated to VTU, Belagavi)


CERTIFICATE

Certified that the project titled CFD simulation of vertical Thermosyphon Reboiler and Estimation of

Heat Transfer Coefficient is carried out by Ashwanth Subramanian (1RV12CH007), Nikhil S

(1RV12CH022), Rishabh Khurana (1RV12CH024) and Suddhadeep Sarkar (1RV12CH032), who are

bonafide students of R.V. College of Engineering, Bengaluru, in partial fulfillment for the award of degree of

Bachelor of Engineering in Chemical Engineering of the Visvesvaraya Technological University, Belagavi

during the year 2015-2016. It is certified that all corrections/suggestions indicated for the internal assessment

have been incorporated in the report deposited in the department library. The project report has been approved

as it satisfies the academic requirements in respect of project work prescribed by the institution for the said

degree.

Signature of Guide Signature of Head of the Department Signature of Principal

(Dr. Vinod Kallur) (Dr. M.A. Lourdu Antony Raj) (Dr. K.N. Subramanya)

External Viva

Name of examiners Signature with date

1.

2.

R.V. COLLEGE OF ENGINEERING, BENGALURU - 560059

(Autonomous Institution Affiliated to VTU, Belagavi)


DECLARATION

We, Ashwanth Subramanian, Nikhil S, Rishabh Khurana and Suddhadeep Sarkar, students of Eighth

semester B.E., Department of Chemical Engineering, R V College of Engineering, Bengaluru-560059, bearing

USN: 1RV12CH007, 1RV12CH022, 1RV12CH024 and 1RV12CH032 hereby declare that the project titled

CFD simulation of vertical Thermosyphon Reboiler and Estimation of Heat Transfer Coefficient has

been carried out by us and submitted in partial fulfilment of the program requirements for the award of degree

in Bachelor of Engineering in Chemical Engineering of the Visvesvaraya Technological University,

Belagavi during the year 2015-2016.

Further We declare that the content of the dissertation has not been submitted previously by anybody for the

award of any degree or diploma to any other University.

We also declare that any Intellectual property rights generated out of this project carried out at

R.V.C.E. will be the property of R.V.College of Engineering, Bengaluru and we will be among the

authors of the same.

Place: Bengaluru

Date:

Signature

Ashwanth Subramanian ----------------------

Nikhil S ----------------------

Rishabh Khurana ----------------------

Suddhadeep Sarkar ----------------------

i

ACKNOWLEDGEMENTS

With a deep sense of gratitude, veneration and grateful heart we express our sincere thanks to our esteemed

project guide, Dr. Vinod Kallur, Associate Professor, Department of Chemical Engineering, R.V. College of

Engineering, Bengaluru for offering his valuable help in all possible ways, continuous encouragement and

sharing of his deep knowledge throughout the course of our project. He influenced not only this research

work, but also taught us how to accomplish even a routine task with consciousness, patience and dedication.

We wish to express our warm and sincere thanks to Dr. Lourdu Antony Raj, HOD, Department of Chemical

Engineering, for providing us this wonderful opportunity of working in Department of Chemical Engineering.

His kind support and guidance have been of great value in this research.

We would also like to thank, Mrs. Anupama V Joshi, Assistant Professor, and Department of Chemical

Engineering, who was a guiding light throughout our project.

We warmly thank Mr. Harsha, Lab in-charge, department of Chemical Engineering, R. V. College of

Engineering, for constantly helping us in obtaining our objectives.

We thank department for providing us the license for STAR CCM+.

We would also like to thank all the professors of our department as well as our classmate who gave us the

motivation and support to complete this project.

ii

ABSTRACT

Thermosyphon reboilers are extensively used for chemical engineering applications in various

industries. They comprise of 70% of evaporation duties in all process industries. The reason for the

extensive use of this type of reboiler is due to the low operating and maintenance cost, absence of a

pump and its adjunct controllers, since it works on the principle of density gradient induced by

temperature gradient along the length of the tube, no additional pump is required and hence the energy

required for pumping can be saved. Also, addition of valves and gauges required in pumping circuits

can be avoided. Thermosyphon reboilers are majorly used in petroleum refining, petrochemical and

chemical industries. 95% of the reboilers in petroleum industries are horizontal type, 70% are vertical

type in petrochemical industries and in chemical while nearly 100% are vertical type in chemical

industries. Though Thermosyphon reboilers are widely used in various chemical process industries, there

are no methods available in the literature either for the design of thermosyphon reboiler or prediction of

its performance. Models developed so far in the literature ignore the interfacial shear stress, the

compressibility of vapour or assume one-dimensional steady-state Newtonian flows. Instability in two

phase can affect performance which has not been addressed.

A 3D geometrical model of the thermosyphon reboiler was built for this study. A 3D mesh was generated

for the same model to allow setting up simulations. These 3D model and corresponding mesh were

converted into 2D axisymmetric versions in order to reduce computational time. The simulations were

carried out using STAR CCM+ version 9.06.011. Study of Heat Transfer Coefficient which is crucial

for the design of thermosyphon reboiler to study the effect of various parameters on heat transfer. The

instabilities occurring in the reboiler were also analysed in terms of vapour velocity and bubble diameter.

It was found that for increasing wall temperature from 403 K to 473 K, the volume fraction of vapour at

outlet was higher and inception of boiling was noticed earlier for higher wall temperatures. The volume

fraction of vapour was seen to not exceed 0.9 at all conditions because of superheating. It was also seen

that the instabilities in bubble diameter and velocity of vapour reduced with increasing wall

temperatures. Simulation results for lower diameter of 2/3 inch and 3/4 inch showed instabilities.

However, results of simulation for larger pipe diameters of 1 inch and 1.25 inch had negligible

instabilities. Further, instabilities were lower for low inlet velocities of 1 ms-1 and 1.5 ms-1 but, at higher

inlet velocities of 2.5 ms-1 and 3 ms-1, instabilities were not negligible. The variation of heat transfer

coefficient with vapour velocity and bubble Reynolds number prove that heat transfer coefficient in the

vertical thermosyphon reboiler is a function of bubble Reynolds number and Prandtl number but not the

Reynolds number. For higher temperatures, it is observed that some factor other than bubble Reynolds

number is also important which requires further investigation.

iii

TABLE OF CONTENTS

NO. TITLE PAGE

ACKNOWLEDGEMENTS

ABSTRACT

i

ii

TABLE OF CONTENTS iii

LIST OF TABLES vi

LIST OF FIGURES vii

LIST OF SYMBOLS AND ABBREVIATIONS xiii

1.0 Chapter 1: Introduction 1

1.1 Vertical thermosyphon reboilers 2

1.2 Limitations of thermosyphon reboilers 3

1.3 Applications of Computational Fluid Dynamics 3

1.4 Multiphase Flow for Vertical system 4

1.5 Flow pattern transitions in vertical flow 5

1.6 Correlations of boiling heat transfer data 7

1.7 Analysis of thermosyphon systems-Literature Survey 8

1.8 Emerging Technologies 13

1.9 Motivation 13

1.10 Objective and scope of the project 14

1.11 Methodology 14

1.12 Organization of the report 15

2.0 Chapter 2: Physics Models 17

2.1 Axisymmetric Model 17

2.2 Multiphase Models 18

2.2.1 Sub-models under Multiphase Model 18

2.3 Multiphase Interaction 19

2.3.1 Sub-models under Multiphase Interaction 19

3.0 Chapter 3: Design of Thermosyphon Reboiler STAR CCM+ 25

3.1 General Sequence followed in STAR CCM+ 25

3.1.1 Building 3D Geometry 25

3.1.2 Adding Physics Models 27

3.1.3 Generation of Mesh 27

3.1.4 Conversion of 3D mesh to 2D Mesh 28

3.1.5 Analysis Scheme 29

iv

4.0 Chapter 4: Formulation of Cases for Simulation 30

4.1 Simulation Conditions 30

4.2 Simulation Parameters 30

4.3 Meshing Parameters 31

4.4 Selection of Physics Models 32

4.5 Setting up material properties 33

4.6 Simulation cases for a tube of 2/3 inch diameter 34

4.7 Simulation cases for tubes of 1 inch and 1.25 inch diameter 35

5.0 Chapter 5: Results and Analysis 37

5.1 Influence of Mesh type on simulation results 37

5.2 Results and Analysis for Pipe diameter of 2/3 inch 40

5.2.1 Axial variation of vapour fraction for different

velocities(Set-1)

40

5.2.2 Influence of wall temperature on volume fraction of vapour 45

5.2.3 Axial variation of vapour velocity for different inlet

velocities (Set-2)

