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:
2H2(g)+O2(g)→2H2O(g)+energy">2H2(g)+O2(g)→2H2O(g)+energy2�2(�)+�2(�)→2�2�(�)+������
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:
Quality:
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.
Species
This introduces 2 more materials, they are: O2 and H2O
Boundary Conditions
inlet-air - velocity-inlet, 1040 m/s�/� velocity, 600K temperature, and O2�2 with a mole fraction of 0.21 (to simulate atmospheric air).
inlet-fuel - velocity-inlet, 200m/s�/� velocity, 500 bar pressure (5×10^7Pa5×107��), and H2O�2� with a mole fraction of 1 (pure hydrogen).
outlet - pressure-outlet, 0Pa.
walls-bottom, walls-top - stationary wall.
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 formula:ηcombustion=1-mfracH2,outlet×mfoutletmfinlet
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 H2 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 H2�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 H2�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 H2�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 H2�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% .
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 H2�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.
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