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STUDY OF THE COMPARISON BETWEEN SINGLE
CAVITY AND DOUBLE CAVITY OF TRAPPED VORTEX COMBUSTOR USING COLD
FLOW
ANALYSIS. A Project Report
Submitted by
Adhvaryu Jay (100410101005) HinguBhavik (100410101016)
Brahmbhatt Meghali (100410101015)
In partial fulfillment of the award of the degree
Of
Bachelor of Engineering
In
AERONAUTICS
SARDAR VALLABHBHAI PATEL INSTITUTE OF TECHNOLOGY,
VASAD
Gujarat Technological University, Ahmedabad
December, 2013
& SARDAR VALLABHBHAI PATEL INSTITUTE OF TECHNOLOGY
Aeronautical Engineering 2013
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CERTIFICATE
Date:
This is to certify that the dissertation entitled study of the
comparison
between single cavity and double cavity of trapped vortex
combustor using
cold flow analysis has been carried out by Jay Adhvaryu, Bhavik
Hingu and
Meghali Brahmbhatt under my guidance in fulfilment of the degree
of
Bachelor of Engineering in Aeronautics(7th Semester) of
Gujarat
Technological University, Ahmedabad during the academic year
2013-14.
Guides:
Mr. Vivek C. Joshi Niyati Shah Assistant Professor Mechanical
Department Assitant Professor Parul Institute of Engineering and
Technology SVIT, Vasad
Dr. DipaliThakkar I/C HOD, Department of Aeronautical Engg.
SVIT, Vasad
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ACKNOWLEDGEMENT
We owe a debt of gratitude to Mr. Vivek C. Joshi and Mrs. Niyati
Shah asour guide for the vision and foresight which inspired us to
conceive the project. Besides being an advisor, its necessary to
appreciate Mr. Vivek Joshi for his encouragement, insightful
comments, and hard questions which kept the team motivated to
accomplish the project study.
We would like to express our sincere gratitude to Mrs. Niyati
Shah for the continuous support as faculty guide in our Project
study, for her patience, motivation, enthusiasm, and immense
knowledge. Her guidance helped us all the time of project and
writing of this project. The team could not have imagined having a
better advisors and mentors for our Project study. We also thank
our friends and parents who have been involved with the project in
ways that may not seem greatly significant. It would have been
impossible for us to complete the project without your support.
With Regards, Adhvaryu Jay (100410101005) Hingu Bhavik
(100410101016)
Brahmbhatt Meghali (100410101015)
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ABSTRACT
A new combustor concept referred as the trapped vortex combustor
(TVC)
employs a vortex that is trapped inside a cavity to stabilize
the flame. The cavity
is formed between two axis-symmetric disks mounted in tandem.
TVC offers
many advantages when compared to conventional swirl stabilizers.
In the
present work, numerical investigation of cold flow
(non-reacting) through single
cavity and double cavity trapped vortex combustor is performed.
Commercial
CFD software Fluent has been used for this study. We are
comparing the single
cavity TVC and double cavity TVC with the help of total pressure
plot,
streamline plot and vorticity plot. The other main objective of
our study is to
evaluate the performance and combustion stability of the single
cavity trapped
vortex combustor and double cavity trapped vortex combustor by
varying the
mass flow rate through the single cavity TVC and double cavity
TVC. From the
mass flow rate study, it is inferred that as the mass flow rate
increases
combustion stability is increased in both the single cavity TVC
and double cavity
TVC.
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LIST OF TABLES
5.1- Grid independence study of fore body
5.2-Grd independence study of single cavity
5.3- Grid independence study of double cavity
5.4- Reduction in drag coefficient
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LIST OF FIGURES
Figure 1.1: Trapped Vortex Combustor
Figure 1.2: Swirl vanes
Figure 1.3: Cavities in TVC
Figure 1.4: NOx v/s combustor pressure for different
combustors
Figure 1.5: Combustor efficiency v/s FAR for conventional
combustor and TVC
Figure 2.1: Change in drag coefficient resulting from the
addition of afterbody
to forebody- spindle combination obtained for different cavity
lengths.
Figure 2.2: Change in drag coefficient due to the second
cavity
Figure 3.1: Difference between real experiment and CFD
simulation.
Figure 4.1: Single Cavity TVC Geometry
Figure 4.2: Double Cavity TVC Geometry
Fig 4.3: Forebody mesh
Fig 4.4: Single Cavity mesh
Fig 4.5: Double Cavity mesh
Fig 4.6: Block diagram of Eddy Dissipative Model
Fig 5.1: Total pressure plot of Single Cavity at 40 m/s
Fig 5.2: Total pressure plot of Double Cavity at 40m/s
Fig 5.3: Total pressure plot for Single Cavity at 50m/s
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Fig 5.4: Total pressure plot for Double Cavity at 50m/s
Fig 5.5: Total pressure plot for Single Cavity at 60m/s
Fig 5.6: Total pressure plot for Double Cavity at 60m/s
Fig 5.7: Total pressure plot for Single Cavity at 70m/s
Fig 5.8: Total pressure plot for Double Cavity at 70m/s
Fig 5.9: Streamline plot for Single Cavity at 40 m/s
Fig 5.10: Streamline plot for Double Cavity at 40 m/s
Fig 5.11: Streamline plot for Single Cavity at 50 m/s
Fig 5.12: Streamline plot for Double Cavity at 50 m/s
Fig 5.13: Streamline plot for Single Cavity at 60 m/s
Fig 5.14: Streamline plot for Double Cavity at 60 m/s
Fig 5.15: Streamline plot for Single Cavity at 70 m/s
Fig 5.16: Streamline plot for Double Cavity at 70 m/s
Fig 5.17: Vorticity plot for Single cavity at 40 m/s
Fig 5.18: Vorticity plot for double cavity at 40 m/s
Fig 5.19: Vorticity plot for Single cavity at 50 m/s
Fig 5.20: Vorticity plot for double cavity at 50 m/s
Fig 5.21: Vorticity plot for Single cavity at 60m/s
Fig 5.22: Vorticity plot for double cavity at 60m/s