45

5.2.4 Axial variation of bubble diameter for different inlet velocities

(Set-3)

50

5.2.5 Variation of Heat Transfer Coefficient at wall along the

length of the tube (Set-4)

55

5.2.6 Variation of Heat Flux at Wall with Delta T along the tube

(Set-5)

60

5.2.7 Variation of Heat Transfer Coefficient with Velocity of

Vapour in the tube (Set-6)

65

5.2.8 Variation of Heat Transfer Coefficient with Bubble Reynolds

Number (Set-7)

69

5.3 Analysis of tube of 1-inch and 1.25 inch diameter 74

5.3.1 Variation of Velocity of Vapour Along the axis (1 inch

diameter pipe) (Set-8)

74

5.3.2 Variation of Velocity of Vapour Along the axis (1.25 inch


79

5.3.3 Variation of Bubble diameter along the axis (1 inch diameter

pipe) (Set-10)

83

5.3.4 Variation of Bubble diameter along the axis (1.25 inch


88

5.3.5 Variation of Heat Transfer Coefficient with Bubble Reynolds

Number (1 inch diameter pipe) (Set-12)

93

5.3.6

Variation of Heat Transfer Coefficient with Bubble Reynolds

Number (1.25 inch diameter pipe) (Set-13)

98

v

5.4 Significance of findings 103

6.0 Chapter 6: Conclusions 104

6.1 Scope for future work 105

References 106

vi

LIST OF TABLES

Table No. Title of the Table Page No.

1.1 Existing equations for heat transfer coefficient from literature 8

4.1 Simulation Conditions 30

4.2 Simulation Parameters 31

4.3 Meshing Parameters 31

4.4 Selection of Physics Models 32

4.5 Eulerian Multiphase Models 32

4.6 Phase Interaction Physics Models 33

4.7 Material Properties of liquid at inlet 34

4.8 Material Properties of vapour 34

4.9 Simulation Sets and Descriptions for a tube of 2/3 inch diameter 35

4.10 Simulation Sets and Descriptions for a tube of 1 inch and 1.25 inch diameter 36

5.1 The condition set for checking effect of meshing 38

vii

LIST OF FIGURES

Figure No. Title of the Figure Page No.

1.1(a) Vertical thermosyphon reboiler 2

1.1(b) Horizontal thermosyphon reboiler 2

1.2 Types of flow regimes for vertical flow 4

1.3 Data for pressure gradient in fully developed air-water flux in a vertical tube 6

2.1(a) Sample Geometry 17

2.1(b) Axisymmetric Model of Sample Geometry 17

2.2 Illustration of Cole bubble departure frequency parameter 21

2.3 Illustration of Hibiki Ishii Nucleation Site Number Density parameter 22

3.1 General steps of operation in STAR CCM + 25

3.2 The 2D Sketch creation in STAR CCM+ of tube 26

3.3 The full 3D tube created in STAR CCM+ 26

3.4 Final 3D geometry created 27

3.5 The mesh scene of the 3D model 28

3.6 The converted 2D mesh of the tube 29

5.1(a) Trimmer meshing in STAR CCM+ 37

5.1(b) Tetrahedral meshing in STAR CCM+ 38

5.1(c) Polyhedral Meshing in STAR CCM+ 38

5.2(a) Simulation results of trimmer meshing 39

5.2(b) Simulation results of polyhedral meshing 39

5.2(c) Simulation results of tetrahedral meshing 40

5.3(a) Vapour Fraction along the axis for wall temperature: 403 K 41

5.3(b) Vapour Fraction along the axis for wall temperature: 413 K 41

5.3(c) Vapour Fraction along the axis for wall temperature: 423 K 42

5.3(d) Vapour Fraction along the axis for wall temperature: 433 K 42

5.3(e) Vapour Fraction along the axis for wall temperature: 443 K 43

5.3(f) Vapour Fraction along the axis for wall temperature: 453 K 43

5.3(g) Vapour Fraction along the axis for wall temperature: 463 K 44

5.3(h) Vapour Fraction along the axis for wall temperature: 473K 44

5.4 Scalar scene representation - increasing wall temperature at inlet velocity = 1

ms-1

45

5.5(a) Vapour velocity (ms-1) variation along the axis for wall temperature: 403 K 46

5.5(b) Vapour velocity (ms-1) variation along the axis for wall temperature: 413 K 47

5.5(c) Vapour velocity (ms-1) variation along the axis for wall temperature: 423 K 47

file:///C:/Program%20Files/CD-adapco/STAR-CCM+9.06.009-R8/doc/en/online/STARCCMP/usingSegregatedMultiphase.205.049.html%23ww1193994

viii

5.5(d) Vapour velocity (ms-1) variation along the axis for wall temperature: 433 K 48

5.5(e) Vapour velocity (ms-1) variation along the axis for wall temperature: 443 K 48

5.5(f) Vapour velocity (ms-1) variation along the axis for wall temperature: 453 K 49

5.5(g) Vapour velocity (ms-1) variation along the axis for wall temperature: 463 K 49

5.5(h) Vapour velocity (ms-1) variation along the axis for wall temperature: 473 K 50

5.6(a) Bubble diameter (m) variation along the axis for wall temperature: 403 K 51

5.6(b) Bubble diameter (m) variation along the axis for wall temperature: 413 K 51

5.6(c) Bubble diameter (m) variation along the axis for wall temperature: 423 K 52

5.6(d) Bubble diameter (m) variation along the axis for wall temperature: 433 K 52

5.6(e) Bubble diameter (m) variation along the axis for wall temperature: 443 K 53

5.6(f) Bubble diameter (m) variation along the axis for wall temperature: 453 K 53

5.6(g) Bubble diameter (m) variation along the axis for wall temperature: 463 K 54

5.6(h) Bubble diameter(m) variation along the axis for wall temperature: 473 K 54

5.7(a) Heat Transfer Coefficient (Wm-2K-1) variation along the axis for wall

temperature: 403 K

55

5.7(b) Heat Transfer Coefficient (Wm-2K-1) variation along the axis for wall

temperature: 413 K

56

5.7(c) Heat Transfer Coefficient (Wm-2K-1) variation along the axis for wall

temperature: 423 K

56

5.7(d) Heat Transfer Coefficient (Wm-2K-1) variation along the axis for wall

temperature: 433 K

57

5.7(e) Heat Transfer Coefficient (Wm-2K-1) variation along the axis for wall

temperature: 443 K

58

5.7(f) Heat Transfer Coefficient (Wm-2K-1) variation along the axis for wall

temperature: 453 K

58

5.7(g) Heat Transfer Coefficient (Wm-2K-1) variation along the axis for wall

temperature: 463 K

59

5.7(h) Heat Transfer Coefficient (Wm-2K-1) variation along the axis for wall

temperature: 473 K

60

5.8(a) Heat Flux (Wm-2) vs Delta T for wall temperature: 403 K 61

5.8(b) Heat Flux (Wm-2) vs Delta T for wall temperature: 413 K 61

5.8(c) Heat Flux (Wm-2) vs Delta T for wall temperature: 423 K 62

5.8(d) Heat Flux (Wm-2) vs Delta T for wall temperature: 433 K 62

5.8(e) Heat Flux (Wm-2) vs Delta T for wall temperature: 443 K 63

5.8(f) Heat Flux (Wm-2) vs Delta T for wall temperature: 453 K 63

5.8(g) Heat Flux (Wm-2) vs Delta T for wall temperature: 463 K 64

ix

585.8(h) Heat Flux (Wm-2) vs Delta T for wall temperature: 473 K 64

5.9(a) Variation of Heat Transfer Coefficient (Wm-2K-1) with Velocity of Vapour at

wall temperature of 403 K

65

5.9(b) Variation of Heat Transfer Coefficient (Wm-2K-1) with Velocity of Vapour at


66

5.9(c) Variation of Heat Transfer Coefficient (Wm-2K-1) with Velocity of Vapour at


66

5.9(d) Variation of Heat Transfer Coefficient (Wm-2K-1) with Velocity of Vapour at


67

5.9(e) Variation of Heat Transfer Coefficient (Wm-2K-1) with Velocity of Vapour at


67

5.9(f) Variation of Heat Transfer Coefficient (Wm-2K-1) with Velocity of Vapour at


68

5.9(g) Variation of Heat Transfer Coefficient (Wm-2K-1) with Velocity of Vapour at


68

5.9(h) Variation of Heat Transfer Coefficient (Wm-2K-1) with Velocity of Vapour at


