Analysis of Scramjet Combustion Chamber

S.Sridhar1, T.Dinesh 2, V.Bharath 3 ,K.Jayakumar4

1. Assistant Professor, Department of Mechanical Engineering, NSIT, Salem, India 2, 3, 4 Students, Department of Mechanical Engineering, NSIT, Salem, India

1,sridharcadcam@gmail.com 2,dineshtamizharasu@gmail.com 3,bharathramsmart@gmail.com, 4,jayakumark28598@gmail.com

ABSTRACT- Numerical simulations of the scramjet combustor by using the commercial CFD code Fluent

with the coupled implicit method with second-order accurate discretization have been obtained for the reacting

flows with the parallel fuel injection (ramp injection). The k-ε model has been used to examine supersonic flow

in a model scramjet combustor. Scramjet combustor model and it is consists of a one-sided divergent channel

with a Ramp type flame holder at the base of which hydrogen is injected. Ramp injector is re-designed to

increase the combustion efficiency. The cavity is introduced and extended to the nozzle exit. The cavity is

mainly introduced to increase the fuel and air mixture. The presence of cavity after the injector increases the re-

circulation flow inside the cavity. Due to the shape, the rate of mixing and combustion increased, it also results

in increase of pressure. The total pressure losses are minimized. The scramjet combustor model is designed and

analyzed by using GAMBIT and FLUENT software. In the analyzed result is compared with theoretical

calculations. The k-ε computations are capable of predicting mixing and combustion simulations well and good.

Index Terms—supersonic combustion ramjet (SCRAMJET), computational fluid dynamics (CFD)

I.INTRODUCTION

An airframe integrated scramjet propelled vehicle has advantages for application to several missions. In its

simplest form, such a vehicle will combine the features of quick reaction, low vulnerability to counter attack and

better propulsion efficiency. The Supersonic Combustion Ramjet (SCRAMJET) engine has been recognized as

the most promising air breathing propulsion system for the hypersonic flight (Mach number above 5).In recent

years, the research and development of scramjet engine has promoted the study of combustion in supersonic

flows. Extensive research is being carried out over the world for realizing the scramjet technology with hydrogen

fuel with significant attention focused on new generations of space launchers and global fast-reaction

reconnaissance missions. However, application for the scramjet concept using high heat sink and hydrogen fuels

offers significantly enhanced mission potential for future military tactical missiles. Scramjet being an air-

breathing engine, the performance of the missile system based on the scramjet propulsion is envisaged enhance

the payload weight and missile range. Supersonic combustion ramjet engine for an air-breathing propulsion

system has been realized and demonstrated by USA on ground and in flight. X- 43 vehicle used hydrogen fuel.

Hydrocarbon fuel scramjet engine is still under study and research. Mixing, ignition and flame holding in

combustor, ground test facilities and numerical simulation of Scramjet engine are the critical challenges in the

development of scramjet engine. In our project to plan combine Ramp and wall injectors for further increasing

mixing. Analysis process is carried for with cavity and without cavity type combustion chamber. Cavity is mainly

introduced for creating circulation. Circulation helps further mixing of air and fuel.

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Volume 5, Issue 9, September /2018

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II.LITERATURE SURVEY

Among the three critical components of the scramjet engine, the combustor presents the most

formidable problems. The complex phenomenon of supersonic combustion involves turbulent mixing, shock

interaction and heat release in supersonic flow. The flow field within the combustor of scramjet engine is very

complex and poses a considerable challenge in design and development of a supersonic combustor with an

optimized geometry. Such combustor shall promote sufficient mixing of the fuel and air so that the desired

chemical reaction and thus heat release can occur within the residence time of the fuel-air mixture. In order to

accomplish this task, it requires a clear understanding of fuel injection processes and thorough knowledge of the

processes governing supersonic mixing and combustion as well as the factors, which affects the losses within the

combustor. The designer shall keep in mind the following goals namely,

i) Good and rapid fuel air mixing.

ii) Minimization of total pressure loss.

iii) High combustion efficiency.

One of the strategies to solve the aforesaid problems of mixing is generation of axial vortices. Axial vortices

possess a better far field mixing characteristics. Also they are being propagated to a considerable distance, even

with the suppressing characteristics of the supersonic core flow. Ramp injectors are considered to be a key

feature to generate axial vortices. Figure 4 & 4A depicts some of the characteristics of Ramp injectors flow field.

The following are the characteristics of the ramp injectors. The spillage vortices (contra rotating vortices)

generated by Ramp compression.