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Fig 5.23: Vorticity plot for Single cavity at 70m/s
Fig 5.24: Vorticity plot for double cavity at 70m/s
Fig 5.25: Variation in coefficient of drag (CD) with change in
velocity
Fig A1: General Window
Fig A2: Turbulence Model Window
Fig A3: Material Window
Fig A4: Inlet Condition
Fig A5: Outlet Condition
Fig A6: Solution Method
Fig A7: Solution Initialization
Fig A8: Check And Run
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LIST OF SYMBOLS
- Turbulent Specific Dissipation Rate
- Turbulent Dissipation Rate
-Density
- Turbulent Kinetic Energy
ABBREVIATION
DNS- Direct numerical simulation
LES- Large eddy simulation
QUICK- Quadratic upwind interpolation for convective
kinematics
RANS- Reynolds average navier-stokes equation
SIMPLE- Semi implicit method for pressure linked equations
TVC- Trapped vortex combustor
NOMENCLATURE
CD Coefficient of Drag
D0 Diameter of Forebody mm
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D1
Diameter of First Afterbody mm
D2
G
ReHd
Diameter of Second Afterbody mm
Downstream Distance from First
Afterbody mm
Reynolds Number based on hydraulic
diameter
I Turbulence Intensity
L Characteristic Length m
V Characteristic Velocity m/s
h Inlet duct height mm
CD Avg. coefficient of Drag
U0 Free stream Velocity
Ap Projected Area
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TABLE OF CONTENTS
Acknowledgement iii
Abstract iv
List of Tables v
List of figures vi
List of symbol ix
List of Abbreviations ix
Nomenclature ix
Chapter 1. INTRODUCTION TO TVC 01 1.1 Objective 02
1.1.1. What is a combustor? 02
1.1.2. Requirements of combustor 02
1.1.3. Drawbacks of Regular Gas Turbine Combustor 03
1.2. Advantages of TVC 04
1.2.1. Flame stability 04
1.2.2. Low emissions 05
1.2.3. Fuel flexibility 07
1.3. Actual Mechanism of TVC 08
Chapter 2. LITERATURE REVIEW 09 (a) Drag and flow
characteristics of afterbodies 09
(b) Effect of mass injection into the cavity 09
(c) Relation of mass injection and optimum cavity size 09
(d) Characteristics of TVC using k- model 10
(e) Placement of second afterbody for vortex stability 10
Chapter 3. INTRODUCTION TO CFD 12 3.1 CFD Analysis Process
13
3.1.1 Problem Statement 13
3.1.2 Mathematical Model 14
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3.1.3 Discretization Process 14
3.1.4 Iterative Strategy 14
3.1.5 Simulations 15
3.1.6 Post processing and Analysis 15
Chapter 4.METHODOLOGY AND THEORY 16 4.1 Selection of Geometry
and Placement 16
4.1.1 Single Cavity TVC Geometry 16
4.1.2 Double Cavity TVC Geometry 16
4.2 Meshing Process 17
4.2.1 Forebody mesh 19
4.2.2 Single Cavity mesh 19
4.2.3 Double Cavity mesh 22
4.3 Setup in FLUENT 22
4.3.1 Models 22
4.3.2 Material 22
4.3.3 Boundary conditions 23
4.3.4 Solution methods 22
4.3.5 Time step calculation 22
4.3.6 Convergence criteria 22
Chapter 5. RESULTS AND DISCUSSIONS 23 5.1 Total Pressure Plots
23
5.2 Flow Pattern 27
5.3 Vorticity Plots 30
5.4 Validation and verification 35
5.4.1 Validation 35
5.4.2 Verification 36
5.5 Result Summary 36
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Chapter 6. CONCLUSION 38 6.1 Conclusion 38
6.2 Contribution 39
6.3 Future scope 39
Appendix 1: Fluent Setup 40
Appendix 2: List of References 47
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-
STUDY OF THE COMPARISON BETWEEN SINGLE CAVITY AND DOUBLE CAVITY
OF
TRAPPED VORTEX COMBUSTOR USING COLD FLOW ANALYSIS.
A PROJECT REPORT
Submitted by
Adhvaryu Jay (100410101005) Hingu Bhavik (100410101016)
Brahmbhatt Meghali (100410101015)
In fulfillment for the award of the degree of BACHELOR OF
ENGINEERING
in Aeronautics
SARDAR VALLABHBHAI PATEL INSTITUTE OF TECHNOLOGY, VASAD
Gujarat Technological University, Ahmedabad DECEMBER 2013
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CHP I. INTRODUCTION TO TVC
Trapped Vortex Combustor (TVC) is a novel design concept for
potential use in gas turbines wherein cavities are used to trap the
vortex flow structure. TVC offers many advantages when compared to
conventional combustor, the main advantage being flame
stabilization.
Fig 1.1: Trapped Vortex Combustor In conventional combustors,
flame stabilization is achieved with the help of toroidal flow
pattern whereas in TVC physical cavities help in creating
recirculation zones, coupled with direct injection of fuel and air,
thus providing a continuous source of ignition. The recirculation
zones are regions of low velocity where mixing, ignition and
burning can occur without any disturbance. After burning, these hot
products are mixed with the main flow by using wake regions
generated by the bodies placed in main flow. Because of this, the
pilot flame is able to resist high velocity flows and has extended
lean and blowout limits.
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1.1 OBJECTIVE:
1.1.1. WHAT IS A COMBUSTOR? A combustor is a component or area
of a gas turbine, ram jet or scram jet engine where combustion
takes place. It is also known as a burner, combustion chamber or
flame holder. In a gas turbine engine, the combustor or combustion
chamber is fed high-pressure air by the compression system. The
combustor then heats this air at constant pressure. After heating,
air passes from the combustor through the nozzle guide vanes to the
turbine. In the case of a ramjet or scramjet engines, the air is
directly fed to the nozzle.
1.1.2. REQUIREMENTS OF COMBUSTOR: The objective of the combustor
in a gas turbine is to add energy to the system to power the
turbines and produce a high velocity gas to exhaust through the
nozzle in aircraft applications. As with any engineering challenge,
accomplishing this requires balancing many design considerations,
such as the following:
Completely combust the fuel. Otherwise, the engine is just
wasting the unburnt fuel. Low-pressure loss across the combustor.