69

5.10(a) Variation of Heat Transfer Coefficient (Wm-2K-1) with Bubble Reynolds

Number at wall temperature of 403 K

70

5.10(b) Variation of Heat Transfer Coefficient (Wm-2K-1) with Bubble Reynolds


70

5.10(c) Variation of Heat Transfer Coefficient (Wm-2K-1) with Bubble Reynolds


71

5.10(d) Variation of Heat Transfer Coefficient (Wm-2K-1) with Bubble Reynolds


71

5.10(e) Variation of Heat Transfer Coefficient (Wm-2K-1) with Bubble Reynolds


72

5.10(f) Variation of Heat Transfer Coefficient (Wm-2K-1) with Bubble Reynolds


72

5.10(g) Variation of Heat Transfer Coefficient (Wm-2K-1) with Bubble Reynolds


73

5.10(h) Variation of Heat Transfer Coefficient (Wm-2K-1) with Bubble Reynolds


73

5.11(a) Vapour velocity (ms-1) variation along the axis for pipe diameter of 1 inch

and wall temperature: 403K

75

x

5.11(b) Vapour velocity (ms-1) variation along the axis for pipe diameter of 1 inch

and wall temperature: 413 K

75

5.11(c) Vapour velocity (ms-1) variation along the axis for pipe diameter of 1 inch


76

5.11(d) Vapour velocity (ms-1) variation along the axis for pipe diameter of 1 inch


76

5.11(e) Vapour velocity (ms-1) variation along the axis for pipe diameter of 1 inch


77

5.11(f) Vapour velocity (ms-1) variation along the axis for pipe diameter of 1 inch


77

5.11(g) Vapour velocity (ms-1) variation along the axis for pipe diameter of 1 inch


78

5.11(h) Vapour velocity (ms-1) variation along the axis for pipe diameter of 1 inch


78

5.12(a) Vapour velocity (ms-1) variation along the axis for pipe diameter of 1.25 inch


79

5.12(b) Vapour velocity (ms-1) variation along the axis for pipe diameter of 1.25 inch


80

5.12(c) Vapour velocity (ms-1) variation along the axis for pipe diameter of 1.25 inch


80

5.12(d) Vapour velocity (ms-1) variation along the axis for pipe diameter of 1.25 inch


81

5.12(e) Vapour velocity (ms-1) variation along the axis for pipe diameter of 1.25 inch


81

5.12(f) Vapour velocity (ms-1) variation along the axis for pipe diameter of 1.25 inch


82

5.12(g) Vapour velocity (ms-1) variation along the axis for pipe diameter of 1.25 inch


82

5.12(h) Vapour velocity (ms-1) variation along the axis for pipe diameter of 1.25 inch


83

5.13(a) Bubble diameter (m) variation along the axis for pipe diameter of 1 inch and

wall temperature: 403 K

84

5.13(b) Bubble diameter (m) variation along the axis for pipe diameter of 1 inch and


84

5.13(c) Bubble diameter (m) variation along the axis for pipe diameter of 1 inch and


85

xi

5.13(d) Bubble diameter (m) variation along the axis for pipe diameter of 1 inch and


85

5.13(e) Bubble diameter (m) variation along the axis for pipe diameter of 1 inch and


86

5.13(f) Bubble diameter (m) variation along the axis for pipe diameter of 1 inch and


87

5.13(g) Bubble diameter (m) variation along the axis for pipe diameter of 1 inch and


87

5.13(h) Bubble diameter (m) variation along the axis for pipe diameter of 1 inch and


88

5.14(a) Bubble diameter (m) variation along the axis for pipe diameter of 1.25 inch


89

5.14(b) Bubble diameter (m) variation along the axis for pipe diameter of 1.25 inch


90

5.14(c) Bubble diameter (m) variation along the axis for pipe diameter of 1.25 inch


90

5.14(d) Bubble diameter (m) variation along the axis for pipe diameter of 1.25 inch


91

5.14(e) Bubble diameter (m) variation along the axis for pipe diameter of 1.25 inch


91

5.14(f) Bubble diameter (m) variation along the axis for pipe diameter of 1.25 inch


92

5.14(g) Bubble diameter (m) variation along the axis for pipe diameter of 1.25 inch


93

5.14(h) Bubble diameter (m) variation along the axis for pipe diameter of 1.25 inch


93

5.15(a) Heat Transfer Coefficient (Wm-2K-1) Vs Bubble Reynolds Number for pipe

diameter of 1 inch and wall temperature of 403 K

94

5.15(b) Heat Transfer Coefficient (Wm-2K-1) Vs Bubble Reynolds Number for pipe


95

5.15(c) Heat Transfer Coefficient (Wm-2K-1) Vs Bubble Reynolds Number for pipe


95

5.15(d) Heat Transfer Coefficient (Wm-2K-1) Vs Bubble Reynolds Number for pipe


96

5.15(e) Heat Transfer Coefficient (Wm-2K-1) Vs Bubble Reynolds Number for pipe


96

xii

5.15(f) Heat Transfer Coefficient (Wm-2K-1) Vs Bubble Reynolds Number for pipe


97

5.15(g) Heat Transfer Coefficient (Wm-2K-1) Vs Bubble Reynolds Number for pipe


97

5.15(h) Heat Transfer Coefficient (Wm-2K-1) Vs Bubble Reynolds Number for pipe


98

5.16(a) Heat Transfer Coefficient (Wm-2K-1) Vs Bubble Reynolds Number for pipe

diameter of 1.25 inch and wall temperature of 403 K

99

5.16(b) Heat Transfer Coefficient (Wm-2K-1) Vs Bubble Reynolds Number for pipe


99

5.16(c) Heat Transfer Coefficient (Wm-2K-1) Vs Bubble Reynolds Number for pipe


100

5.16(d) Heat Transfer Coefficient (Wm-2K-1) Vs Bubble Reynolds Number for pipe


100

5.16(e) Heat Transfer Coefficient (Wm-2K-1) Vs Bubble Reynolds Number for pipe


101

5.16(f) Heat Transfer Coefficient (Wm-2K-1) Vs Bubble Reynolds Number for pipe


102

5.16(g) Heat Transfer Coefficient (Wm-2K-1) Vs Bubble Reynolds Number for pipe


102

5.16(h) Heat Transfer Coefficient (Wm-2K-1) Vs Bubble Reynolds Number for pipe


103

xiii

LIST OF SYMBOLS AND ABBREVIATIONS

CFD Computational Fluid Dynamics

Liquid specific heat

Wait coefficient

Outer diameter of pipe

The bubble departure frequency

Area coefficient for scaling between the nucleation site area density and the wall area

fraction the bubble-induced quenching

influences.

The force per volume applied to the continuous

phase c momentum equation due to dispersed

phase d.

Acceleration due to gravity

Mass Velocity

h Heat transfer Coefficient

Latent Heat

Liquid conductivity

The bubble influence wall area fraction

Vapour conductivity

The nucleation site number density

Nusselt number for bubble

Prandtl number of liquid

Bubble Reynolds number

Bulk Temperature of continuous phase

Interface temperature

Bulk Temperature of dispersed phase

Surface temperature

Saturated-liquid temperature

Waiting time

Viscosity of vapour

VTR Vertical Thermosyphon Reboiler

v Relative drift velocity due to the use of volume-fraction weighted definitions of phase velocity

Density of Liquid

Liquid phase density

Density of vapour

file:///C:/Program%20Files/CD-adapco/STAR-CCM+9.06.009-R8/doc/en/online/STARCCMP/usingSegregatedMultiphase.205.112.html%23ww1202448

CFD Simulation of Vertical Thermosyphon Reboiler and Estimation of Heat Transfer Coefficient

Department of Chemical Engineering, RVCE, Bengaluru Page 1

CHAPTER 1

INTRODUCTION

Thermosyphon is a method of exchanging heat based on simple principle of natural

convection. This method is commonly used in devices in which liquid circulation

takes place from a heated region to a region heaving relatively lesser temperature.

Main application of this method can be commonly seen in solar heater for domestic

purposes and reboiler in petroleum industries. Similar to heat pumps the thermal cycle

of a thermosyphon system works simultaneous evaporation and condensation. Though

thermosyphon reboilers are widely used in various chemical process industries the

flow and heat transfer characteristics are not completely understood yet. While

experimental analysis is expensive owing to various alternatives, numerical

simulations get very complex as it involves multiphase flow with other heat, mass and

momentum transfer interactions. Computational Fluid Dynamics (CFD) simulation

proves to be a useful tool for analysis of flow and heat transfer characteristics in such

systems [1, 2].

