CFD SIMULATION OF FLOW DYNAMICS CFD SIMULATION OF FLOW DYNAMICS AND HEAT TRANSFER AND HEAT TRANSFER

CHARACTERISTICS OF FREE AND CHARACTERISTICS OF FREE AND IMPINGING SWIRLING FLAME JETSIMPINGING SWIRLING FLAME JETS

Under the supervision of – Dr. Subhash Chander

Presented by –Vikram dang

11209014

1

DEPArTMENT OF MECHANICAL ENGINEErINGDr B r AMBEDKAr NATIONAL INSTITUTE OF TECHNOLOGY

JALANDHAr

CONTENTSCONTENTS

1. Introduction

2. Literature Review

3. Problem Formulation

4. Results and Discussion4. Results and Discussion

5. Conclusions and Scope for Future Research

6. References

2

1. INTRODUCTION1. INTRODUCTION

Motivation of the study

Flame impingement heat transfer

Combustion in swirling flows

Introduction to CFD Introduction to CFD

3

Motivation of the StudyMotivation of the Study

In engineering, it is important to know how things work andhow we can predict them.

Experimental investigation is far more accurate than numericalsimulations, but they are somewhat rigid in geometrical designs,and also generally require expensive flow visualizationtechnologies like PIV to completely study the fluid dynamics.

Computational Fluid Dynamics (CFD) offer flexibility in Computational Fluid Dynamics (CFD) offer flexibility ingeometrical designs and flow visualization is also very cheap andeasy to accomplish. But, accuracy of computational resultsshould be first validated against analytical or experimental data.

In the present project, CFD simulation of swirl burner is carriedout to understand the physics of Swirling flame and heat transfercharacteristics of Swirling Flame impingement. Experimental dataof the same is also available for validation.

4

Flame Impingement Heat TransferFlame Impingement Heat Transfer

To achieve intense localized heating, used in furnaces as it enhances convective heat transfer rate from the combustion products to load, increases productivity , reduced fuel consumption.

Commonly used in material processing and manufacturing, glass heating processes (processing time is short), metal heating and cutting, forming, welding etc. cutting, forming, welding etc.

Major disadvantage – non-uniformity of the heat flux near the stagnation point. It can be improved by using swirling flame.

Very complex process because fluid dynamics, thermodynamics and chemical kinetics, all play an important role.

These phenomena need deeper understanding before the process can be analysed, to avoid high thermal stresses and overheating of the hot spots of the glass products.

5

Combustion in Swirling FlowsCombustion in Swirling Flows

Swirl flows has a spiral motion in the tangential direction in addition to axial and radial directions.

The tangential component enhances the turbulent mixing of fuel and air and the swirl-induced recirculation (CTRZ) stabilizes the flame.

Recirculation zone recirculate heat and active chemical Recirculation zone recirculate heat and active chemical species to the root of the flame, thus reducing velocity requirements for flame stabilization.

Swirl number S, representing axial flux of swirl momentum divided by axial flux of axial momentum, times the equivalent nozzle radius.

6

2/dG

GS

x

Weak swirl, S < 0.2Moderate swirl, 0.2 < S < 0.6Strong swirl, S > 0.6

Central Toroidal Recirculation Zone Central Toroidal Recirculation Zone (CTRZ)(CTRZ)

Strongly swirling flow generates a large radial pressure gradientdue to centrifugal effects, creating a low-pressure central region.

Axial decays of axial velocity, swirl velocity and axis pressure occurs.

Reduction of radial pressure gradient causes a negative axial pressure gradient, which is sufficient to ‘suck’ the flow back towards the jet nozzle.towards the jet nozzle.

7Flow recirculation in a strong swirling flow (Lefebvre, 1983)

Characteristic Regions in Swirling Characteristic Regions in Swirling Impinging Flame JetsImpinging Flame Jets

The free jet region

The stagnation region

(stagnation region)

The wall jet region

(boundary layer flow)(boundary layer flow)

8

Computational Fluid Dynamics (CFD)Computational Fluid Dynamics (CFD)

CFD is the science of predicting fluid flow, heat and mass transfer, chemical reactions, and related phenomena by solving numerically the set of governing PDEs(conservation of mass, momentum, energy, species mass, etc.).