The turbine that the combustor feeds
needs high-pressure flow to operate efficiently. The flame
(combustion) must be held (contained) inside of the combustor.
If
combustion happens further back in the engine, the turbine
stages can easily be damaged. Additionally, as turbine blades
continue to grow more advanced and are able to withstand higher
temperatures, the combustors are being designed to burn at higher
temperatures and the parts of the combustor need to be designed to
withstand those higher temperatures.
Uniform exit temperature profile. If there are hot spots in the
exit flow, the turbine may be subjected to thermal stresses or
other types of damage. Similarly, the temperature profile within
the combustor should avoid hot spots, as those can damage or
destroy a combustor from the inside.
Small physical size and weight. Space and weight is at a premium
in aircraft applications, so a well-designed combustor strives to
be compact. Non-aircraft applications, like power generating gas
turbines, are not as constrained by this factor.
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Wide range of operation. Most combustors must be able to operate
with a variety of inlet pressures, temperatures, and mass flows.
These factors change with both engine settings and environmental
conditions (I.e., full throttle at low altitude can be much
different than idle throttle at high altitude).
Environmental emissions. Although gas turbine combustion systems
operate at extremely high efficiencies, they produce pollutants
such as oxides of nitrogen (NOx), carbon monoxide (CO) and unburned
hydrocarbons (UHC) and these must be controlled to very low
levels.
1.1.3.DRAWBACKS OF REGULAR GAS TURBINE COMBUSTORS: Gas turbines
dont just deliver power, without any side effects. The drawbacks of
conventional combustor are:
Combustion stability
An important property of a combustion chamber is combustion
stability. To have combustion stability, the flame must remain
stable at varying fuel mixtures, inlet temperatures, turbulence
levels, flow speeds and so on. If it doesnt, things can go
wrong.
To simplify the idea of combustion stability, usually only the
mixture is considered. The combustion stability now depends on the
range of the FAR (Fuel Air Ratio) at which the flame remains
stable. If the flame dies due to too much fuel, we have rich
extinction. Similarly, if there is too little fuel, we have weak
extinction.
The two limits mainly depend on the mass flow of air m air
passing through the combustion chamber. Flames have trouble
surviving at high flow velocities. And a high flow velocity is, of
course, linked to a high mass flow. So too high flow
velocities/mass flows arent good. On the other hand, if the flow
velocity is too low, the flame will move upstream. This is called
flashback. Its not very good either. So, low flow velocity either
is also undesirable.
Losses
Several losses occur in the combustion chamber. First, there is
a heat loss. Heat is being spent on heating up/vaporizing the fuel,
and on heating the combustor itself. Another cause is incomplete
combustion. If part of the fuel does not combust, then heat is
wasted.
Next to heat losses are the pressure losses. Skin friction and
turbulence effects cause the so-called cold losses.
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Pollutants
Hydrocarbon-fueled gas turbines usually have several unwanted
combustion products. The most important combustion products are
H2O, CO2, O2 and N2. These are the so-called products of complete
combustion, and make up 99% of the combustion products.
The remainder of the combustion products can be split up into
two groups. The group of gaseous pollutants consists of nitrogen
oxides NOx, carbon monoxide CO and a variety of unburned
hydrocarbons UHCs. The amount of gaseous pollutants is usually
given by the emission index (EI).
The second group of remaining combustion products is called
smoke. It mainly consists of soot particles, which are particles
with a high amount of carbon in them.
1.2 ADVANTAGES OF TVC:
LOWER EMISSIONS GREATER FLAME STABILITY ADDED FUEL FLEXIBILITY
LONGER LIFE REDUCED CAPITAL COSTS
These advantages will be discussed in the subsequent sections.
1.2.1 FLAME STABILITY: The requirements of low fuel consumption and
low pollutant emissions are paramount for all types of combustors,
with the combustor primary zone airflow pattern of prime importance
to flame stability, combustion efficiency, and low emissions. Many
different types of airflow patterns are employed by non-TVC
concepts, but one common feature to all is the creation of a
toroidal flow reversal that recirculates and entrains a portion of
the hot combustion products to mix with the incoming air and fuel
to stabilize the flame. Although these designs have long been used
in many practical combustion devices, there are limitations,
especially for lean premixed applications. Flame stability is
achieved though the use of recirculation zones to provide a
continuous ignition source which facilitates the mixing of hot
combustion products with the incoming fuel and air mixture. Swirl
vanes (figure 1.2) are commonly employed to establish the
recirculation zones. This method creates a low
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velocity zone of sufficient residence time and turbulence levels
such that the combustion process becomes self-sustaining. The
challenge, however, is selection of a flame stabilizer that ensures
that both performance (emissions, combustor acoustic and pattern
factor) and cost goals are met.
Fig 1.2: Swirl vanes
In contrast to conventional combustion systems which rely on
swirl stabilization, the TVC employs cavities (figure 1.3) to
stabilize the flame and grows from the wealth of literature on
cavity flows. Much of the historical effort examines the flow field
dynamics established by the cavities, as demonstrated in aircraft
wheel wells, bomb bay doors and other external cavity structures.
Cavities have also been studied as a means of cooling and reducing
drag on projectiles and for scramjets and waste incineration.
Fig 1.3: Flow through TVC
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1.2.2 LOW EMISSIONS:
With increasing concern on environmental issues, control of
pollutant emissions and extension of service life of combustor is a
dire need. Low emissions can be achieved by controlling fuel-air
mixing process and temperature of each combustion zone. TVC adopts
the staged combustion technology. Structurally, it can be divided
into two zones: pilot combustion zone and primary combustion zone.
Pilot combustion zone stabilizes flame and primary zone produces
thrust.
Due to the efficient mixing provided by recirculation zones, TVC
allows lean blowout limit that ensures low NOx and CO emissions.