1.1 Vertical thermosyphon reboilers

A thermosyphon reboiler is a type of shell and tube heat exchanger which works on

the principle of natural convection. A reboiler is typically used to provide heat at the

bottom of industrial distillation columns. Proper reboiler operation is important for

effective distillation. In a distillation column, the vapour driving the separation comes

from the reboiler. The reboiler receives a stream of liquid from the bottom of the

column and it partially or completely vaporizes the stream. The boiling of liquid

occurs inside the tubes in a vertical type thermosyphon reboiler, which is shown in

Figure 1.1(a). In case of a horizontal type thermosyphon reboiler shown in Figure 1.1

(b) the boiling occurs on the shell side.



Figure 1.1(a) Vertical thermosyphon reboiler

Figure 1.1(b) Horizontal thermosyphon reboiler [2]

In a vertical thermosyphon reboiler, the liquid circulation occurs due to density

difference between vapour-liquid mixture in the heat exchanger and the liquid flowing

through the downcomer to the reboiler. Movement of liquid starts as the liquid gets

heated and causes it to expand. Hot liquid at the top becomes less dense than the

cooler liquid present at the bottom. Heated liquid entering inside the tube rises upward

and gets replaced by cooler liquid returning downwards.

Natural circulation systems are widely used in chemical processing industries.

Thermosyphon reboilers are used in more than 70% of evaporation duties occurring in

process industries [2]. About 70% of the thermosyphon reboilers used in

petrochemical industries have vertical configuration and 30% are horizontal.

Thermosyphon reboilers are most economic systems as they need no pump. The major

advantage of thermosyphon reboiler is that there are no auxiliary equipments; no

pumps, no drives, no pump control systems. This results in significant reduction in

overall cost of investment. In addition to this, they offer lesser maintenance and



operating costs by elimination of pump driver energy consumption and pump

maintenance costs [2, 3].

1.2 Limitations of thermosyphon reboilers

Fouling is a major concern amongst all heat exchangers. Foulants such as corrosion

products and dirt form scale on heat transfer surfaces or block the tubes by forming a

plug. In case of vertical thermosyphon reboilers excessive circulation may occur when

reboiler sump level is high and cannot be lowered. Insufficient circulation might occur

due to plugging of tubes and insufficient liquid head which may lead to poor heat

transfer and possible tube over-heating. Surging may occur if the reboiler temperature

difference is small and column pressure is not controlled. When the column pressure

rises, it increases the bottom pressure. Boiling decreases or stops which results in

bottom liquid level to build up. Dumping will occur, causing the pressure to fall. This

in turn increases the boiling and the pressure increases.

Oscillations have been identified which cause instabilities in reboiler. These

oscillations may be caused by pressure drop limitation in the reboiler outlet or outlet

piping system. The generated vapour cannot find its way out in sufficient quantity and

some accumulates as a pocket near the top of the reboiler. Expansion of the vapour

pocket can momentarily reverse the process flow, leading to a drop in pressure, which

in turn causes liquid to rush back in. Thermosyphon failure might be caused by low

heat fluxes. This is common at start-up of a multi component mixture with negligible

reboiler temperature difference. If flow is not adequately started, the reboiler may only

vaporise some of the relatively lighter components in the liquid and leaving behind

heavy liquid [4].

1.3 Applications of Computational Fluid Dynamics

Computational Fluid Dynamics (CFD) provides a qualitative and quantitative

prediction of fluid flows by means of mathematical modelling, numerical methods and

software tools. Computational fluid dynamics gives an insight into flow patterns that

are difficult, expensive or important to study using traditional experimental

techniques. Simulations provide support in quantitative production of flow phenomena

for several desired quantities for virtually any problem and realistic operating



conditions. Computational fluid dynamics makes predictions by using a computer to

solve the mathematical equations for the problem at hand.

The advantages of CFD lie in the fact that it is not restricted to fluid flow alone but is

sufficiently generic in solving equations of fluid flow, energy transfer, reaction

kinetics and mass transfer simultaneously under a single domain. CFD also proves to

be a useful tool in analyzing a complicated systems and varying geometries. CFD is

also useful in analysis in case of failures, troubleshooting, optimization, de-

bottlenecking and revamping situations. CFD as a technique involves use of

simulations of chemical processes that are generic in nature. CFD is gaining

increasing popularity for industrial simulation of chemical reactors and a variety of

combustion devices. CFD handles solution of fluid dynamic equations on digital

computers and they require fewer restrictive assumptions relatively and helps in

obtaining a complete description of flow field for all the variables. Configurations that

are quite complex in nature can be analyzed using CFD and relatively these

methodologies can be applied easily.

1.4 Multiphase Flow for Vertical system

Multiphase flow is simultaneous flow of materials with different phases, or materials

with different chemical properties but in the same state or phase. One of the type of

two phase flow system is gas-liquid flow. There are various types of flow regimes

detected in vertical flow which are shown in Figure 1.2.

Figure 1.2 Types of flow regimes for vertical flow system [5]



One of the types of flow is bubbly type where there is dispersion of bubble in the

liquid and the liquid is continuous. Another type of flow is Slug Flow, also known as

Plug flow where smaller bubbles come together to form a larger coalescence which

equivalent to the diameter of tube. In case of Churn Flow, the Plug Flow bubbles

break down to give churn regime. Annular flow is that type of flow in which liquid

flows close to the inner wall of tube in the form of a film, with some liquid at the

centre and gas dispersed in it in the form of bubbles. Another type of flow is

Dispersed flow where the bubble concentration is relatively high than bubbly flow as

the liquid flow rate is increased [5].

1.5 Flow pattern transitions in vertical flow

Bubble-coalescence leads to gradual bubble growth in Bubble-Plug transition which

occupies the whole pipe cross-section. The transition to slug flow occurs when the

void fraction is around 25-30%. In highly turbulent flows, break-up of the bubbles

may be postulated to occur to offset the progression of the coalescence. It seems more

likely that void waves are formed in the flow, and that, within these waves, the

bubbles become closely packed and are better able to coalesce, leading to plug flow

[6].

Churn flow is essentially defined as developing plug or slug flow. However, churn

flow as defined here does exist in fully developed flow, and has the following unique

characteristics:

The regime is entered from slug flow by the formation of flooding-type waves, and

these persist as a characteristic of the regime throughout. Such waves are absent in

both slug flow and annular flow but are formed repeatedly in the churn flow regime

and transport the liquid upwards. In between successive flooding waves, the flow of

the liquid phase in the film region near the wall reverses direction, and is eventually

entrained by the next upward-moving wave.

The onset of churn flow is accompanied by a sharp increase in pressure gradient, as

illustrated in the results obtained depicted in Figure 1.3. A detailed evaluation of the

plug to churn flow transition in terms of this flooding mechanism is given [6].



As the gas velocity is increased after the churn flow regime has been entered, the

pressure gradient initially decreases and the passes through a minimum value. The

flooding waves (and their associated intensive gas-liquid interactions promote large

pressure gradients, and as they disappear, the pressure gradient reduces.

Figure 1.3 Data for pressure gradient in fully developed air-water flux in a vertical

tube [6]

Eventually, the pressure gradient increases again as the gas flow rate increases. The

onset of true annular flow corresponds to the point where there is no flow reversal

within the liquid film. This might correspond approximately to the pressure drop

minimum. Another definition might be the flow reversal point. It is clear that,

though both churn and annular flow have the characteristic of having a liquid layer

at the wall and a gas core in the centre of the pipe, their flow behaviour is quite

different. The definition of the exact point of transition is, nevertheless difficult.

Annular to wispy-annular transition is supposed to occur approximately at a critical

liquid momentum flux though, again, identification of the transition is to some

extent subjective.



1.6 Correlations of boiling heat transfer data

Various Correlations of experimental data have been achieved for different boiling

regimes [6]. The nucleate boiling regime is of great engineering importance because

of very high heat fluxes possible with moderate temperature differences. The data for

this regime are correlated by Equation (1.1)

= ( , ) Equation (1.1)

The parameter in Equation (1.1) is Nusselt number defined in Equation (1.2)

Equation (1.2)

Where q/A is the total heat flux, is the maximum bubble diameter as it leaves the

surface, is the difference between the surface and saturated-liquid

temperatures, and is the thermal conductivity of the liquid. The bubble Reynolds

number R, is defined in Equation (1.3)

Equation (1.3)

where is the average mass velocity of the vapour leaving the surface, and is the

liquid viscosity. The mass velocity, is defined in Equation (1.4)

=

Equation (1.4)

Where is the latent heat of vaporization. Rohsenow has used Equation (1.1) to

correlate Addoms pool boiling data, and is expressed in Equation (1.5)

Equation (1.5)

Where denotes the heat capacity for the liquid, and other terms have their usual

meanings. The coefficient in Equation (1.5) varies with the surface-fluid

combination. Existing applied for heat transfer coefficient for two-phase systems is

shown in Table 1.1.