1. Pre-compression by the Ramp face produces favorable region for injection.

2. Stagnation region near the leading edge of the Ramp injector improves ignition.

Generation of acoustic oscillations is also considered to be a better candidate to achieve better mixing.

Unsteady shear layers generate acoustic oscillations. Wall mounted cavities generates these oscillations to aid the

mixing enhancement. The Cavity parameters in figure 1, Cavities are characterized by their L/D ratio. There are

three regimes of cavity behavior, categorized by the shear layer separation and its reattachment. For cavities of

L/d less than 1, the shear layer reattaches way past the trailing edge of the cavity it generates transverse

oscillations. These cavities are called as „Open Cavities‟. This type of oscillations aid in penetration of fuel For

L/D more than 2, the separated shear layer attaches to the bottom wall of the cavity, it generates longitudinal

oscillations, which aid in flame holding characteristics.

Figure: 1 Combination of ramp and wall injector.

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III. COMBUSTION CHAMBER DESIGN

We have referred more than ten journals and gathered information is about the injectors, combustion

chamber design and cavity creating. We compared all these journals and we made a model of combustion

chamber with the combination of ramp and wall injectors. Which is placed before the cavity? We design the

model in CATIA software Mesh generation is performed in a Fluent pre-processing program called Gambit.

The current model is combining of Ramp and wall injector combustion chamber type as shown in figure. The

boundary conditions are such that, the air inlet and fuel inlet surfaces are defined as pressure inlets and the

outlet is defined as pressure outlet. In this particular model the walls of the combustor duct do not have

thicknesses. The domain is completely contained by the combustor itself; therefore there is actually no heat

transfer through the walls of the combustor.

1) Air inlet-30mm.

2) Outlet-50mm.

3) Combustion chamber length-667mm.

4) Cavity length and depth-80,20mm.

Figure: 2. with and without cavity combustion chamber design

Case-1

Figure: 3.Meshed 3D Combustion chamber with cavity model.

Case-2

Figure: 4.Meshed 3D Combustion chamber without cavity model.

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Boundary Conditions: During analysis we have taken same pressure for both fuel and air for all the models.

Pressure inlet and pressure outlet conditions were taken on the left and right boundaries respectively. Pressure

inlet condition was taken for fuel injector. The top and bottom boundaries, which signify the sidewalls of the

isolator, had symmetry conditions on them. The walls, obstacles and other materials were set to standard wall

conditions. The computations were initially carried out with various levels of refinement of mesh. The input

parameters that were for the model is shown in tabulated form,

Table: 1. Boundary conditions

Modeling details: The .msh file obtained from the GAMBIT was exported to FLUENT for subsequent

analysis. The .msh file was read using FLUENT and subsequently its grid checking was done, the grid was

checked with no error and the formation of one default interior. The following models were selected:

1. The pressure based solver,

2. Energy equation

3. Standard k-ε model,

4. RNG k-ε model,

5. Eddy-dissipation criterion in the

species transport section.

IV. ANALYSIS RESULT

Circulation in Cavity: Below figure shows circulation creation in their cavity. Color shows pressure variation

near to the cavity and inside the cavity. Pressure is less inside cavity.

Case-1

Figure: 5. Path lines colored by static pressure

Input parameters Air Fuel

Mach No 3.12 1.5

Temperature 1000k 300k

Pressure 80325Pa 80325Pa

Mass fraction of

O2

0.213 0

Mass fraction of

N2

0.767 0

Mass fraction of

H2

0 1

Mass fraction of

H2O

0.02 0

Inlet length 0.04m 0.002m

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Case-2

Figure: 6. Path lines colored by static pressure

Cavity type combustion chamber:

1) Static Temperature: The static temperature was taken as an indication of combustion efficiency of the fuel

(hydrogen). Higher combustion efficiency means a greater percentage of the injected fuel undergoes combustion

resulting in a higher static temperature at the combustor exit. The maximum temperature is 3443K.

Figure: 7 Static temperature

Figure: 8 X, Y plot of Static temperature

2).Static pressure: Static pressure is the pressure that is exerted by a fluid. Specifically, it is the pressure

measured when the fluid is still, or at rest. The value of static pressure at the outlet is nearly 1.03e+04 Pa. but

the entrance given static pressure is 80325 pa, the pressure is decreased due to circulation creation in cavity area

Figure: 9 Static pressure.

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Figure: 10 .X, Y plot of Static pressure.

3) Total pressure: Figure 11 is giving the details about total pressure variation; it is clear that total pressure is

getting decreased gradually up to sudden level after that the variation is high due to shock wave creation. Figure

4.2.3 shows the variation of total pressure in the top and bottom side of the wall in the top wall total pressure

loss is more compare to bottom wall it is because of the change in area at top side.