(figure 1.4)
Fig1.4: NOx v/s combustor pressure for different combustors
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1.2.3 FUEL FLEXIBILITY: TVC burns a wide variety of medium and
low-BTU gases including hydrogen-rich gasified coal, biomass
products, and landfill gas. These quailities in turn lead to a
higher combustion efficiency than a conventional combustor. (figure
1.5)
Fig1.5: Combustor efficiency v/s FAR for conventional combustor
and TVC
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1.3 THE ACTUAL MECHANISM OF TVC:
The actual stabilization mechanism facilitated by the TVC is
relatively simple. A conventional bluff or fore body is located
upstream of a smaller bluff body - commonly referred to as an aft
body. The flow issuing from around the first bluff body separates
as normal, but instead of developing shear layer instabilities
which in most circumstances is the prime mechanism for initiating
blowout, the alternating array of vortices are conveniently trapped
or locked between the two bodies.
In a TVC concept, the re-circulation of hot products into the
main fuel-air mixture is accomplished by incorporating two critical
features. First, a stable recirculation zone must be generated
adjacent to the main fuel-air flow. If the vortex region, or cavity
region, is designed properly, the vortex will be stable and no
vortex shedding will occur. This stable vortex is generally used as
a source of heat, or hot products of combustion.
The second critical design feature involves transporting and
mixing the heat from the vortex, or cavity, region into the main
flow. This is accomplished by using wake regions generated by
bodies, or struts, immersed in the main flow. This approach ignites
the incoming fuel-air mixture by lateral mixing, instead of a
back-mixing process. By using geometric features to ignite the
incoming fuel-air mixture, instead of pure aerodynamic features,
the TVC concept has the potential to be less sensitive to
instabilities and process upsets. This is particularly important
near the lean flame extinction limit, where small perturbations in
the flow can lead to flame extinction.
The very stable yet more energetic primary/core flame zone is
now very resistant to external flow field perturbations, yielding
extended lean and rich blowout limits relative to its simple bluff
body counterpart. It has been researched that TVC configuration can
withstand velocities near Mach 1. This unique characteristic of the
TVC technology provides a fluid dynamic mechanism that can overcome
the high flame speed of hydrogen-rich syngas and potentially allow
IGCC gas turbines to operate the combustor in premixed mode. This
system configuration also has greater flame holding surface area
and hence will facilitate the more compact primary/core flame zone
essential to promoting high combustion efficiency and reduced CO
emissions.
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Chp II. LITERATURE REVIEW
Little and Whipkey[5] studied drag and flow characteristics of
afterbodies created by
placing disks on spindle. They showed that if the disk is not of
a proper height, backflow occurs. The cavity thus formed should
also be of a size appropriate to capture the vortex and create a
recirculation zone. If the disc is placed further downstream, too
much air is trapped and the recirculation zone is not properly
centered inside the cavity. The condition of a stable recirculation
zone corresponds to a condition of minimum drag.
Hsu and Roquemore[4] revealed the importance of mass injection
directly into the
cavity. Without the injection, the cavity is very lazy and
poorly organized. The required amount of air ranges from 5% to 10%
of the mainstream air. The flow interactions in the cavity are in
general dependent on the cavity length. When the cavity length is
0.59m of the forebody diameter, a stable vortex is trapped in the
cavity. Peak combustion efficiency can be improved by increasing
primary airflow rate and when a second trapped vortex is added to
the combustor via a second afterbody downstream to the first
afterbody.
Viswanath Katta and Roquemore[7] studied how mass injection
affects dynamic characteristics of flow inside the cavity and
surrounding it. Unsteady flow can cause instabilities and by
constructing a cavity with a proper size, such instabilities can be
reduced. Mass injection increases the optimum cavity size and fuel
injected into this cavity increases efficiency by allowing proper
mixing and longer residence time. If the cavity size is
non-optimum, mass injection causes instabilities. Thus, with mass
injection we obtain an optimum sized cavity that can capture a
vortex and stabilize flames
P.Selvaganesh and S.Vengadesan[9] studied the characteristics of
the trapped vortex
combustor under cold flow (non-reacting) condition using k-
family of turbulence models. Time averaged quantities from these
calculations were obtained by averaging the data over a period of
0.46s (twenty flow through time). Calculations were made by five
different two equation models. Figure 2.1 shows the variation in
change in drag coefficient (CD), which is obtained by subtracting
the base drag coefficient (without afterbody) from that obtained
with the afterbody for different cavity lengths. From the
entrainment characteristics, it is inferred that the primary air
needs to be
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injected to accommodate the decrease in oxidizer inside the
cavity to obtain better performance from the TVC.
Fig2.1: Change in drag coefficient resulting from the addition
of afterbody to forebody- spindle combination obtained for
different cavity lengths.
S.Vengadesan and C.Sony[8] enhanced vortex stability in trapped
vortex combustor. For that they optimized the size for second
cavity. For this, they have considered one diameter ratio D2/D0
=0.585[10] and varied the position of second cavity. For every case
they determined CD. CD is obtained by subtracting the base drag
coefficient without second cavity (i.e. with single cavity alone)
and is plotted in Figure 2.2. The drag coefficient decreases
initially with increase in separation distance (G/D0) between the
first afterbody and the second afterbody, reaches minimum and then
increases for large separation. The results show that, the drag
reduction is high when the second body is placed at 0.585D0
downstream of the first afterbody. They found that due to presence
of second cavity second vortex stabilizes the primary vortex.
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Fig2.2: Change in drag coefficient due to the second cavity
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CHP iii. INTRODUCTION TO CFD:
Fluid (gas and liquid) ows are governed by partial dierential
equations which represent conservation laws for the mass, momentum,
and energy. Computational Fluid Dynamics (CFD) is the Art of
replacing such PDE systems by a set of algebraic equations that can
be solved using digital computers. CFD provides a qualitative (and
sometimes even quantitative) prediction of uid ows by means of
Mathematical modeling (partial dierential equations) Numerical
methods (discretization and solution techniques) Software tools
(solvers, pre- and post processing utilities) CFD enables
scientists and engineers to perform numerical experiments (i.e.
computer simulations) in a virtual ow laboratory (figure 3.1)
Fig3.1: Difference between real experiment and CFD simulation.