Table 1.1 Existing Equations for heat transfer coefficient from literature [7]

Sl

No.

Name of

Equation

Equation Description

1 Bromleys

expression = 0.62 [

3( )( + 0.4)

( )]

1 4

Stable film boiling on

the surface of

horizontal tubes and

vertical plates has

been studied by

Bromley. The theory

was developed

considering

conduction alone

through the film on a

horizontal tube,

Bromley obtained the

given equation

2 Berensons

Equation h= 0.425[

3 ()(+0.4)

() ()]

1 4

A modification of

Bromleys equation

has been proposed by

Berenson to provide a

similar correlation for

stable film boiling on

horizontal surface.

3 Hsu and

Westwaters

Equation

h[

2

()3]

1 3 = 0.0020R0.6

Re=4

Hsu and Westwater

considered film

boiling for the case of

a vertical tube.

1.7 Analysis of thermosyphon systems Literature Survey

This section highlights some of the earlier research done in this field of thermosyphon

reboilers. A major part of literature related to TSR show analysis of instabilities in two

phase flow in a vertical thermosyphon reboiler (VTR). Existing work attempts to

identify instabilities while operating reboilers under various conditions like

atmospheric, sub-atmospheric and above atmospheric pressures the focus has been

more on eliminating the instabilities and less on finding what caused these

instabilities. This set of work has been attempted using numerical analysis,

experimental studies and simulation analysis. One of the most important attempts was

carried out on a at sub-atmospheric pressure. An industrial sized free convection

thermosyphon reboiler replicated to its full scale was used for experimental setup in

existing work. Water is used as working fluid while steam was used as heating source.



According to earlier research that were carried out helped to recognize the range in

which reboilers could be utilized with stability. The performance in the flow-induced

unstable region, the heat induced in stable region and in unstable region was

considered. It was noticed that the range of stable operation has significant

dependence on process pressures. The range becomes smaller as vacuum becomes

lesser. Flow induced Flow induced instabilities can be reduced by increasing heat

loads or flooding the reboilers. Thermosyphon reboilers have been used for analysis

from many years.

Previous works predict the performance of thermosyphon reboilers, numerical

modelling of thermosyphon reboiler and other systems. The explanation of this

existing work has been explained in this section [6]. An experimental and theoretical

study was carried out to recognize operational characteristics of thermosyphon

reboilers. Performance of reboiler operation to variation of driving temperature

difference driving, liquid head and operating pressure in the inlet line in a VTR were

examined. The effect of all the above parameter was explained by developing a simple

model. The model developed has been subdivided in the tube as heating zone and

evaporating zone and the operational characteristics of these zones with the influence

of length was noticed [7].

The existing research involved an analysis on two-phase flow carried out for boiling

characteristics inside high pressure system in presented focusing on non-uniform axial

heating profiles. After presenting the current status of the use of CFD techniques in

flow boiling predications the detailed numerical model was developed. The effect of

heat flux profile in the axial direction investigated while maintaining the same total

power. Sine and cosine shapes were considered with linearly increasing, linearly

decreasing the heat flux at different profiles [8]. Capabilities of CFD were looked into

wall boiling. The computational model used here incorporates the Eulerian two phase

flow description with splitting of heat flux. Trials under various conditions regarding

liquid sub cooling flow rate and heat flux were conducted [9].

In current literature it is shown that experimental investigation were carried out on

thermosyphon loop consisting of condensation and an evaporation system.

Evaporators with and without wick structure were considered with water and methanol



as the working fluid. Effect of varying evaporator types, fluid fill and cooling water

temperatures ratio were examined and it was noticed that wick structure can increase

the heat transfer rate and water as a working fluid can carry large heating range [10].

Earlier works has also involved CFD modelling of sub cooled flow boiling for nuclear

engineering applications. General basis CFD codes CFX-5 was used to non-drug

forces under wall boundary conditions and model two-phase flow turbulence were

investigated [11]. Conceptualizations of subcooled boiling in CFD, experimental data

of model are validated and the model was registered to design of fuel assembly. Wall

boiling with wall heat flux algorithm was used for prototype with version of CFD

code CFX. The model was used in inspection of the phenomena inside a hot channel

of a fuel assembly for critical heat flux phenomena [12].

Experiments were conducted for previous research work using a obtained data from

model which is used for VTR operation under vacuum. This experiment focused at

coupled problem to get better estimates of the heat transfer coefficient for the

condensing steam in the heated process fluid and the shell in the tubes of a

thermosyphon under a steady state [13]. To study incipience of nucleate boiling in

VTR an experiment was performed. The maximum valve of superheat found around

the onset of boiling was obtained from distributions of correlated with submergence

and wall temperature, physical properties of working liquids and heat flux [14].

Thermosyphon systems were considered as segment of Next Generation Nuclear

Plant. This study describes performance of two phase thermosyphon heat transfer with

alkali metals as working fluid [15].

Many investigations have been carried out previously to analyse and improve thermal

performance of a thermosyphon for various working fluids, chosen specific to

application of thermosyphon systems. The analysis of cryogenic thermosyphon was

studied and charged with Nitrogen-Argon mixture as working fluid. The mixture of

components in heat transfer has been considered that mass transfer of that components

has been discussed theoretically. It was noticed that a binary mixture of Nitrogen-

Argon can increase the temperature range for operation in a cryogenic thermosyphon.

The rate of heat transfer increase in Argon mole fraction with increase in dry-out limit

until film boiling appears at the top of condenser [16].



An inclined two phase closed thermal performance of an inclined two phase closed

thermosyphon is considered. Examination was carried out with aqueous solutions of

n-Butanol and distilled water as working fluids. Investigations were carried out with

filling ratio of 60% and various inclinations of 450,600 and 900 to horizontal for

different flow rates and varying heat fluxes. It was noticed that thermosyphon when

with aqueous solution of n-Butanol shows maximum thermal performance charged

with distilled water [17].

To estimate the thermal performance of two phase closed thermosyphon experiment

was conducted. Experiments were done under stationary and vibratory condition with

R134a and water as working fluids. Thermosyphon was experimental for input heat

flux, various adiabatic lengths of vibrations. Their results showed that highest heat

flux is provided liquid filling ratio of 50% and 350mm of adiabatic length [18].

To study the effect of aspect ratio and filling ratio and on the thermal performance

under normal operating conditions experimental investigations were performed on

inclined two-phase closed thermosyphon with working fluid being distilled water. It is

absorbed that at an inclination angle of 60 degrees thermosyphon showed maximum

thermal performance for all aspect ratios [19]. R134a was used as the working fluid to

perform an experimental analysis of thermosyphon to study the effect of temperature

difference between condenser section and bath. Parameters like coolant mass flow rate

and fill ratio were also studied. It is noticed that with increase in all the above three

parameters showed improve heat flux [20].

A study was carried out on closed two-phase thermosyphon. With the use of nano-

fluids there were many attempts made to evaluate execution of thermosyphon. Water

and different water based nano-fluids like CuO,23 and laponite clay were used as

working fluid. Comparing with water as working fluid and other working fluids, water

as working fluids showed good performance in the study [21]. Distribution of

temperature in a thermosyphon and comparing the rate of heat transfer with nano-fluid

at different concentrations in distilled water. Terminations were obtained that with the

addition of 5.3% of iron oxide nanoparticles by volume in water conformed that

enhancing the thermal performance of thermosyphon [22].



An experimentation was carried out of both mathematically and experimentally on

two-phase closed out by varying inclination angles and filling ratios. Maximum

performance of thermosyphon happened at inclination angle 50o and filling ratio 45%.

Mathematical model showed tolerable relationship between the investigation

parameter [23]. Investigations were performed on the performance of two-phase

closed thermosyphon. Investigation on thermosyphon performed by varying

mechanical parameter and by varying geometries. Investigation on effect of flow rate

of coolant, heat duty and length of condenser were carried out [24].

