Week 12- Major Project (CFD Analysis on Combustion in a SCRAMJET engine)

In this project, I will be calculcating the most efficient slot-fuel injector and cavity-flame holder for a scramjet in ANSYS Fluent.

A scramjet engine is an improvement over the traditional ramjet engine as it efficiently operates at hypersonic speeds and allows supersonic combustion. Thus it is known as Supersonic Combustion Ramjet or Scramjet. The engine uses the velocity of supersonic flow to compress the incoming air.

This is the schematic diagram:

A fuel slot is a small tube attached on the tube of the main engine, which serves as the inlet for fuel.

The angle visible is called the fuel-injector angle, this angle will be varied through the simulation and the results of the angle will be seen.

A cavity is an extension from the tube of the main engine, and serves as a side-stop for combustion so that the main airflow isn't affected. A single cavity is as seen below:

The angle visible is called the angle of cavity. This angle will be varied throughout this simulation, and its effects will be seen.

The fuel used in this simulation is Hydrogen and the combustion reaction as follows:

`2H_2 (g) + O_2 (g) → 2H_2O (g) + en ergy`

Combustion Efficiency

It is defined as the ratio of burnt fuel to the total fuel for an engine.

In this case, it case the efficiency can be said as: `eta_(combustion) = (m_(burnt H^2))/(m_(i nl et H^2))`

We know, `m_(burnt H^2)= m_(i nl et lH^2) - m_(unburnt H^2)`

`eta_(combustion) = ( m_(i nl et H^2) - m_(unburnt H^2))/(m_(t otal H^2))`

`eta_(combustion) = 1 -( m_(unburnt H^2))/(m_(i nl et H^2))`

We can say, `m_(unburnt H^2) = mf r a c_(H^2,outl et) xx mf_(outl et)`

Therefore, `eta_(combustion) =1 -( mf r a c_(H^2,outl et) xx mf_(outl et))/(mf_(i nl e t)`

The project is divided into three cases, each case is further divided into a number of sub-cases. In each case, the Combustion Efficiency will be calculated.

Case 1: The fuel-injector angle is fixed at 90 degrees, while the angle of cavity is set to 30, 45 and 60 degrees.

Case 2: The angle of cavity is set to 60 degrees, while the fuel-injector angle is set to 20, 30, 45, 60 and 90 degrees.

Case 3: The fuel-injector is placed inside the cavity, for different angles of cavity and fuel-injector angles.

Since finding the combustion efficiency for 15 cases is infeasable and the goal for the project is to find the most efficient configuration for the engine. The angles for case 3 will be chosen on the basis of the results from Case 1 and Case 2.

Geometry

The geometry is a sketch created in SpaceClaim with the appropriate angles of cavity and fuel-injector, this geometry is converted into a surface using the pull tool. Named selections are defined for all the boundaries. This is the final geometry for a case:

And its named selections,

Meshing

The default quadrilateral mesh is used for all simulations. Capture Proximity is turned on to ensure the region near the fuel injector is more refined.

This is the Mesh before exporting to Fluent:

Mesh Metrics

As quality of most elements is greater than 0.9, this mesh can be said to be of satisfactory quality.

The mesh roughly contains 16324 nodes and 15901 elements, and this number will vary for each case.

Simulation Setup

General

The simulations used a pressure based, 2D, steady state solver.

Materials

Air and Hydrogen gas (fluid) were used for this simulation.

The properties were the default ones as shown below,

Hydrogen,

Turbulence Model

While doing literature review for this project, the two most commonly used models were: k-omega SST and k-epsilon. In the end, the model chosed was k-omega SST due to its flexibility in capturing boundary layers for any y+/velocity. [1]

Species

This introduces 2 more materials, they are: `O_2` and `H_2 O`

Boundary Conditions

inlet-air - velocity-inlet, 1040 `m//s` velocity, 600K temperature, and `O_2` with a mole fraction of 0.21 (to simulate atmospheric air).

inlet-fuel - velocity-inlet, 200`m//s` velocity, 500 bar pressure (`5 xx 10^7 Pa`), and `H_2 O` with a mole fraction of 1 (pure hydrogen).

outlet - pressure-outlet, 0Pa.

walls-bottom, walls-top - stationary wall.

Boundary Conditions were decided after extensive literature review and were chosen from Reference [1]

Solution Methods

Reports

Four reports were generated for each iteration, they are:

outlet-mass-flow - this report generates the mass flow rate at the outlet.

mass-flow-rate-inlet-fuel - this report generates the mass flow rate of the fuel at the inlet.

mf-h2-outlet - this generates the mass fraction of fuel at the outlet.

combustion-efficiency - this generates the combustion efficiency at the outlet

User Defined Functions - Custom Field Functions

One function was used to calculate the combustion efficiency using the following formula:

`eta_(combustion) =1 -( mf r a c_(H^2,outl et) xx mf_(outl et))/(mf_(i nl e t)`

The solution was hybrid intialised and performed for 200 iterations or until the residuals dropped below 1e-4.

Results

Qualitative Results

Temperature Contour

The combustion produces heat, most of the heat is generated in the cavity. This is expected as the cavity is designed for this purpose.

O2 Contour

There is no oxygen present in the fuel inlet and in the cavity, this means that combustion has taken place in the cavity. Oxygen is presnet in the gap between fuel inlet and cavity, this also says that fuel does not immediately combust after entering the freestream.

H2 Contour

All of the hydrogen fuel dissolves immediately after entering the freestream flow, this is because the fuel ratio is extremely low for this simulation (as only 1 fuel inlet is being simulated). The fuel ratio is about 0.001.