Pre-Processing

9

Problem Identification1. Define goals2. Identify domain

Pre-Processing3. Geometry4. Mesh5. Physics6. Solver Settings

Solve7. Compute Solution

Post-Processing8. Examine Results

2. LITERATURE SURVEY2. LITERATURE SURVEY

Studies are characterised on the basis of various issues:

Swirling flame structure

Heat transfer characteristics of impinging Heat transfer characteristics of impinging swirling flame

Stability

Emission analysis

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Swirling Flame StructureSwirling Flame StructureREFERENCE WORK DONE/CONCLUSION

Chen and Driscoll, (1988),Tangirala et al., (1987)

•Introduction of swirl reduces the combustion lengths by producing high rates of entrainment of ambient fluid and fast mixing particularly near the boundaries of recirculation zone. •Due to central toroidal vortex, the overall fuel-air mixing rate within a swirl-stabilized flame is found to be five times greater than that of a simple jet resulting in fivefold shortening of flame length.

Syred et al., (1971) •Experimental results based on pitot tube showed that combustion induces comparatively small change in aerodynamics of swirling flow field

Domkundwar et al. (1978)

•Did analysis of swirling recirculating reacting turbulent jets .passing through diffusers.•Suggested that with increase in swirl parameter, the flow reversal is intensified and the forward stagnation point moves upstream.

11

Claypole et al. (1981)

•Suggested flow of hot species around the eye of recirculation plays an increasingly important role in the flame stabilization mechanism.

Masseeh et al. (1990)

•Studied the temperature contours of swirling and non-swirling flames. •They found that the contours of swirling flames were similar to non-swirling flame but substantially shorter and thicker.

Owaki et al.(2007)

•Examined the fluid dynamics underlying swirl combustion experimentally using a nozzle burner connected to a larger diameter duct. •It was found that rich combustion modes are produced by an increase or decrease in the central axial velocity induced by the increase or decrease in the central axial velocity induced by the cross-sectional area change in the vortex core.

Cheng et al. (1998)

•Showed that the maximum mean flame temperature is located at the edge of the recirculation zone, indicating violent combustion and strong mixing of fuel, air and hot products in this region.

Widmann et al. (2000)

•A three-dimensional model to simulate the aerodynamics in the 12-vane cascade swirl .•Numerical simulation using (RNG) k-ε turbulence model results in a velocity profile consistent with experimental measurements, and correctly predicts a recirculation zone that is experimentally observed at the exit of the annular passage. 12

Zhao et al. (2004)

•Studied numerically open swirl-stabilized turbulent premixed flame. •Better understanding of the flame properties by simulating the flow field, combustion and heat transfer. •Spalding’s stretch-cut-slide model is extended to determine the mixing controlled fuel burning rate

Bazdidi-Tehraniet al. (2006)

•Carried out numerical investigation of a confined swirling flow in a model combustion chamber. •Reveal superiority of the RSM over the standard k-ε model for prediction of swirling flows. •For instance RSM predicts the central toroidal recirculation zone well but k-ε does notzone well but k-ε does not

Al-khafaji et al. (2012)

•Presented the application of CFD in the design of combustor.•A can type combustor with non-swirling and swirling flows at inlet is considered for the analysis under isothermal environment. • The commercially available code ‘FLUENT’, based on finite volume technique and incorporates the standard k- epsilon turbulence model has been used to carry out the predictions. •The numerical results are validated against experimental results and a reasonable matching is observed.

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Swirling Flame ImpingementSwirling Flame ImpingementREFERENCE WORK DONE/CONCLUSION

Huang et al. (2006)

•Observed that swirling flames have capability to produce more uniform heat flux distributions with higher local heat flux at the stagnation point plate than the premixed flame jet without swirl.

Zhao et al. (2009)

•Investigated the heat transfer from a turbulent swirling inverse diffusion flame (IDF) to a flat surface.•Observed that impinging swirling IDF achieves complete and •Observed that impinging swirling IDF achieves complete and intense combustion compared to impinging non-swirling IDF.• This was due to better mixing between the fuel and air induced by the swirl and produce stretched heat fluxes.

Singh et al. (2012)

•Observed a dip in heat flux at the stagnation point and this dip is more pronounced at smaller separation distances. •This suppressed heat fluxes at stagnation point is due to the outward movement of the fluid in swirling flow because of increase in tangential velocity resulting in the creation of weak flow region around the central axis.

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Swirling Flame StabilitySwirling Flame StabilityREFERENCE WORK DONE/CONCLUSION

Chigier et al., (1970)

•High rates of mixing near the nozzle exit results in increased stability of swirling flames

Claypole et al., (1981)

•Flow of hot species around the eye of recirculation plays an increasingly important role in the flame stabilization mechanism

Gupta et al. (1977)

•Studied the stability limits and emission characteristics of concentric multi-annular swirl burner. (1977) concentric multi-annular swirl burner. •They observed wider stability limits for this burner. •A better controlled aerodynamics and distribution of reactants lead to overlapping of high fuel concentration regions of large shear stresses.