Experiments on partial-vacuumed thermosyphon were conducted Performance of

thermosyphon were studied with the effect of filling ratio, extra volume and heat loads

and it is noticed that these variables have significant effect on performance of

thermosyphon [25]. The effect of evaporator and condenser re-surfacing on overall

performance is another work on thermosyphon was reported. They described that on

making the condenser and evaporator more hydrophobic the thermal performance of

thermosyphon is increased by 15.27% and thermal resistance decreases by 2.35 times

compared with plane one [26].

Water to air thermosyphon heat pipe to evaluate the performance experimentally and

theoretically was selected [27]. The effect of filling ratio and varying heat loads and

on the performance of thermosyphon using CFD modelling in FLUENT version 6.2

experimental values were investigated [28].

A complete analysis of literature [6-28] suggests that the studies carried on CFD

analysis on thermosyphon reboilers is meagre. Some work has been done on

experimental and numerical analysis on VTR that these studies describe that

performance of thermosyphon reboilers under particular conditions but very less work

has been done in parametric correction of thermosyphon performance. Literature

associated with thermosyphon has less numerical simulation till now, few data is

available on simulation using CFD, the bottle neck being experimental analysis and

complex numerical as it involves multiphase flow with other heat, mass and

momentum transfer interactions. Therefore there is a need to evaluate the performance

of vertical thermosyphon by changing flow characteristics. Heat transfer to subcooled

water which is used as working fluid flowing inside the heated tube with uniform wall



temperature is to be simulated. There is a need to covey inspection on net vapour

generated, formation of boiling in the tube and rate of condensation of vapours. The

effect of flowing velocities, operating pressure, varying wall temperature and inlet

subcooling need to be analysed. This analysis assists to optimize boiling of fluid

inside the reboiler tube in maximizing length of vaporization and minimizing sensible

heating length inside the tube.

1.8 Emerging Technologies

The upcoming technologies mainly focus on enhancing the efficiency of a

thermosyphon reboiler. One of such systems are the two phase closed thermosyphons.

The two phase closed thermosyphon can provide reliable and effective thermal control

for energy conservation, energy recovery and renewable energy applications. This

device does not contain capillaries unlike other heat pumps and works on a two phase

close cycle where latent heat of evaporation and condensation is used to transfer heat.

One of the major advantages of using a closed loop thermosyphon is that it can be

used across a wider range of temperatures since flow resistance is less as condensate is

returned to heated side of the system under the effect of gravity.

Closed loop thermosyphon is a very effective heat treansfer device and it has several

operating limits: viscous, sonic, dryout, boiling and flooding and other phenomena

called geyser boiling. Important factors affecting closed loop thermosyphon

performance are the working fluid and related filling ratio. This device can mainly be

applied for several thermal control and energy storing applications.

1.9 Motivation

Thermosyphon systems are those which work on principal of natural convection

which circulates fluids without the necessity of a mechanical pump. Thermosyphon

reboiler is very popular in Chemical engineering industries for pumping fluids at low

costs. Though very popular, they have not been well studied. Thermosyphon reboiler

has not been understood well because of the complicated multiphase flows which

occur in them along with other experimental errors and limitations which are

inevitable when studying them. Analysing a thermosyphon reboiler and



understanding its flow and heat transfer characteristics can lead to designing a

thermosyphon reboiler to operate at its maximum efficiency.

The experimental limitations such as finding the effect of change of the diameter,

finding the inception of boiling point in a thermosyphon operating is practically very

difficult and expensive. Using a CFD, these difficulties can be overcomed. CFD

simulation of thermosyphon also can be used to study effect of each affecting

parameter. Number of design parameters such as diameter and length of tube can be

varied with no extra costs. Flowrates can be changed without any operating valves.

Many other expenses that could be incurred during experimental analysis can be saved

if CFD is used. Estimation of Heat transfer coefficient is key to understand the

thermosyphon reboiler and hence design it to show maximum efficiency. CFD

simulation package STAR CCM+ is such a tool which can be used to completely

understand the reboiler by estimating heat transfer coefficient and to generate data

which can be later analysed to confirm the finding using experimental analysis.

1.10 Objective and scope of the project

This project aims to use the available models to account for multiphase effects during

boiling of the fluid in a tube heated at the wall. The project also aims to measure

volume fraction of vapour, velocity of vapour, bubble diameter, bubble Reynolds

number and study effects of flow velocities, varying wall temperatures and varying

geometries and to estimate the heat transfer coefficient.

The objective of this project is to create a three dimensional model and a two

dimensional axisymmetric model of a tube of a vertical thermosyphon reboiler

using STAR CCM+ V9.06 as the simulation tool.

Determine the most effective meshing model for modelling the tube.

Find optimal conditions for maximizing volume fraction of vapour.

Estimation of heat transfer coefficient accounting for multiphase interaction

between vapour and liquid.

Determine the effect of change in wall temperature on the flow and heat

transfer characteristics of the system.

Determine the effect of change in diameter on the simulation results.



1.11 Methodology

This project involves analysis of a single tube in the thermosyphon system using CFD.

This involves creation of the tube geometry in STAR CCM+ using 3D CAD. Physics

models relevant to the case are defined in the physics continua. An appropriate mesh

is applied on the geometry to set up an environment for computation. Since use of 3D

mesh takes valuable computation time, the 3D mesh is converted into a 2D one and

linked with the appropriate physics continua. The simulation is run for 5000-10000

iterations and the convergence is checked for convergence. Data generated in each

simulation run is compiled and analysed using excel. Trends observed are reported.

1.12 Organization of the report

The report is divided into the following six chapters

Chapter 1 presents an introduction the principle of Thermosyphon Reboiler, and

explains various flow regimes existing in vertical flow. It also explains the transition

in flow regimes that occur during multiphase flow. Along with objective of project it

includes literature survey.

Chapter 2 presents the methodology used in creating the model and simulating it. It

shows the creation of three dimensional model, adding physics to the physics

continua, meshing the created geometry, conversion of three dimensional to two

dimensional mesh, simulating the model and about scheme of analysis followed.

Chapter 3 presents the detailed explanation of physics models available in brief for

modelling two phase systems and also the detailed explanation of models chosen for

the created two dimensional model.

Chapter 4 explains the formulation of simulation cases for validation of model and

analysis of thermosyphon system. Various simulation runs formulated have been

tabulated for sets of parametric analysis carried out.

Chapter 5 consolidates the results derived from the simulations and also discusses the

observations made.



Chapter 6 enlists the conclusions derived from the analysis of the results. The scope of

future work is also highlighted in this section.



CHAPTER 2

PHYSICS MODELS

In this chapter, the operation physics models selected for the simulation are explained in

detail. The description of material properties, set the region type, application of initial and

boundary conditions are given here. STAR-CCM+ contains a wide range of physics models

and methods for the simulation of single and multi-phase fluid flow, heat transfer,

turbulence, solid stress, dynamic fluid body interaction, aero acoustics, and related

phenomena.

2.1 Axisymmetric Model

Axisymmetric is a two-dimensional representation of a three-dimensional system symmetric

about its axis. The geometry shown in Figure 3.1(a) can be modelled using a simple model

shown in Figure 3.1(b).

Figure 2.1(a) Sample geometry

Figure 2.1(b) Axisymmetric model of sample geometry



Axisymmetric model has the following advantages:

Smaller and simpler models

Faster execution

Easier post-processing

2.2 Multiphase Model

Multiphase flow is a term which refers to the flow and interaction of several phases within

the same system where distinct interfaces exist between the phases. The term phase usually

refers to the thermodynamic state of the matter: solid, liquid, or gas. In modelling terms, a

phase is defined in broader terms, and can be defined as a quantity of matter within a system

that has its own physical properties to distinguish it from other phases within the system [29,

46]. For example:

Liquids of different density

Bubbles of different size

Particles of different shape

Multiphase flows can be classified into two categories:

Dispersed flows, such as bubbly, droplet, and particle flows

Stratified flows, such as free surface flows, or annular film flow in pipes

The sub-models selected to model the vertical thermosyphon reboiler are described in the

following section.

