Trapped vortex combustor

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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)


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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 5.9: Streamline plot for Single Cavity at 40 m/s

Fig 5.10: Streamline plot for Double Cavity 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

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)


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 afterbodies 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



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At 60m/s



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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:



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At 60 m/s:



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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



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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



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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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