Rawe and Kremer (1981)

•Investigated stability limits of gas diffusion flame with swirl•Concluded that the flame stability depends on the location of the reaction zone.•Listed that radial shift of the flame front in regions of excessive local fluid velocities and lifting of the flame by exceeding the maximum possible fuel concentration within the stabilization region, are the reasons for flame extinction.

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Derksen et al., (2000)

•Performed a large-eddy simulation (LES) of the single-phase turbulent flow in a model cyclone geometry on a uniform, cubic computational grid consisting of 4.9 X 106 cells. •The Navier-Stokes equations were discretized according to a lattice-Boltzmann scheme.•The Reynolds number, based on the inlet velocity and the cyclone body diameter was 14,000. •A standard Smagorinsky subgrid-scale model with Cs = 0.1, including wall-damping functions, was applied. •The simulations exhibit vortex-core precession is observed to move about the geometrical axis of the cyclone in a quasi-periodic manner.periodic manner.

Ranga dineshet al., (2009)

•Investigated turbulent isothermal swirling flows with a strong emphasis on vortex breakdown, recirculation and instability behaviour using LES. •LES successfully predicts both the upstream first recirculation zone generated by the bluff body and the downstream vortex breakdown bubble. •The results also highlight the precession mode of instability in the centre jet and the oscillations of the central jet precession, which forms a precessing vortex core.

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Emission AnalysisEmission AnalysisREFERENCE WORK DONE/CONCLUSION

Claypole et al. (1981)

•Studied the effect of swirl burner aerodynamics on NOx

formation. •The major area of NOx formation is the reaction zone. •With increase in swirl number, the reaction zone moves upstream.

Coghe et al. (2004)

•Moderate increase of swirl number from 0.7 to 0.82 could reduce NOx formation up to 30% and promotes the generation of the recirculation phenomena.recirculation phenomena.

Schmittel et al. (2000)

•Observed reduction in total NOx emission with increase in swirl number as higher swirl caused faster mixing and therefore lower mean temperatures in the main reaction zone and hence lower NO emissions.

Alecci et al., (2005)

•Did analysis and modeling of a low NOx swirl burner. •A 3D numerical simulation of the fluid dynamics of a turbulent swirling gas jet was proposed. •Points out the existence of a reverse flow zone (RFZ), in keeping with the real system behaviour

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Conclusions from Literature ReviewedConclusions from Literature Reviewed

Most of the studies are either for free open flames or for confined swirling flames for appliances like combustors.

No numerical studies are available on heat transfer characteristics of swirling impinging flames.

Very few studies are available on heat transfer characteristics of swirling partially premixed/ diffusion flames impinging on a flat surface.swirling partially premixed/ diffusion flames impinging on a flat surface.

No detailed study was found on the effect on flame structure by varying separation distance of plate from the burner exit.

For swirling flames impinging normal to plane surface, CH4 air flames have not been studied. This is important because methane is important fuel for industrial applications (as natural gas) and is also a fuel for which the chemical reaction mechanism is well established. Studies will be very much helpful for validating the numerical codes.

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3. PROBLEM FORMULATION3. PROBLEM FORMULATION

In the present study, ANSYS FLUENT 14 (commercial CFD software) is used to model the flow field and heat transfer of swirling premixed methane/air flame impinging on a flat surface.

Aim is to analyze computationally the steady state heat Aim is to analyze computationally the steady state heat transfer from a methane/air premixed, turbulent and axisymmetric flame jet impinging on a flat plate.

19

Governing EquationsGoverning Equations

For flows with heat transfer, FLUENT solves

Conservation equation for mass

Conservation equation for momentum

Conservation equation for energy.

For flows involving species mixing or reactions

Species conservation equation is solved.

20

Mass Conservation EquationMass Conservation Equation

Continuity equation

Source Sm is the mass added to the continuous phase from the dispersed second phase (e.g., due to vaporization of liquid droplets) and any user defined sources

mSvt

liquid droplets) and any user defined sources

For 2D axisymmetric geometries, the continuity equation is given by:

21

0

r

v

r

v

x

v

trrx

Momentum Conservation EquationsMomentum Conservation Equations

Conservation of momentum in an inertial (non-accelerating) reference frame is described by:

Axial momentum conservation equation for 2D axisymmetric flows :

Fgpvvt

v

vpvvrvvrv 2111

22

v

x

vr

xrr

p

r

vvr

rx

vvr

rt

v xxrxxx

3

22

111

xrx Fx

v

r

vr

rr

1

The radial momentum conservation equation for 2D axisymmetric flows is:

r

v

x

vr

xrr

p

r

vvr

rx

vvr

rt

v xrrrrxr 111

2

23

22

1

r

vv

r

vr

rrrr

The tangential momentum conservation equation for 2D swirling flows may be written as:

23

rz Fr

vv

r

2

3

2

r

vv

r

v

rr

rrx

vr

xrr

vvr

rx

vvr

rv

tzrzzzrzx

r

3

2

1111

Energy & Species Transport EquationsEnergy & Species Transport Equations

ANSYS FLUENT solves the energy equation in the following form:

ANSYS FLUENT predicts the local mass fraction of each species, Y ,

hj

effjjeff SvJhTkpEvt

E

ANSYS FLUENT predicts the local mass fraction of each species, Yi, through the solution of a convection-diffusion equation for the ith species. This conservation equation takes the following general form:

24

iiii

i SRJYvt

Y

where Ri is the net rate of production of species i by chemical reaction (eddy dissipation model) and Si denote sources.

Computational Procedure EmployedComputational Procedure Employed

Full simulation of swirling flame was divided into two simulations:

1. Simulation of 3D swirl burner domain

2. Simulation using 2D axisymmetric-swirl solver for flame (assuming zero circumferential gradients)(assuming zero circumferential gradients)

Firstly, simulation for 3D swirl burner domain was done and the velocity profile generated in this simulation was used as an input for second simulation involving 2D

axisymmetric-swirl model.

25

Computational DomainComputational Domain

The modeling of the problem is divided in two parts:

1. Burner domain

2. Flame domain

26

Burner Domain & Grid (ANSYS ICEM Burner Domain & Grid (ANSYS ICEM CFD)CFD)

Air entry

Ø20

Ø25

Ø47 (ID) 53 (OD)

Ø 72 (ID) 76 (OD)

25

140

Polyhedral mesh containing 386867 cells and 1925599 nodes

27

Fuel entry

Fuel exit holes

Helical vane swirler

200

25

Boundary Conditions for Burner Domain Boundary Conditions for Burner Domain (Re = 5000 and (Re = 5000 and ϕϕ= 1)= 1)

Boundary zone Velocity Inlet (m/s) Temperature (K) Species (mass

fraction)

Turbulent intensity

(%) and Hydraulic

Diameter (m)

AIR INLET 17.93277 300 O2 = 0.2329175 5.517581 % and

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0.009 m

FUEL INLET 1.846983 300 CH4 = 1

mix

mixexit du

Re

ii

iii

mixMY

MY

actual

stoic

FA

FA

/

/ 8

1'

Re16.0

HD

avgu

uI

Flame Domain (GAMBIT 2.4)Flame Domain (GAMBIT 2.4)

Boundary 1 – impingement surface (300K)

Boundary 2 – pressure outlet(Patm)

Boundary 3 – pressure inletBoundary 4 – burner rim

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Boundary 4 – burner rim (300K)

Boundary 5 – velocity-inletBoundary 6 – burner axis

Computational Grid used for Flame Computational Grid used for Flame SimulationSimulation

Open Flame and

Impingement cases

(H/d)

Number of Nodes Number of

Quadrilateral

(Structured) Cells

OPEN 251316 250250

30

OPEN 251316 250250

2 266311 265000

3 398936 397500

4 531561 530000

6 796811 795000

Solution Methods and Discretization Solution Methods and Discretization Schemes for Burner SimulationSchemes for Burner Simulation

MODEL SETTING

Space 3D

Time Steady

Type Pressure-Based

Gravity X=0, Y=0, Z(m/s2) = -9.81

Viscous RNG k-ε with swirl

Solution Methods Option

Pressure-Velocity

Coupling

SIMPLE

Spatial Discretization Option

Gradient Least Squares Cell

Based

31

Viscous RNG k-ε with swirl

dominated flow and

standard wall functions

Heat Transfer Disabled

Species Transport Enabled, volumetric –

disabled, inlet diffusion and

diffusion energy source -

enabled

Pressure-Velocity Coupling SIMPLE

Pressure PRESTO!

Momentum Second Order Upwind

Turbulent Kinetic

Energy

Second Order Upwind

Turbulent Dissipation

Rate

Second Order Upwind

Species Second Order Upwind

Energy Second Order Upwind

Solution Methods and Discretization Solution Methods and Discretization Schemes for Flame SimulationSchemes for Flame Simulation

MODEL SETTING

Space 2D Axisymmetric swirl

Time Steady

Type Pressure-Based

Gravity X(m/s2) = -9.81, Y = 0

Viscous Stress – omega RSM

Heat Transfer Enabled

Species Transport Enabled, volumetric –

Solution Methods Option

Pressure-Velocity

Coupling

SIMPLE

Spatial Discretization Option

Gradient Least Squares Cell Based

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Species Transport Enabled, volumetric –

enabled, inlet diffusion and

diffusion energy source -

enabled

Mixture Material and solid

material

Methane-air having 5

volumetric species ( CH4,

O2, H2O,CO2 and N2) and

Copper

Turbulence-Chemistry

Interaction

Eddy - Dissipation

Variable-Density

parameters

Operating density = 1.225

kg/m3

Pressure PRESTO!