Figure: 11 Total pressure

Figure: 12.X, Y plot of total pressure

4)Density: Plot of density distribution at air in, out shows that density increases with H2 injection and then, it

decreases gradually with mixing and combustion of air and hydrogen fuel mixture and the subsequent expansion

of the combustion products.

Figure13.Density

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Figure14.X, Y plot of Density

5).Mass Fraction of H2: The below graph shows the distribution of H2 In the air in, out of the combustor. As

can be seen, the mass fraction of hydrogen is maximum at the fuel injection port and continues to decrease

along the length of the combustor due to combustion. Thus, the graph provides evidence of combustion.

Figure: 15 Mass Fraction of H2

Figure: 16.X, Y plot of Mass Fraction of H2

6) Mass Fraction of H2O: The contour and XY Plot of water Mass fraction for the flow field downstream of

the air in, out is shown in the figure. Typically, when dealing the chemical reaction, it is important to remember

that mass is conserved, so the mass of product is same as the mass of reactance. Even though the element exists

in different the total mass of each chemical element must be same on the both side of equation

Figure: 17 Mass Fraction of H2O

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Figure: 18 X, Y plot of Mass Fraction of H2O

7) Mass Fraction of O2: The contour and XY Plot of O2 Mass fraction for the flow field downstream of the

air in, out is shown in the figure. Oxygen is decreased in every combustion reaction. Decreasing of oxygen it

shows burning rate is increased inside the combustion chamber

Figure: 19 Mass Fraction of O2

Figure: 20 X, Y plot of Mass Fraction of O2

V. RESULT COMPARISION

The table 2 shows Comparison between with and without cavity combustion chamber. Pressure is high in with

cavity type combustion chamber compare to without cavity type combustion chamber. Temperature is high in

without cavity type combustion chamber compare to with cavity type combustion chamber. Mass friction of H2,

H2Oand O2 is slightly varying in the both combustion process.

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Table: 2. Result comparison

V.CONCLUSION

In the scramjet engine the air fuel mixture is less in the combustion chamber due to the supersonic flow

which takes limited time to escape from the combustion chamber. To increase the time of air to stay in

combustion chamber we have modified the injector design with the cavity in the combustion chamber.

The reason is why the cavity is introduced to create circulation of air. So that the air fuel mixing can be

obtained maximum. The circulation reduces the pressure inside the combustion chamber. The total pressure loss

will simultaneously maximum due to the circulation. The efficiency will be maximum only when the total

pressure loss is maintained minimum. The total pressure loss is maintained by providing cavity place at a specific

angle 300.

The combustion chamber design is analyzed by using fluent software. The standard fluent conditions are

used for analysis. The K epsilon turbulence model is selected. We referred journals for the Boundary conditions.

In this we are analyzed two cases. One is with cavity type combustion chamber and another one is without

cavity type combustion chamber.

In with cavity type combustion chamber pressure is obtained 75012.32pa. The pressure is high in the

upper wall compared to bottom wall. Pressure is decreased in the cavity, so the pressure loss is more at bottom

wall compared to top wall. Temperature is obtained 3443k.Increasing of temperature it shows increasing of

burning rate.

Temperature is high at the bottom wall compared to top wall. Along the combustion chamber

temperature is increased. Mass fraction of H2is 0.9995785, Mass fraction of H2O is 0.2513, Mass fraction of

O2is 0.23.In without cavity type combustion chamber we got maximum pressure level is 73476.26pa. Pressure

loses is less in both the walls due to the no circulation inside the combustion chamber. Temperature is obtained

4357k. In bottom wall temperature is high compared to top wall. The maximum temperature shows the

increased burning rate.

Parameters

Inlet Conditions Combustion Chamber

With Cavity Without

Cavity

Pressure 80325pa 75012.32pa 73476.26pa

Total Pressure 80325pa 80857.75pa 80408.41pa

Temperature 1000k 3443k 4357k

Density( kg/m3) 1.225

0.35146 0.351

Mass Fraction of

H2

1

0.9995785

0.9999699

Mass Fraction of

H2O

0.02

0.2513 0.25155

Mass Fraction of

O2

0.213

0.23

0.23

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Compare to both the cases, pressure is high in cavity type combustion chamber. Pressure range is

75012.32Pa. Temperature range is high in cavity type combustion chamber compare to without cavity type

combustion chamber. Maximum Temperature is 4357k. The mass fraction of H2, H2O and O2 is slightly

varying.

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