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

AIM: To calculate the most efficient slot-fuel injector and cavity-flame holder for a scramjet in ANSYS Fluent.

THEORY:

• 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 to 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.

• The angle visible is called the angle of the 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 is as follows:

  2H2(g)+O2(g)→2H2O(g)+energy">2H2(g)+O2(g)→2H2O(g)+energy

Combustion Efficiency:

• It is defined as the ratio of burnt fuel to the total fuel for an engine.
• In this case, its case the efficiency can be said as:

• 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 the cavity is set to 30, 45 and 60 degrees.

Case 2: The angle of the 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.

• Finding the combustion efficiency for 15 cases is infeasible and the goal of 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:

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:

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

• The mesh roughly contains 16257 nodes and 15847 elements, and this number will vary for each case.

• And its named selections,

SIMULATION SETUP:

General:

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

Materials:

• Air and Hydrogen gas (fluid) was used for this simulation.
• The properties were the default ones as shown below,

Hydrogen,

Turbulence Model:

• While doing a literature review for this project, the two most commonly used models were: k-omega SST and k-epsilon. In the end, the model chosen 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: O2 and H2O

Boundary Conditions:

• inlet-air - velocity-inlet, 1040 m/s velocity, 600K temperature, and O2 with a mole fraction of 0.21 (to simulate atmospheric air).
• inlet-fuel - velocity-inlet, 200 m/s velocities, 500 bar pressure (5×107Pa), and H2O with a mole fraction of 1 (pure hydrogen).
• outlet - pressure-outlet, 0Pa.
• walls-bottom, walls-top - stationary wall.
• Boundary Conditions were decided after an extensive literature review and were chosen from Reference [1]

Solution Methods:

Reports:

Three 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:

To calculate the combustion efficiency using the following formula:

RESULTS:

1.CASE-1 - Angle of cavity: 30° Fuel Injector Angle: 90°

Residual:

Mass flow rate - Outlet:

• Number of iterations: 110
• Combustion Efficiency: 0.99932621

Temperature Contour:

H2 Contour:

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

MESH:

• Number of iterations: 120
• Combustion Efficiency: 0.99947846.

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

MESH:

• Number of iterations: 120

• Combustion Efficiency: 0.99969267

2. CASE-2. The angle of cavity: 60° Fuel Injector Angle: 20°

MESH:

• Number of Iterations: 120

• Combustion Efficiency: 0.99999005

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

MESH:

• Number of Iterations: 120

• Combustion Efficiency: 0.99999022

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

Mesh/Geometry:

• The temperature contour looks similar in some cases the mesh has been displayed.

• Number of Iterations: 120

• Combustion Efficiency: 0.99998677

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

Mesh/Geometry:

• Number of Iterations: 120

• Combustion Efficiency: 0.99988288

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

Mesh/Geometry:

• Number of Iterations: 120
• Combustion Efficiency: 0.99969113
• Note: This simulation is the same as Case 1/Subcase 3, hence the same data is being used.

3. Case-3 - Fuel inside Cavity:

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

Mesh/Geometry:

• Number of Iterations: 120

• 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: 120

• Combustion Efficiency: 1

• Mass Fraction of H2 at outlet: 3.4076226e-09.

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

Mesh/Geometry:

• Number of Iterations: 120
• Combustion Efficiency: 1
• Mass Fraction of H2 at outlet: 1.0374389e-10.

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

Mesh/Geometry:

• Number of Iterations: 120

• Combustion Efficiency: 1

• Mass Fraction of H2 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 RESULT:

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

Angle of Cavity (°)	Combustion Efficiency (%)
30	99.932621
45	99.947846
60	99.969267

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

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

Case 3 - Fuel Injector inside Cavity:

Angle of Fuel Injector (°)	Angle of Cavity (°)	Combustion Efficiency (%)	Mass Fraction of H2">H2H2 at outlet
20	60	100	1.10E-10
30	60	100	3.41E-09
20	45	100	1.04E-10
30	45	100	8.40E-09

CONCLUSION:

1. The simulation was run, and all qualitative results were suitable.

2. The obtained efficiency is extremely high, but this is due to the presence of a single inlet for the entire engine. Because all efficiencies were in the 99.99% range, the convergence criteria were changed to 1e-4 to obtain more precise results.

3. The combustion efficiency cannot be compared because only one fuel inlet was present during the simulation. Actual engine efficiencies range between 80 and 90% [2].

4. In Case 2, efficiency increases as the cavity angle increases, possibly due to increased area during combustion as the cavity angle increases. Increased surface area leads to more complete combustion.

5. In Case 2, the efficiency decreases as the angle of the 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.

6. For Case 3, due to extremely high efficiencies when the fuel inlet is placed in the cavity, the efficiencies could no longer be compared. Instead, the outlet mass fraction of H2 gas was compared.

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

REFERENCE:

• https://www.researchgate.net/publication/228371858_Dynamic_Combustion_Characteristics_in_Scramjet_Combustors_with_Transverse_Fuel_Injection
• https://iopscience.iop.org/article/10.1088/1742-6596/1276/1/012041/pdf