Momentum Second Order Upwind

Turbulent Kinetic Energy Second Order Upwind

Turbulent Dissipation

Rate

Second Order Upwind

Species Second Order Upwind

Energy Second Order Upwind

4. RESULTS AND 4. RESULTS AND DISCUSSIONSDISCUSSIONS

Divided into 4 sections based on the following objectives:

1. Validation of simulation result with experiments.

2. To understand the structure of free swirling flame.

3. To understand structure of swirling flame impinging 3. To understand structure of swirling flame impinging on flat surface.

4. To understand heat transfer characteristics of swirling flame impinging on flat surface.

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Comparison between Experimental and Comparison between Experimental and Simulation Stagnation point Heat FluxSimulation Stagnation point Heat Flux

Different Cases

simulated

Experimental heat flux

at stagnation point

(kW/m2)

Simulation heat flux at

stagnation point

(kW/m2)

Difference between

experiment and

simulation values (%)

H/d = 2 139.37 186.30 33.67 %

H/d = 3 91.64 110.60 20.69 %

H/d = 4 59.00 76.00 28.81 %

H/d = 6 49.16 65.35 32.93 %

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Experimental heat flux distribution obtained by Singh et al. (2012)

Heat flux distribution obtained by simulation

Re = 5000, and

Radial Distance (m)

0.00 0.02 0.04 0.06 0.08 0.10 0.12

He

at

Flu

x (

kW

m2)

0

50

100

150

200H/d = 2H/d = 3H/d = 4H/d = 6

Re = 5000, = 60 and = 1.0

Radial Distance (m)

0.00 0.02 0.04 0.06 0.08 0.10 0.12

He

at

Flu

x (

kW/m

2)

0

50

100

150

200H/d = 2H/d = 3H/d = 4H/d = 6

Possible reasons for Possible reasons for differencedifference

Assumption of 2D axisymmetric-swirl model

Turbulence was modeled using RANS

Single-step global reaction mechanism was used for

Re = 5000, = 60o and = 1.0

Radial Distance (m)

0.00 0.02 0.04 0.06 0.08 0.10 0.12

He

at

Flu

x (

kW

/m2

)

0

50

100

150

200H/d = 2 (Simulated)H/d = 6 (Simulated)H/d = 2 (Experimental)H/d = 6 (Experimental)

mechanism was used for methane-air reaction

Experiments were done using CNG

Radiation modeling was not done

35

Re = 5000, = 60o and = 1.0

Radial Distance (m)

0.00 0.02 0.04 0.06 0.08 0.10 0.12

Hea

t F

lux (

kW

/m2)

0

20

40

60

80

100

120

140 H/d = 3 (Simulated)H/d = 4 (Simulated)H/d = 3 (Experimental)H/d = 4 (Experimental)

STRUCTURE OF FREE SWIRLING STRUCTURE OF FREE SWIRLING FLAMEFLAME

Central Toroidal Recirculation Zone (CTRZ)Recirculation bubble plays an important role in flame stabilization.

Provides hot flow of recirculated combustion products and a reduced velocity region where flame velocity and and a reduced velocity region where flame velocity and flow velocity can be matched.

Slanting Reaction ZoneReaction zone can be characterised as a region where bulk of chemical energy is released.

36

Central Toroidal Recirculation zone (CTRZ)Central Toroidal Recirculation zone (CTRZ)

•The recirculation zone extends up to 0.55d in the radial direction and up to 1.76d in the downstream axial direction.

•Eye of the recirculation

37

Eye of CTRZ

•Eye of the recirculation zone is located near the slanting flame reaction zone.

•The recirculation zone almost touches the burner exit in the upstream direction. This indicates that the flame could be stabilized right at the burner rim.

Centerline Axial Velocity, Temperature and Centerline Axial Velocity, Temperature and CHCH44 mass fractionmass fraction

Re = 5000, = 60o and = 1.0

Axia

l V

elo

city (

m/s

)

-5

0

5

10

Tem

pera

ture

(K

)

1000

1200

1400

1600

1800

2000

2200

Mass F

ractio

n o

f C

H4

0.02

0.03

0.04

0.05

0.06

Axial Velocity

•Sharp decrease in axial velocity is caused due to presence of recirculation zone.

•Maximum negative axial velocity is13.3 m/s.

