Two Stream Combustion

Two Stream Combustion, CFD Simulation ANSYS Fluent Training

Description

Combustion reactions occur when air combines with hydrocarbon fuel, converting fuel energy into heat energy. In some applications, two separate fuel streams are used to carry out this reaction — a configuration known as two-stream combustion, where two fuel streams react with a common oxidizer. This project models a horizontal cylindrical combustion chamber using methane (CH₄) and diesel (C₁₂H₂₃) as fuels, combined with an oxidizer stream to produce combustion. The chamber includes four separate inlets: one each for the primary fuel, secondary fuel, and oxidizing stream. The CH₄ stream enters at a flow rate of 0.02 kg/s and 810 K, the C₁₂H₂₃ stream at 0.02 kg/s and 530 K, and the oxidizer at 1.2 kg/s and 723 K.

The Species model was used to define the combustion reaction, configured in non-premixed combustion mode — meaning fuel and oxidizer enter the reaction zone through separate paths without mixing beforehand. This model relies on a mixture fraction representing the mass fraction derived from the fuel stream. Since a secondary fuel is present, the secondary stream option was also activated. The 3D geometry was built in SpaceClaim, representing the interior of a horizontal cylindrical combustion chamber with four inlets for primary fuel, secondary fuel, and airflow. The domain was meshed in ANSYS Meshing using an unstructured mesh totaling 689,854 cells.

Methodology

The simulation was run under steady-state conditions using a pressure-based solver, with gravitational effects excluded.

Key model settings included:

• Turbulence model: Realizable k-epsilon with standard wall functions

• Species model: Non-premixed combustion with non-adiabatic energy treatment and secondary stream enabled

• Species definitions: Fuel as CH₄, oxidizer as 0.79 N₂/0.21 O₂, secondary stream as C₁₂H₂₃

• Boundary conditions: Mass flow inlets for fuel (0.02 kg/s, 810 K, mean mixture fraction 1), oxidizer (1.2 kg/s, 723 K, mean mixture fraction 0), and secondary fuel (0.02 kg/s, 530 K, secondary mean mixture fraction 1); pressure outlet at 0 Pa gauge; stationary walls with zero heat flux

• Solution methods: SIMPLE pressure-velocity coupling, standard pressure discretization, and first-order upwind schemes applied across momentum, turbulence, mixture fraction, and energy equations

• Initialization: Hybrid method

Conclusion

Results include 2D and 3D contours of pressure, velocity, temperature, and species mass fractions (CH₄, CO₂, C₁₂H₂₃, O₂, H₂O, H₂, CO), along with water-liquid and water-vapor volume fractions. Temperature rises significantly within the reaction zone, where the primary and secondary fuel streams combine with the oxidizing stream inside the chamber, releasing substantial thermal energy. Near the chamber inlet, reactant mass fractions (CH₄, C₁₂H₂₃, and O₂) decrease, while reaction product concentrations (CO₂, CO, H₂O, and others) correspondingly increase — confirming that the combustion reaction proceeds correctly, converting fuel and oxidizer into combustion products.