2.2.1 Sub-Models under Multiphase Model

Multiphase Segregated Fluid model is commonly known as the Eulerian multiphase model

in the literature. The multiphase segregated fluid model solves conservation equations for

mass, momentum, and energy for each phase. Phase interaction models are provided to

define the influence that one phase exerts upon another across the interfacial area between

them [30, 46]. Lagrangian multiphase model solves the equation of motion for representative

parcels of the dispersed phase as they pass through the system. It is intended for systems

that consist mainly of a single continuous phase carrying a relatively small volume of

discrete particles, droplets, or bubbles. It is suited where the interaction of the discrete phase

with physical boundaries is important [31, 46]. Dispersed multiphase model simulates

dispersed phases in a Eulerian manner. This model combines aspects of both the Lagrangian



multiphase model and the Segregated (Eulerian) multiphase models. This model and the

Volume of Fluid model can be activated in the same simulation [32, 46]. Discrete element

model is an extension of the Lagrangian multiphase model, but where individual particles

are modelled rather than representative parcels, and where inter-particle contact forces are

explicitly accounted for [33, 46]. Fluid Film model predicts the dynamic characteristics of

wall films using boundary layer approximations and assumed velocity and temperature

profiles across the depth of the film. Film transport was predicted using thin shells that lie

across the surface of solid walls on which the film is formed [34, 46]. Volume of fluid model

was provided for systems containing two or more immiscible fluid phases, where each phase

constituted a large structure within the system (such as typical free surface flows). This

approach captures the movement of the interface between the fluid phases, and is often used

for marine applications [35, 46].

2.3 Multiphase Interaction

The interaction between vapour and liquid phase is described using multiphase interaction

models. STAR CCM+ provides various models to account for interaction between vapour

and liquid. Each model under Multiphase Interaction model has competing sub-models from

which suitable selections can be made. The sub-models chosen for modelling the vertical

thermosyphon reboiler are explained briefly in this section.

2.3.1 Sub-models under Multiphase Interaction

The wall dry out area fraction specifies how much of the heat flux that is applied at the wall

is directed towards the vapour convection, as opposed to liquid convection and evaporation.

The heat fluxes can be represented as shown in Equation (3.1).

= (

+ +

)(1 ) + Equation (3.1)

Where,

is the convective heat flux which describes the removal of heat by single-phase

turbulent convection on those parts of the wall that is not affected by boiling. In applications

with fixed wall heat flux, this term defines the point at which the wall first exceeds saturation

temperature.

is the evaporative heat flux, which describes the power that is used to produce bubbles

from nucleation to departure. This term is a strong function of wall superheat . In



fixed heat flux applications, once boiling has started, this term is responsible maintaining a

wall temperature that is slightly higher than the saturation temperature.

is the quenching heat flux, which describes the enhancement of heat transfer, due to

the replacement of a departing bubble by an influx of cooler liquid farther away from the

wall. Bubble-induced quenching is also known in literature as liquid agitation or pumping.

This term is less important when the liquid is close to saturation temperature, but in highly

subcooled flows, , it makes an important contribution to the enhanced heat transfer

due to boiling.

is the vapour contribution to convective heat flux based on single-phase turbulent

convection by the vapour. is the wall contact area fraction for the vapour, based on

either an expression for , or a transition volume fraction representing the start of wall

dry out.

It can be used to improve robustness during initial convergence, or for indicating approach

to departure from nucleate boiling conditions in converged solutions. The basic model

defines the area fraction as zero until the volume fraction of the vapour near the wall exceeds

a specified value. It then transitions the area fraction to unity as the vapour volume fraction

approaches unity. To reduce grid-dependency, the near-wall volume fraction that is used for

the area fraction calculation can optimally be defined as an average over a prescribed

thickness of notional bubbly layer next to the wall [36, 46].

The Boiling Mass Transfer rate model is only available when the Phase Coupled Fluid

Energy model is active in the physics continuum. It is used to model the rate of bulk boiling

or condensation between phases. The properties of three child nodes, the Continuous Phase

Nusselt Number, the Disperse Phase Nusselt Number and the Interface Temperature, are

used to define the parameters that are required by the model equation [37, 46].

Continuous Phase Nusselt number is a dimensionless number that can be defined as the ratio

of convective heat transfer to the conductive heat transfer across the boundary. For each

Eulerian multiphase phase interaction that is not between particles in the Granular Pressure

model, it is necessary to define which phase is continuous and which is dispersed. This

property controls the heat transfer rate to the boiling interface from the liquid side. Disperse

Phase Nusselt Number is a dimensionless number which controls the heat transfer rate to

the boiling interface from the vapour side. The heat transfer from the phase-change interface

file:///C:/Program%20Files/CD-adapco/STAR-CCM+9.06.009-R8/doc/en/online/STARCCMP/usingSegregatedMultiphase.205.040.htmlfile:///C:/Program%20Files/CD-adapco/STAR-CCM+9.06.009-R8/doc/en/online/STARCCMP/usingSegregatedMultiphase.205.040.htmlfile:///C:/Program%20Files/CD-adapco/STAR-CCM+9.06.009-R8/doc/en/online/STARCCMP/usingSegregatedMultiphase.205.044.html%23ww1193453file:///C:/Program%20Files/CD-adapco/STAR-CCM+9.06.009-R8/doc/en/online/STARCCMP/usingSegregatedMultiphase.205.044.html%23ww1193453file:///C:/Program%20Files/CD-adapco/STAR-CCM+9.06.009-R8/doc/en/online/STARCCMP/usingSegregatedMultiphase.205.044.html%23ww1193495file:///C:/Program%20Files/CD-adapco/STAR-CCM+9.06.009-R8/doc/en/online/STARCCMP/usingSegregatedMultiphase.205.044.html%23ww1193533



to each of the two phases on either side of the interface is modelled as shown in Equation

(3.2) and (3.3) [37, 46].

()

= ()

( ) Equation (3.2)

()

= ()

( ) Equation (3.3)

The interface temperature is often defined as a constant saturation temperature for a

particular system pressure. In STAR-CCM+, a field function can optionally be used for the

interface temperature, so that depth can be accounted for, or over pressure due to surface

tension in micron-sized bubbles [38, 46].

Bubble Induced quenching temperature model corrects the Bubble Induced Quenching Heat

Flux so that it uses the temperature of the liquid brought to the wall by the action of the

departing bubble. This temperature can be different to the liquid temperature computed at

the cell centre next to the wall. The correction is taken from a location on the undisturbed

liquid phase temperature profile away from the departure site [39, 46].

The bubble departure frequency determines how many bubbles leave a nucleation site per

second. This is last of three key factors determining the evaporation rate in subcooled

boiling. Figure 3.2 illustrates this parameter.

Figure 2.2 Illustration of Cole bubble departure frequency parameter [46]

This frequency is equivalent to a terminal velocity scale over bubble departure diameter, and

the overall evaporation rate is calibrated around this assumption. The standard model that is

implemented for bubble departure frequency is the Cole model. This is equivalent to taking

a typical bubble rise velocity (estimated using unit drag coefficient) as the velocity scale,

over bubble diameter as the length scale using Equation (3.4) [40, 46].

file:///C:/Program%20Files/CD-adapco/STAR-CCM+9.06.009-R8/doc/en/online/STARCCMP/usingSegregatedMultiphase.205.112.html%23ww1202873file:///C:/Program%20Files/CD-adapco/STAR-CCM+9.06.009-R8/doc/en/online/STARCCMP/usingSegregatedMultiphase.205.112.html%23ww1202873



= 4

3

( )

Equation (3.4)

Del Valle Kenning Bubble Induced Quenching Heat Transfer Coefficient model calculates

the bubble induced quenching heat transfer coefficient When a bubble leaves the heated

surface, cooler liquid fills the space that it occupied. The heat transfer during this process is

known as quenching heat transfer. The Quenching Heat Transfer Coefficient is used to

calculate the quenching heat flux. This heat transfer coefficient is calculated using Equation

(3.5).

= 2

Equation (3.5)

Here is the waiting time between bubble departure and the nucleation of the next bubble

given by Equation (3.6)

=

Equation (3.6)

the wait coefficient. The default value is 0.8. This value comes from an assumption by

Kurul and Podowski that quenching occurs between the departure of one bubble and the

nucleation of next. This period is 80% of the departure cycle [41, 46].

Hibiki Ishii nucleation site number density determines the number of locations on the heated

surface where bubbles form, per unit area. This is the leading factor determining the

evaporation rate in a mechanistic model of subcooled boiling. Figure 3.3 illustrates this

parameter.

Figure 2.3 Illustration of Hibiki Ishii Nucleation Site Number Density parameter [46]

file:///C:/Program%20Files/CD-adapco/STAR-CCM+9.06.009-R8/doc/en/online/STARCCMP/usingSegregatedMultiphase.205.053.html%23ww1194318file:///C:/Program%20Files/CD-adapco/STAR-CCM+9.06.009-R8/doc/en/online/STARCCMP/usingSegregatedMultiphase.205.053.html%23ww1194327file:///C:/Program%20Files/CD-adapco/STAR-CCM+9.06.009-R8/doc/en/online/STARCCMP/usingSegregatedMultiphase.205.049.html%23ww1193994



The Hibiki Ishii model is a more advanced nucleation site number density model. It is

applicable for pressures up to 198 bar. This model is used with the Kocamustafaogullari

model for calculating bubble departure diameter [42, 46].