38

Axial Distance (m)

0.00 0.02 0.04 0.06 0.08 0.10

Axia

l V

elo

city (

m/s

)

-15

-10

-5

Tem

pera

ture

(K

)

200

400

600

800

1000

Mass F

ractio

n o

f C

H

0.00

0.01

0.02Axial Velocity

TemperatureMethane Mass Fraction

velocity is13.3 m/s.

•After that the flow has started accelerating downstream.

•Temperature peak of 2070 K is located very close to the burner exit due to the impingement effect between the reverse flow of hot gases and the main flow.

•After the region of intense reaction zone, temperature is dropping slowly axially and value of mass fraction of methane is very close to zero

Axial, Radial and Tangential Velocity Axial, Radial and Tangential Velocity Variation at different Axial HeightsVariation at different Axial Heights

Re = 5000, = 1.0 and open flam e

Axia

l V

elo

city (

m/s

)

-15

-10

-5

0

5

10

15

Z = 5 mmZ = 10 mmZ = 15 mmZ = 20 mmZ = 25 mmZ = 30 mmZ = 50 mm

Re = 5000, = 1.0 and open flame

Ra

dia

l V

elo

city (

m/s

)

-2

0

2

4

6

8

10

12

Z = 5 mmZ = 10 mmZ = 15 mmZ = 20 mmZ = 25 mmZ = 30 mmZ = 50 mm

39

Radial D istance (m )

0.00 0.01 0.02 0.03 0.04 0.05

-15

Radial Distance (m)

0.00 0.01 0.02 0.03 0.04 0.05

-2

Re = 5000, = 1.0 and open flame

Radial Distance (m)

0.00 0.01 0.02 0.03 0.04 0.05

Ta

nge

ntia

l V

elo

city (

m/s

)

0

1

2

3

4

5

6

Z = 5 mmZ = 10 mmZ = 15 mmZ = 20 mmZ = 25 mmZ = 30 mmZ = 50 mm

•Reaction zone is divergent since the peak of axial velocity gets shifted radially outward from the central axis. •The existence of sharp peaks in the radial velocity profiles shows that flow expansion exists due to heat release in the combustion zone. •Positive values of tangential velocity at radial distance indicate the rotation in the flow. Thus, swirl decay is there

Temperature VariationTemperature Variation

•Largest temperature gradient (i.e., closely spaced isotherms) is the region where actual burning takes place (reaction zone). Thus, the intense combustion starts very close to the burner exit.•The maximum temperature reached is 2070 K whereas corresponding stoichiometeric adiabatic flame

40

Re = 5000, open flame

Radial Distance (m)

0.00 0.01 0.02 0.03 0.04 0.05

Te

mpe

ratu

re (

K)

500

1000

1500

2000

Z = 5 mmZ = 10 mmZ = 15 mmZ = 20 mmZ = 25 mmZ = 30 mmZ = 50 mm

stoichiometeric adiabatic flame temperature is 2210 K (Glassman, 1996).•This difference in temperature value can be attributed to diffused reaction zone and strong entrainment effect due to swirl in the present case. •The temperature was peaking above the recirculation bubble due to after burning of the combustion unstable species (oxidation of CO, H2 and OH) in that region (Red colour region).

STRUCTURE OF IMPINGING STRUCTURE OF IMPINGING SWIRLING FLAMESWIRLING FLAME

Central Toroidal Recirculation Zone (CTRZ) and Movement of Eye.

Effect of Plate on Flame structure (Velocity and Temperature Contours, (Velocity and Temperature Contours, Velocity vectors).

41

CTRZ and Movement of EyeCTRZ and Movement of Eye

42

Different cases (in sequence) H/d = 2H/d = 3H/d = 4H/d = 6Open flame

Locus of Movement of EyeLocus of Movement of Eye•For the cases of H/d = 6 and open flame, location of eye almost coincides depicting that when the plate is too far then it doesn’t cause any change in flow structure of the flame. •Eye comes a little back towards the burner exit in case of H/d = 3. However, H/d = 4 case shows

43

However, H/d = 4 case shows particularly different nature in terms of CTRZ and eye location, as the eye got shifted a lot closer to the plate and too far from the burner exit.

It has been revealed in the literature (Syred, 1974) that the location of the eye significantly affects the stability of the flame. Closer the location of the eye to burner rim more stable is the flame. Here in the simulations it has been noticed that the separation distance of H/d = 4 makes the eye to move farthest from the burner rim which could cause the flame to unstable.

Effect of Plate on Flame StructureEffect of Plate on Flame Structure

Velocity magnitude contours.

Axial velocity contours.

r = 0

Impingement Plate

contours.

Velocity vectors.

Temperature contours.