The general process for performing a CFD analysis is outlined below
so as to provide a reference for understanding the various aspects
of a CFD simulation.
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3.1 CFD ANALYSIS PROCESS: 1. Problem statement Information about
the ow 2. Mathematical model IBVP = PDE + IC + BC 3. Mesh
generation Nodes/cells, time instants 4. Space discretization
Coupled ODE/DAE systems 5. Time discretization Algebraic system
Ax=b 6. Iterative solver Discrete function values 7. CFD software
Implementation, debugging 8. Simulation run Parameters, stopping
criteria 9. Post processing Visualization, analysis of data 10.
Verication Model validation / adjustment 3.1.1. PROBLEM
STATEMENT
The first step of the analysis process is to formulate the flow
problem by seeking answers to the following questions:
o What is the objective of the analysis? o What is the easiest
way to obtain those objectives? o Whatgeometry should be included?
o What are the freestream and/or operating conditions? o
Whatdimensionality of the spatial model is required? (1D, quasi-1D,
2D,
axisymmetric, 3D) o What should the flow domain look like? o
What temporal modeling is appropriate? (Steady or Unsteady) o What
is the nature of the viscous flow? (Inviscid, Laminar, Turbulent) o
How should the gas be modeled?
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3.1.2. MATHEMATICAL MODEL
Choose a suitable flow model (viewpoint) and reference
frame.
Identify the forces which cause and inuence the uid motion.
Dene the computational domain in which to solve the problem.
Formulate conservation laws for the mass, momentum, and
energy.
Simplify the governing equations to reduce the computational
eort.
Add constitutive relations and specify initial/boundary
conditions. 3.1.3. DISCRETIZATION PROCESS
The PDE system is transformed into a set of algebraic equations
1.Mesh generation (decomposition into cells/elements)
Structured or unstructured, triangular or quadrilateral? CAD
tools + grid generators (Delaunay, advancing front) Mesh size,
adaptive refinement in interesting flow regions
2.Space discretization (approximation of spatial
derivatives)
Finite dierences/volumes/elements High- vs. low-order
approximations
3.Time discretization (approximation of temporal
derivatives)
Explicit vs. implicit schemes, stability constraints Local time
stepping, adaptive time step control.
3.1.4. ITERATIVE STRATEGY
The strategy for performing the simulation involves determining
such things as the use of space-marching or time-marching, the
choice of turbulence or chemistry model, and the choice of
algorithms.
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3.1.5. SIMULATIONS
The simulation is performed with various possible with options
for interactive or batch processing and distributed processing.
3.1.6. POST PROCESSING AND ANALYSIS
Postprocessing of the simulation results is performed in order
toextract the desired information from the computed flow field
Calculation of derived quantities (stream function,
vorticity)
Calculation of integral parameters (lift, drag, total mass)
Visualization (representation of numbers as images)
Systematic data analysis by means of statistical tools
Verification and validation of the CFD mode
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CHP IV. METHODOLOGY AND THEORY
4.1 SELECTION OF GEOMETRY AND PLACEMENT: The geometry chosen for
the study is the same used by Viswanath [10]for experimental
studies.
4.1.1. SINGLE CAVITY TVC GEOMETRY:
The single cavity TVC geometry consists of 70mm diameter flat
cylindrical forebody (D0) surrounded by a cylinder of inner
diameter 80mm (outer body). The fore-body spans a length of
30mm.The fore-body and the single after- bodies are connected
through a 9mm diameter cylindrical pipe. (figure 4.1)
Fig 4.1: Single Cavity TVC Geometry
4.1.2. DOUBLE CAVITY TVC GEOMETRY:
S. Vengadesan and C. Sony[8]optimized the second cavity size in
order to obtain a stable vortex inside the cavity. As seen in the
figure, the drag coefficient decreases initially with increase in
separation distance (G/D0) between the first afterbody and the
second afterbody, reaches minimum and then increases for large
separation. The results show that, the drag reduction is high when
the second body is placed at 0585D0 downstream of the first
afterbody. (figure 2.2)
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The two-cavity TVC geometry consists of 70mm diameter flat
cylindrical forebody (D0) surrounded by a cylinder of inner
diameter 80mm (outer body). The fore-body spans a length of 30mm.
It is concluded from Selvaganesh and Vengadesanthat a cavity with
aspect ratio of 06 is optimum with regard to minimum change in mean
drag coefficient (CD) we maintain the same aspect ratio. The
coefficient of drag CD = D/(05U20 Ap), where is density of air, U0
is the free stream velocity, and Ap is the projected area. Hence at
40mm downstream of the forebody, first afterbody of diameter (D1)
508mm and width 20mm is placed and a second afterbody (D2) of
4095mm diameter and width 20mm is placed 9975mm (G) downstream from
the first afterbody. Forebody and the two afterbodies are connected
through a 9mm diameter cylindrical pipe. (figure 4.2)
Fig 4.2: Double Cavity TVC Geometry
4.2 MESHING PROCESS:
Meshing of the single cavity as well as double cavity is done in
ICEM CFD. 2D Monoblock structure mesh is used for it.
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Fig 4.3: Forebody mesh
Fig 4.4: Single Cavity mesh
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Fig 4.5: Double Cavity mesh
4.3. SETUP IN FLUENT:
Unsteady Fluent code for incompressible turbulent flow through
the above explained TVC geometry is considered. The steps followed
in FLUENT are:
Models Material Boundary condition Solution methods Time step
calculation Convergence criteria
4.3.1. MODELS DNS resolves the flow to sufficiently fine detail
to capture the motion of the smallest eddies and the briefest
time-scales. For most practical combustion systems with high
Reynolds number, this is extremely computationally expensive.
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Fig 4.6: Block diagram of Eddy Dissipative model
LES uses the supposition that most of the flow energy is
contained in the largest eddies, and only flow features on the
scale of the largest eddies are calculated.