The model has the following characteristics:

It considers a boundary condition for wall superheating.

It is validated against numerous sets of experimental data.

It has a wide range of applicability in terms of mass flow, pressure, and contact

angle.

Interaction area density specifies the interfacial area available for drag, heat, and mass

transfer between each pair of phases in an interaction. Heat and mass transfer models use

the interaction area density directly, while drag models use one quarter of the interfacial area

as an estimate of the projected area. Any correction factors, such as for non-spherical particle

shapes or particle crowding, are assumed to be covered in correlations for the drag, heat, and

mass transfer coefficients [43, 46].

Interaction length scale is used to define non-dimensional parameters such as the Reynolds

number for a phase interaction, and also to compute an interaction area density. The

interaction length scale for a continuous-dispersed interaction is chosen to be the mean

particle size [45, 46].

Kocamustafaogullari Bubble Departure Diameter model is more recent, general and based

on force balance with an adjustment for pressure dependence. This model must be used

whenever the Hibiki Ishii model is selected for nucleation site number density [29, 46].

The Kurul Podowski Bubble Influence Wall Area Fraction model assumes that the wall area

influenced by bubble-induced quenching is larger than the nucleation site area, by a specified

factor. When an individual bubble departs from the wall with a diameter , subcooled

liquid flows in to fill the space underneath the detached bubble. The wall area influenced by

this quenching flow is larger than the basic footprint of the bubble [29].

in this model is given by Equation (3.7).

=

2

4

Equation (3.7)

The effect of turbulence in redistribution of non-uniformities in phase concentration is

modelled by an additional Turbulent Dispersion Force model in the phase momentum

file:///C:/Program%20Files/CD-adapco/STAR-CCM+9.06.009-R8/doc/en/online/STARCCMP/usingSegregatedMultiphase.205.050.html%23ww1194193file:///C:/Program%20Files/CD-adapco/STAR-CCM+9.06.009-R8/doc/en/online/STARCCMP/usingSegregatedMultiphase.205.050.htmlfile:///C:/Program%20Files/CD-adapco/STAR-CCM+9.06.009-R8/doc/en/online/STARCCMP/usingSegregatedMultiphase.205.049.html%23ww1193994file:///C:/Program%20Files/CD-adapco/STAR-CCM+9.06.009-R8/doc/en/online/STARCCMP/usingSegregatedMultiphase.205.049.html



equations. This term arises naturally when Reynolds averaging is applied to the

instantaneous drag force. For a simple derivation and further references, see Contribution of

Drag to Turbulent Dispersion. The form of the term is represented by Equation (3.8) and

(3.9) [29].

=

v

v = . {

}

Equation (3.8)

Equation (3.9)

file:///C:/Program%20Files/CD-adapco/STAR-CCM+9.06.009-R8/doc/en/online/STARCCMP/usingSegregatedMultiphase.205.103.html%23ww1200123file:///C:/Program%20Files/CD-adapco/STAR-CCM+9.06.009-R8/doc/en/online/STARCCMP/usingSegregatedMultiphase.205.103.html%23ww1200123



CHAPTER 3

DESIGN OF THERMOSYPHON REBOILER IN STAR CCM+

This Chapter is about the general methodology followed in carrying out the simulation.

Sequence of steps starting with geometry creation, adding physics models, meshing the

geometry, converting 3D model to 2D, processing and post processing are discussed here in

detail.

3.1 General Sequence followed in STAR CCM+

The flowchart in Figure 3.1 depicts the general steps in sequence followed when carrying out

analysis using STAR CCM+:

Figure 3.1 General steps of in STAR CCM + [1, 46]

3.1.1 Building 3D Geometry

The 3D cylinder which represents the tube of the Thermosyphon is created using the 3D CAD

features available in STAR CCM+. Figure 3.2 shows the 2D sketch creation using the draw

feature in STAR CCM+.



Figure 3.2 The 2D Sketch creation in STAR CCM+ of tube

Once the 2D sketch of the tube is created, Using STAR CCM+s Revolve feature, the full

3D tube is created as shown in Figure 3.3.

Figure 3.3 The full 3D tube created in STAR CCM+

In STAR CCM+, to create a 2D axisymmetric model, two conditions must be met [46]:

1) The grid must be aligned with the X-Y plane.

2) The grid must have a boundary plane at the Z = 0 location.



To fulfil these conditions, modification was done to the geometry by reducing the revolve

angle to 90o from 360o. The final 3D geometry before converting to 2D is shown in Figure

3.4.

Figure 3.4 Final 3D geometry created

Geometry scene shown in Figure 3.4 was of diameter 2/3 inch and was 2 m length.

3.1.2 Adding Physics Models

In this step, the required models of physics are selected and added into a physics continuum.

This includes selecting the type of space, state of system, material properties, multiphase

model, viscous regime and other optional models to account for interaction of the phases,

turbulence, heat transfer, stresses and related phenomena from the wide range of models

available in STAR CCM+ [1, 46].

3.1.3 Generation of mesh

Generation of mesh involves creation of a mesh continuum and adding required physics models

to it. There are models for surface mesh, volume mesh and other optional mesh models. Surface

remesher and surface wrapper are the options under surface mesh models. Volume mesh has

the following models Advanced layer mesher, polyhedral mesher, tetrahedral mesher, Thin

mesher and Trimmer mesh model. The model added to the above geometry was Trimmer mesh

model alone and no surface mesh was added. In mesh continuum, the size of the meshing must

also be set. The base size set was in reference to the work done previously by taking the cell

aspect ratio to be of the same order [1]. The meshed 3D geometry is shown in Figure 3.5.



Figure 3.5 The mesh scene of the 3D model

Meshing for the 3D model created shown in Figure 3.5 had a base size of 0.01 and was

meshed with trimmer meshing model.

3.1.4 Conversion of 3D mesh to 2D mesh

The created 3D model is converted to 2D model by using the convert to 2D mesh option

available in STAR CCM+. The 2D axisymmetric model will simulate only if the 3D is

converted to 2D. On conversion to 2D, STAR CCM+ considers the 2D section to be symmetric

about the axis. Hence the results obtained in this section, if multiplied by 360o, the result for

the entire tube can be estimated. The converted 2D mesh scene is shown in Figure 3.6. The

required plots and monitors are created for observing convergence. The plots and monitors

created show the convergence of the model and the stopping criteria for iteration can be set for

the simulation. The model is run for specified number of iterations after creation of

convergence monitors.



Figure 3.6 The converted 2D mesh of the tube

The 2D axisymmetric model with trimmer meshing on which the simulation was run is

shown in Figure 3.6.

3.1.5 Analysis Scheme

As suggested in 3.1.3 and 3.1.4, a geometry and an appropriate mesh for a tube of a

thermosyphon reboiler was selected. Only after selection of mesh, the simulation may be

carried out.

1. The tube was modelled with a constant wall temperature and water entering at

constant temperature of 353 K.

2. Appropriate physics models were chosen to show the flow characteristics and

multiphase interactions in the tube.

3. The results obtained in the simulation runs were subjected to post processing where

appropriate data were extracted, tabulated and analysed.



CHAPTER 4

FORMULATION OF CASES FOR SIMULATION

The thermosyphon reboiler needs to be analysed for flow and heat transfer

characteristics for a wide range of conditions. Thus, it is of utmost importance to have

a clear idea of what is to be simulated before starting the simulation. The project

involves study on a single tube of a thermosyphon reboiler. The same is analysed for

different inlet velocities and different wall temperatures. This chapter showcases the

variation cases to be simulated.

4.1 Simulation Conditions

The conditions that were taken as basis for our simulations are shown in Table 4.1.

Table 4.1 Simulation Conditions

Parameter Value

Length 2 m

Inner Diameter 2/3 inch, 1 inch, 1.25 inch

Heat Flux 4411.75 kW/m2

Mass flux 998 2896 kg/m2s

Inlet Temperature 353K

System Pressure 1 bar

4.2 Simulation Parameters

Setting up a simulation with the conditions listed in Table 4.1 required input of various

simulation parameters to the system. These simulation parameters are the ASCII values

that we enter in the computer. Those simulation parameters are shown in Table 4.2.



Table 4.2 Simulation Parameters

P

batch 1 final report draft 6

Documents