H2O Contour

Water is a byproduct of the combustion reaction is so only seen to be generated in the cavity, this confirms that combustion takes place only in the cavity and not anywhere else.

 

Case 1

Angle of cavity: 30° Fuel Injector Angle: 90°

Mesh/Geometry

Number of iterations: 114

Combustion Efficiency: 0.99932903

2. Angle of cavity: 45° Fuel Injector Angle: 90°

Mesh/Geometry

Number of iterations: 166

Combustion Efficiency: 0.99947512

3. Angle of cavity: 60° Fuel Injector Angle: 90°

Mesh/Geometry

Number of iterations: 200

Combustion Efficiency: 0.99969113

 

Case 2

 

1. Angle of cavity: 60° Fuel Injector Angle: 20°

Mesh/Geometry

 

Number of Iterations: 105

 

Combustion Efficiency: 0.99999005

 

2. Angle of cavity: 60° Fuel Injector Angle: 30°

Mesh/Geometry

 

 

Number of Iterations: 112

 

Combustion Efficiency: 0.99999022

 

3. Angle of cavity: 60° Fuel Injector Angle: 45°

Mesh/Geometry

 

Number of Iterations: 200

 

Combustion Efficiency: 0.99998677

 

4. Angle of cavity: 60° Fuel Injector Angle: 60° (Ramp Injection)

Mesh/Geometry

 

Number of Iterations: 112

 

Combustion Efficiency: 0.99988288

 

5. Angle of cavity: 60° Fuel Injector Angle: 90° (Transverse Injection)

Mesh/Geometry

Number of Iterations: 200

Combustion Efficiency: 0.99969113

Note: This simulation is the same as Case 1/Subcase 3, hence the same data is being used.

 

Case 3 - Fuel inside Cavity

 

1. Angle of cavity: 60° Fuel Injector Angle: 20°

Mesh/Geometry

 

Number of Iterations: 169

 

Combustion Efficiency: 1

 

Mass Fraction of `H_2` at outlet: 1.1043768e-10

 

2. Angle of cavity: 60° Fuel Injector Angle: 30°

Mesh/Geometry

 

Number of Iterations: 113

 

Combustion Efficiency: 1

 

Mass Fraction of `H_2` at outlet: 3.4076226e-09

3. Angle of cavity: 45° Fuel Injector Angle: 20°

Mesh/Geometry

 

Number of Iterations: 166

 

Combustion Efficiency: 1

 

Mass Fraction of `H_2` at outlet: 1.0374389e-10

 

4. Angle of cavity: 45° Fuel Injector Angle: 30°

Mesh/Geometry

 

Number of Iterations: 126

 

Combustion Efficiency: 1

 

Mass Fraction of `H_2` at outlet: 8.3984144e-09

 

The combustion efficiency is extremely high, this can be explained by saying all combustion of the fuel occurs in the cavity itself.

 

Summary of Results

 

Case 1 - Combustion Efficiency vs Angle of Cavity (Fuel Injector Angle: 90°)

 

Angle of Cavity (°)	Combustion Efficiency (%)	Iterations
30	99.932903	114
45	99.947512	166
60	99.969113	200

 

 

 

Case 2 - Combustion Efficiency vs Angle of Fuel Injector (Angle of Cavity: 60°)

 

Angle of Fuel Injector (°)	Combustion Efficiency (%)	Iterations
20	99.999005	105
30	99.999022	112
45	99.998677	200
60	99.988288	112
90	99.969113	200

 

 

Case 3 - Fuel Injector inside Cavity

 

Angle of Fuel Injector (°)	Angle of Cavity (°)	Combustion Efficiency (%)	Mass Fraction of `H_2` at outlet	Iterations
20	60	100	1.10E-10	169
30	60	100	3.41E-09	113
20	45	100	1.04E-10	166
30	45	100	8.40E-09	126

 

 

Conclusions

 

1. The simulation was performed and all qualitative results were found to be satisfactory.

 

2. Efficiency obtained is extremely high, but this is because of the presence of a single inlet for the entire engine. Since all efficiencies were in the order of 99.99%, the convergence criteria was adjusted to 1e-4 to obtain more precise results.

 

3. The combustion efficiency cannot be used in any comparison without context that only 1 fuel inlet was present during the simulation. Actual engine efficiencies range from 80-90% [2].

 

3. In Case 2, the efficiency increases as the cavity angle increases, this can be due to increased area during combustion when the angle of cavity is increased. Increased area results in more complete combustion.

 

4. In Case 2, the efficiency decreases as angle of fuel injector is increased. This can be explained that as the angle decreases, there is less obstruction of flow faced by the fuel, hence combustion occurs more quickly and combusting particles are dissipated quickly.

 

5. For Case 3, due to extremely high efficiences when the fuel inlet is placed in the cavity, the efficiencies could no longer be compared. Instead, the outlet mass fraction of `H_2` gas was compared.

 

6. To summarise, if a suggestion must be made regarding which setup is the most efficient, using the data above. I would suggest a 20° angle of fuel injector inside the cavity with angle of 60°. The fuel injector angle provides the least resistance to flow/combustion and cavity angle provides the most space for complete combustion.

 

 

 

 

References

 

1. Choi, Jeong-Yeol & Ma, Fuhua & Yang, Vigor. (2005). Dynamic Combustion Characteristics in Scramjet Combustors with Transverse Fuel Injection. 41st AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit. 10.2514/6.2005-4428. 

              2. Sukanta Roga 2019 J. Phys.: Conf. Ser. 1276 012041