44

Air Entry

H

Z = 0

Swirling Flame

Velocity Magnitude ContoursVelocity Magnitude Contours

45

Axial Velocity ContoursAxial Velocity Contours

46

Velocity VectorsVelocity Vectors

47

Temperature ContoursTemperature Contours

48

IMPINGEMENT HEAT TRANSFERIMPINGEMENT HEAT TRANSFER

Heat Flux Distribution (Analysis very close to the wall is done).

Axial velocity gradient analysis on radial planes.planes.

Heat Flux DistributionHeat Flux Distribution Peak in heat flux is observed at some location radially away from the

central axis for all separation distances

This peak in heat flux corresponds to direct striking of the flame boundary on the impingement surface (Zhen et al., 2009)

Further a dip in heat flux is observed at the stagnation point and this dip is more pronounced at smaller separation distances.

The dip in heat flux at stagnation point can be attributed to the The dip in heat flux at stagnation point can be attributed to the outward movement of the fluid in swirling flow due to increase in tangential velocity resulting in the creation of weak flow region around the central axis (Yuan et al., 2006)

There was significant difference in the heat flux at and around stagnation point at various separation distances. This can be attributed to strong influence of flame structure on heat transfer distribution in that region (Chander, 2006).

In the present set of simulations dip in the heat flux distribution is explained with flow dynamics of swirling flame

50

Variation of Temperature and Velocity Variation of Temperature and Velocity Components for H/d = 2.Components for H/d = 2.

Re = 5000, = 1.0, H/d = 2

Axia

l Ve

locity (

m/s

)

-2

-1

0

1

2

3

He

at

Flu

x (

kW

/m2)

0

50

100

150

200

Z = 35 mmZ = 36 mmZ = 37 mmZ = 38 mmZ = 39 mmHeat Flux

Re = 5000,

d = 2

Tan

gen

tia

l V

elo

city (

m/s

)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

He

at

Flu

x (

kW

/m2)

0

50

100

150

200

Z = 35 mm

Z = 36 mm

Z = 37 mm

Z = 38 mm

Z = 39 mm

heat flux

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Radial Distance (m)

0.00 0.01 0.02 0.03 0.04 0.05

Radial Distance ( m)

0.00 0.01 0.02 0.03 0.04 0.05

0.0 0

Re = 5000, d = 2

Radial Distance (m)

0.00 0.01 0.02 0.03 0.04 0.05

Radia

l V

elo

city (

m/s

)

-2

-1

0

1

2

3

4

5

Hea

t F

lux (

kW

/m2)

0

50

100

150

200

Z = 35 mmZ = 36 mmZ = 37 mmZ = 38 mmZ = 39 mm

heat flux

Re = 5000,H/d = 2 = 1.0

Radial Distance (m)

0.00 0.01 0.02 0.03 0.04 0.05

Tem

pera

ture

(K

)

200

400

600

800

1000

1200

1400

1600

1800

2000

Hea

t F

lux (

kW

/m2)

0

50

100

150

200

Z = 35 mmZ = 36 mm

Z = 37 mmZ = 38 mm

Z = 39 mmheat flux

Impingement heat flux distribution depends Impingement heat flux distribution depends upon Flow dynamics (Axial velocity)upon Flow dynamics (Axial velocity)

Re = 5000, = 1.0, H/d = 2

Axia

l V

elo

city

(m

/s)

-2

-1

0

1

2

3

Hea

t F

lux (

kW

/m2)

0

50

100

150

200

Z = 35 mmZ = 36 mmZ = 37 mmZ = 38 mmZ = 39 mmHeat Flux

Re = 5000, = 1.0,H/d = 3

Axia

l V

elo

city (

m/s

)

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Heat F

lux (

kW

/m2)

2.0e+4

4.0e+4

6.0e+4

8.0e+4

1.0e+5

1.2e+5

Z = 55 mm

Z = 56 mm

Z = 57 mm

Z = 58 mm

Z = 59 mm

52

Radial Distance (m)

0.00 0.01 0.02 0.03 0.04 0.05

-2 Heat Flux

Radial Distance (m)

0.00 0.01 0.02 0.03 0.04 0.05

-0.4 0.0

heat flux

Re = 5000, = 1.0H/D = 4

Radial Distance (m)

0.00 0.02 0.04 0.06 0.08 0.10

Axi

al V

elo

city (

m/s

)

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

Heat F

lux (

kW

/m2)

0

2e+4

4e+4

6e+4

8e+4

1e+5

Z = 75 mm

Z = 76 mm

Z = 77 mm

Z = 78 mm

Z = 79 mm

heat flux

Re = 5000, = 1.0, H/d = 6

Radial Distance (m)

0.00 0.02 0.04 0.06 0.08 0.10

Axia

l V

elo

city

(m

/s)

0.0

0.1

0.2

0.3

0.4

0.5

Heat

Flu

x (

kW

/m2)

20000

30000

40000

50000

60000

70000

Z = 115 mm

Z = 116 mm

Z = 117 mm

Z = 118 mm

Z = 119 mm

heat flux

Axial Velocity Gradients on Radial Axial Velocity Gradients on Radial PlanesPlanes

r = 0

Impingement Plate

Plan

es

The peak in the heat flux on the impingement surface is due to maximum in the axial velocity gradient in the boundary layer

53

Air Entry

H

Z = 0

Swirling Flamegradient in the boundary layer region.