A model is used to account for the stresses generated in the
flow by eddies which are smaller than this scale. RANS calculate
mean flow parameters. In the averaging process used to generate
mean flow equations, information regarding turbulent fluctuations
is lost. This loss of information is manifested by the second
moments of fluctuating velocity or Reynolds stresses that appear
explicitly in the mean flow equations. The task of a turbulence
model is to find an adequate numerical representation of these
Reynolds stresses. DNS and LES are valuable for providing flow
details that are difficult or impossible to measure experimentally.
But it is more preferable to work with mean quantities.
In the RANS models, These are turbulence models in which the
Reynolds stresses, as obtained from a Reynolds averaging of the
Navier-Stokes equations, are modelled by a linear constitutive
relationship with the mean flow straining field,we have one
equation and two equation models. One-equation turbulence models
solve one turbulent transport equation, usually the turbulent
kinetic energy. In two equation models, most often one of the
transported variables is the turbulent kinetic energy, . The second
transported variable varies depending on what type of two-equation
model it is. Common choices are the turbulent dissipation, , or the
specific dissipation, . The second variable can be thought of as
the variable that determines the scale of the turbulence. The K-
Epsilon model performs poorly when it comes to pressure gradients.
Hence, our next choice was K-Omega models.
Eddy dissipative model
SST KO Two equation model
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K- MODELS: In these models, the second variable is specific
dissipation that determines scale of turbulence.
These models have gained popularity because Can be integrated to
the wall without using any damping functions Accurate and robust
for a wide range of boundary layer flows with pressure
gradient.
STANDARD K-:
Pros:
More accurate near wall treatment, superior performance for low
Re and predicts transition.
For transitional, shear and compressible flows. High numerical
stability.
Cons:
Very severe pressure gradient is under predicted. It is
predicted to be excessive and early separation of flow.
Mesh resolution needed near wall.
SST K- MODEL:
Pros:
STD K-for near walls region and STD K- for regions away from
walls. Eddy viscosity is modified to account for transport effects
of turbulent shear stress. Highly accurate boundary layer
simulations and for high pressure gradients. Highly accurate for
separated flow prediction.
Cons:
Dependency on distance from wall is high so resolution near wall
is required.
Based on all these parameters, we selected our model as SST K-
model.
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4.3.2. MATERIAL
The material selected is incompressible ideal gas. Viscosity is
calculated by Sutherlands three-coefficient method.
4.3.3. BOUNDARY CONDITIONS
Different flat velocity profiles of 40ms1, 50ms1, 60ms1,
70ms1are applied at the inlet. The Reynolds number based on
hydraulic diameter (ReHd) for the incoming
velocity and the forebody diameter (D0) is 1916 105.
The turbulent intensity (I) of 45% based on the formula I = 016
(ReHd)(-1/8)for fully developed pipe flow and a length scale of
inlet duct height (h = 5mm) are used as the boundary condition at
the inlet.
Neumann condition (/x = 0) is prescribed at the outlet and axis
boundary condition along the centre line is applied. The usual no
slip is applied at the walls along the forebody-spindle-afterbody
combination and the outer tube.
4.3.4. SOLUTION METHODS
The QUICK (Quadratic Upwind Interpolation for Convective
Kinematics) scheme is used for the momentum equations and second
order upwind differencing scheme for turbulent quantities. SIMPLE
(Semi Implicit Method for Pressure Linked Equation) algorithm is
used for coupling pressure and velocity terms. Second order
implicit scheme is used for time advancement. PRESTO spatial
discretization scheme is used for pressure.
4.3.5. TIME STEP CALCULATION
For time step calculation, for the usual case,
A smaller time step will typically improve convergence.
4.3.6. CONVERGENCE CRITERIA
When the Drag coefficient attains a constant value with respect
to flow time, it proves the termination of FLUENT program.
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Chp V. RESULTS AND DISCUSSIONS
In this section, results from Fluent like total pressure plot,
vorticity magnitude and streamline plot at different mass flow
rates will be shown and discussed. The cold flow inside the
combustor can be described by the total pressure, streamlines and
vorticity magnitude throughout the flow field. 5.1. TOTAL PRESSURE
PLOT: One of the main objectives of any combustor is to maintain
the minimum pressure drop. The pressure drop is strongly influenced
by the fluid dynamics and geometry of the cavity (aspect ratio,
blockage ratio, and length). At 40 m/s
Fig 5.1: Total pressure plot of Single Cavity at 40m/s
Fig 5.2: Total pressure plot of Double Cavity at 40m/s
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At 50m/s
Fig 5.3: Total pressure plot for Single Cavity at 50m/s
Fig 5.4: Total pressure plot for Double Cavity at 50m/s
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At 60m/s
Fig 5.5: Total pressure plot for Single Cavity at 60m/s
Fig 5.6: Total pressure plot for Double Cavity at 60m/s
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At 70m/s
Fig 5.7: Total pressure plot for Single Cavity at 70m/s
Fig 5.8: Total pressure plot for Double Cavity at 70m/s From the
above plot of total pressure at different velocity, it is inferred
that pressure drop increases in both cases, single cavity TVC and
double cavity TVC as the velocity increases. The increase in
pressure drop is due to the increase in velocity (i.e. increase in
the kinetic energy of gases). The total pressure drop in single
cavity TVC will be more compare to the double cavity TVC i.e. the
pressure recovery of double cavity TVC is higher than single cavity
TVC. One can observe from the pressure plot that the low pressure
region inside the cavity is due to flow separation which results in
vortex formation.
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5.2. FLOW PATTERN: For the cold flow inside combustor, flow
pattern is demonstrated by the streamline plots of the single
cavity TVC and double cavity TVC. At 40m/s
Fig 5.9:Streamline plot for Single Cavity at 40 m/s
Fig 5.10: Streamline plot for Double Cavity at 40 m/s From the
streamline plot of single cavity and double cavity TVC, the
recirculation region inside the cavity is small. So, the flow
experiences less drag. Recirculation region of double cavity is
less than that of single cavity TVC. Due to this decrement, the
flow experiences drag, which is inturn less than that of single
cavity.