H/d = 2 and 3H/d = 2 and 3

Re = 5000, = 1.0 and H/d = 2

Axial Distance (m)

0.00 0.01 0.02 0.03 0.04

Axia

l V

elo

city (

m/s

)

0

2

4

6

8

10r = 15 mmr = 20 mmr = 25 mmr = 30 mm

Re = 5000, = 1.0 and H/d = 3

0.00 0.01 0.02 0.03 0.04 0.05 0.06

Axia

l V

elo

city (

m/s

)

0

2

4

6

8

10

r = 15 mmr = 24 mmr = 30 mmr = 40 mm

54

Axial Distance (m)Axial Distance (m)

Re = 5000, = 1.0 and H/d = 2

Axial Distance (m)

0.035 0.036 0.037 0.038 0.039 0.040

Axia

l V

elo

city (

m/s

)

0.0

0.5

1.0

1.5

2.0

2.5

3.0r = 15 mmr = 20 mmr = 25 mmr = 30 mm

Re = 5000, =1.0 and H/d = 3

Axial Distance (m)

0.055 0.056 0.057 0.058 0.059 0.060

Axia

l V

elo

city (

m/s

)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

r = 15 mmr = 24 mmr = 30 mmr = 40 mm

H/d = 4 and 6H/d = 4 and 6

Re = 5000, = 1.0 and H/d = 4

0.00 0.02 0.04 0.06 0.08

Axia

l V

elo

city (

m/s

)

0

1

2

3

4r = 40 mmr = 48 mmr = 55 mm

r = 65 mmRe = 5000, = 1.0 and H/d = 6

Axia

l V

elo

city (

m/s

)

-6

-4

-2

0

2

4

6

8

10

r = 5 mmr = 16 mmr = 25 mmr = 35 mm

55

Axial Distance (m)

0.00 0.02 0.04 0.06 0.08

Re = 5000, = 1.0 and H/d = 4

Axial Distance (m)

0.075 0.076 0.077 0.078 0.079 0.080

Radia

l V

elo

city (

m/s

)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

r = 40 mmr = 48 mmr = 55 mmr = 65 mm

Axial Distance (m)

0.00 0.02 0.04 0.06 0.08 0.10 0.12

-6

Re = 5000, = 1.0 and H/d = 6

Axial Distance (m)

0.115 0.116 0.117 0.118 0.119 0.120

Axia

l V

elo

city (

m/s

)

0.0

0.1

0.2

0.3

0.4

0.5

r = 5 mmr = 16 mmr = 25 mmr = 35 mm

Summary of ResultsSummary of Results Streamlines of free and impinging flames depict the variation in

swirling flame structure in terms of CTRZ and location of eye of recirculation zone.

Swirling flames has divergent reaction zone which was ascertained by studying the variation in axial velocity at different axial heights.

There was significant change in flame structure with presence and location of plate.and location of plate.

A dip in the heat flux was observed in almost all cases which is the major reason for non-uniformity in heating even for swirling impinging flames. The dip in heat flux can be explained by the variation in axial velocity in the radial direction, which is purely characteristic of flow due to the presence of swirl.

The peak in the heat flux in the radial direction corresponds to maximum value of the axial velocity and temperature gradients in the boundary layer region

56

5. SCOPE FOR FUTURE 5. SCOPE FOR FUTURE RESEARCHRESEARCH

LES and DNS approach could be used for more accurate turbulence modeling of these complex swirl flames.

Zimont (2010) has suggested a strategy of solution consisting of two steps: the first step is to use one FLUENT turbulence methods (k-epslion, k-omega, etc....) and then these results could be a starting point for large eddy simulation (LES) or direct numerical method (DNS).direct numerical method (DNS).

Full 3D simulations could be carried out along with utilisation of detailed combustion reaction mechanisms (e.g., GRI mechanisms).

Further work can be extended to study the peculiar instabilities present like precessing vortex core (PVC) by using unsteady 3D simulations.

Flame speed of fuel mixtures is an important factor that needs to be included for future work.

57

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