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At 50 m/s:
Fig 5.11: Streamline plot for Double Cavity at 50 m/s
Fig 5.12: Streamline plot for Single Cavity at 50 m/s
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At 60 m/s:
Fig 5.13: Streamline plot for Single Cavity at 60 m/s
Fig 5.14: Streamline plot for Double Cavity at 60 m/s
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At 70 m/s:
Fig 5.15: Streamline plot for Single Cavity at 70 m/s
Fig 5.16: Streamline plot for Double Cavity at 70 m/s From the
streamline plot at different velocities, one can observe that
recirculation region increases as the flow velocity increases. So,
in both single and double cavity TVC, drag also increases as flow
velocity increases. 5.3. VORTICITY PLOTS: In cold flow inside the
combustor, the mixing parameter is dependent on the strength of the
vorticity. So, as vorticity increases, circulation strength also
increases. Hence, we get better mixing in TVC.
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At 40 m/s
Fig 5.17: Vorticity plot for Single Cavity at 40 m/s
Fig 5.18: Vorticity plot for Double Cavity at 40 m/s
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In the two-cavity TVC the overall circulation is predominant
which leads to better mixing, when compared with the single cavity.
Mean vorticity magnitude were collected at different axial
locations. For a two-cavity TVC the vorticity magnitude at x = 0m
is 2017.42s1, x = -003m is 3171.07s1 and at x = -007m is 10413.3s1
and for a single cavity the vorticity magnitude at x = -004m is
10408.9s1 and at x = 0.01 m is 1001.4s1. From these values one can
say that the vorticity magnitude is high in case of the two-cavity
TVC. In Fig 5.17-5.18, we notice that primary vortex is being shed
in the single cavity and the vortex is trapped inside double
cavity. From this it can be concluded that the vortex stability can
been achieved when a second afterbody is placed.
At 50 m/s
Fig 5.19: Vorticity plot for Single Cavity at 50m/s
Fig 5.20: Vorticity plot for Double Cavity at 50m/s
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Mean vorticity magnitude were collected at different axial
locations for the case of at 50m/s. For a two-cavity TVC the
vorticity magnitude at x = 0m is 2555.61s1, x = -003m is 3926.9s1
and at x = -007m is 11730.3s1 and for a single cavity the vorticity
magnitude at x = -004m is 12879.2s1 and at x = 0.01 m is 1243.26s1.
At 60 m/s
Fig 5.21: Vorticity plot for Single Cavity at 60m/s
Fig 5.22: Vorticity plot for Double Cavity at 60m/s
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Mean vorticity magnitude were collected at different axial
locations for the case of at 60m/s. For a two-cavity TVC the
vorticity magnitude at x = 0m is 3092.6s1, x = -003m is 4679.78s1
and at x = -007m is 13981s1 and for a single cavity the vorticity
magnitude at x = -004m is 15349.5s1 and at x = 0.01 m is 1485.12s1.
At 70 m/s
Fig 5.23: Vorticity plot for Single Cavity at 70m/s
Fig 5.24: Vorticity plot for Double Cavity at 70m/s
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Mean vorticity magnitude were collected at different axial
locations for the case of at 70m/s. For a two-cavity TVC the
vorticity magnitude at x = 0m is 3619.19s1, x = -003m is 5435.15s1
and at x = -007m is 16233.6s1 and for a single cavity the vorticity
magnitude at x = -004m is 17794.8s1 and at x = 0.01 m is 1725.16s1.
From the vorticity plot of single cavity and double cavity TVC, as
velocity increases rate of rotation (circulation) also increases.
So, it shows that mixing is still better. Variation of velocity
does not affect the performance of TVC. 5.4. VERIFICATION AND
VALIDATION:
As discussed in the Ch-3 CFD simulation result needs
verification and validation of the result. For validation of our
result we used grid independence study. 5.4.1 VALIDATION: In the
computational study there is different type of error encounter in
solution i.e round off error, discretization error etc. so that we
have to validate our result whether it is dependent on grid or not.
Grid Independence Study: FORE BODY: No of cell Node spacing Cd
20733 0.50 0.366067 26268 0.44 0.364152 32242 0.40 0.362612 39064
0.36 0.360910
Table 5.1- Grid Independence study of Fore Body
SINGLE CAVITY:
No of cell Node spacing Cd 17777 0.50 0.31136 22004 0.44 0.32161
26757 0.40 0.312223 31526 0.36 0.311930
Table 5.2- Grid Independence study of Single Cavity
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DOUBLE CAVITY:
No of cell Node spacing Cd 15557 0.50 0.301740 19753 0.44
0.301793 24323 0.40 0.301830 29994 0.36 0.301584
Table 5.3 Grid Independence Study of Double Cavity From the
above grid independence study we can say that there is negligible
difference in the result. All the result is nearer to node spacing
0.5. So we can take optimum grid spacing is 0.5.
5.4.2 VERIFICATION:
For the verification of our CFD result we compare our result
with the result, which is obtained by the S.Vengadesan and C.Sony
for the cold flow through single cavity TVC and double cavity TVC
at 40m/s velocity profile. This is mentioned below:
DOUBLE CAVITY:
Cd-single cavity Cd-double cavity CD 0.31136 0.301740 0.00962
0.32161 0.301793 0.01036 0.312223 0.301830 0.01039 0.311930
0.301584 0.01034
Table 5.4 Reduction in Drag Coefficient
And the result from the S.Vengdesan and C.Sony[8] Avg.CD= 0.0103
which is almost equal to our result.
5.5. RESULT SUMMARY:
The cold flow through the single cavity TVC and double cavity
TVC with change in operating condition i.e. varying mass flow rates
how the performance of single cavity TVC and double cavity TVC is
affected is shown by the graph of Cd vs. varying mass flow rate for
both Single and Double Cavity TVC (figure 5.26)
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From the Fig. 5.26 one can say that as the velocity increases
coefficient of drag increases. But there is still less drag in the
double cavity TVC compare to the single cavity TVC.
Fig 5.25: Variation in coefficient of drag (CD) with change in
velocity
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Chp VI. Conclusion
6.1 CONCLUSION: In the present work, numerical investigation on
single cavity TVC and double cavity TVC for comparison between them
and also how the performance of the single cavity TVC and double
cavity TVC affected by varying mass flow rate. At present detailed
cold flow analysis is carried out. From our present work we get
following conclusion:
Total pressure drop and observation flow patterns indicate that
we get stable vortex in double cavity TVC as compare to the single
cavity TVC, due to second cavity second vortex stabilize the
primary vortex. Vortex stability has been achieved by the
circulation of fluid in both cavities.
From total pressure plot we can say that total pressure drop
(pressure
recovery is high) in double cavity TVC is less as compare to the
single cavity TVC.
Residence Time is high in double cavity TVC as compare to
single
cavity TVC. That leads to a better mixing of fuel and air in
combustor.
From the CFD analysis, we can say that drag experienced by
double cavity TVC is less as compare to single cavity TVC.
From the vorticity plot, we can say that magnitude of vorticity
is high in
double cavity TVC as compare to single cavity TVC. That will
lead to a better mixing when compare to single cavity.
As the mass flow rate through the single cavity TVC and double
cavity TVC
increases drag experienced by the both cavities also increased.
But in case of double cavity TVC percentage of coefficient of drag
decreases as compared to the single cavity TVC as the mass flow
rate is increased.
Both single cavity TVC and Double cavity TVC are less sensitive
to the
engine operating condition.
By increasing the mass flow rate combustor stability is
increased.
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6.2 CONTRIBUTION: The contributions resulting from this research
can be highlighted as follows:
Normally thrust requirement of aircraft engine is vary at
different flight regime. At the time of take-off aircraft engine
needs more mass flow rate to produce take-off thrust and at
cruising aircraft needs less mass flow rate to sustain in those
flight regime. So that at varying mass flow rate condition how the
performance of single cavity TVC and double cavity TVC affected is
studied in this research. And data which we get at varying mass
flow rate condition also investigated by us for their feasibility
in the modern transport, military as well as commercial aircraft
because of their good combustor efficiency, low emission of
pollutants, flame stability, and low cost compare to swirl based
combustor.
Improvement in combustion stability.
6.3 FUTURE SCOPE: There is much that can be done to improve the
Trapped Vortex Combustor concept.Possible future work for this
research can be stated as follow: Reactive flow analysis of single
cavity TVC and double cavity TVC with
varying mass flow rate. Cold flow analysis of double cavity TVC
at different operating condition.
Cold flow analysis of TVC with varying mass flow rate and adding
swirl.
Reactive flow analysis of TVC with varying mass flow rate and
adding
swirl.
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Appendix-1 Fluent Setup Step 1:File Read Case. Navigate to the
working directory and select the .msh file Step 2:Grid Check. Any
errors in the grid would be reported at this time. Step 3:Grid Info
Size. Step 4:Grid Scale. We must define grid units.
Fig A1: General window Step 5:Grid Display. We can look at
specific parts of the grid by choosing the boundariesyou wish to
view under Surfaces. Step 6:Define Models Solver. We tick Unsteady
under Time, 2nd-Order Implicit under Unsteady Formulation and Least
square cell Based under Gradient Options.
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Fig A2: Turbulence Model Window Step 7:Define Models Energy.
Step 8:Define Models Viscous. We must use k- SST (Shear Stress
Transport). Step 9:Define Materials. Under Properties, we pick
incompressible ideal-gas.. We select sutherland into Viscosity.
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Fig A3: Material Window Step 10:Define Operating Conditions.
Operating Pressure equals 101325 Pa. Step 11:Define Boundary
Conditions.
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Fig A4: Inlet Condition
Fig A5: Outlet Condition Step 12: 12. Solve Controls Solution.
Under Discretization, Second Order.
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Fig A6: Solution Method Step 13:Solve Initialize Initialize.
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Fig A7: Solution Initialization Step 14:Solve Monitors Residual.
Under Options, select Print and Plot. Step 15:Solve Monitors Force.
Under Options, select Print and Plot. Under coefficient, select
Drag (Force Vector [1,0,0]) . step 16:Report Reference Values. Step
17:Solve Case Check. Any errors in the previous steps will be
reported at this time.
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Fig A8: Check And Run Step 18:File Write Case Step 19:Solve
Iterate. Time step size: it is the time that happens between
consecutives calculus. Number of Time Steps: it is the amount of
times that the software makes the same calculus, decreasing mistake
made. Step 20:File Write Case & Data. To save the file.
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Appendix-2 References
1. FLUENT user guide and Manual 2014.
2. ICEMCFD user guide and Manual 2014.
3. TECHPLOT user guide and Manual 2012.
4. HSU, K.Y., GOSS, L.P. and ROQUEMORE, W.M. Characteristics of
a trapped-vortex combustor, J of Propulsion and Power, 1998,
14,(1), pp 57-65.
5. LITTLE, JR., B.H. and WHIPKEY, R.R. Locked-vortex
afterbodies, J Aircr, 1979, 16, pp 296-302. 6. NANDAKUMAR, V. and
VENGADESAN, S. Reactive flow analysis of Trapped vortex combustor
using two equation turbulence models, 2008, Master Thesis,
Department of Applied Mechanics, IIT Madras, India. 7. VISWANATH R.
KATTA AND W. M. ROQUEMORE. "Study on Trapped-Vortex
Combustor-Effect of Injection on Flow Dynamics", Journal of
Propulsion and Power, Vol. 14, No. 3 (1998), pp. 273-281 8.S.
VENGADESAN AND C. SONY. Enhanced vortex stability in trapped vortex
combustor, Department of Applied Mechanics Indian Institute of
Technology of Madras Chennai, India. 2010.
9. SELVAGANESH, P. and VENGADESAN, S. Cold flow analysis of
Trapped vortex combustor using two equation turbulence
models.Aeronaut J, 2008, 112, (1136), pp 569-580. 10. VISWANATH,
P.R. Flow management techniques for base andafterbody drag
reduction, Prog Aerospace Sci, 1996, 32, pp 79-129. 11. CFD-POST
user guide and Manual 2014. 12. An innovative combustion chamber
Architecture, www.tecc-project.eu
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