Challenge on Combustion

• Objectives :

1. To perform a combustion simulation on the combustor model and plot the variation of the mass fraction of the different species’ in the simulation using line probes at different locations of the combustor. To plot for CO2, H2O, CH4, N2, O2, NOx emissions & Soot formation.
2. To add the water content in the fuel from 5% to 30% by mole and observe the effect of it on the results.

• Introduction :

Combustion is a chemical process in which a substance reacts rapidly with oxygen and gives off heat. The original substance is called the fuel, and the source of oxygen is called the oxidizer.

During combustion, new chemical substances are created from the fuel and the oxidizer. These substances are called exhaust. Most of the exhaust comes from chemical combinations of the fuel and oxygen. When a hydrogen-carbon-based fuel (like gasoline) burns, the exhaust includes water (hydrogen + oxygen) and carbon dioxide (carbon + oxygen). But the exhaust can also include chemical combinations from the oxidizer alone. If the gasoline is burned in air, which contains 21% oxygen and 78% nitrogen, the exhaust can also include nitrous oxides (NOX, nitrogen + oxygen). The temperature of the exhaust is high because of the heat that is transferred to the exhaust during combustion. Because of the high temperatures, exhaust usually occurs as a gas, but there can be liquid or solid exhaust products as well. Soot, for example, is a form of solid exhaust that occurs in some combustion processes.

To summarize, for combustion to occur three things must be present: a fuel to be burned, a source of oxygen, and a source of heat. As a result of combustion, exhausts are created and heat is released. You can control or stop the combustion process by controlling the amount of the fuel available, the amount of oxygen available, or the source of heat.

• Geometry :

1. A 3D geometry is provided, but since it would take lot of time for solving the simulation we modify it and use the 2D version for analysis.
2. Using split body feature two times to get a singular face of the combustion chamber. And copying that face into new design model.
3. Using combine feature to have a single component in design.

Below is seqence of images as the geomwtry is edited step by step.

• Mesh :

Element Size = 2.5 mm.

Number of Nodes = 11869

Number of Elements = 23023

• Case Setup :

1. Pressure based steady state Solver with absolute Velocity formulation. Axisymmetric geometry.
2. Standard K-Epsilon model with energy equation solved.
5. Boundary Conditions : Air inlet - 0.5 m/s (Mass fraction O2 = 0.23 )

Fuel inlet - 80 m/s (Mass fraction CH4 = 0.95 to 0.7 and H2O = 0.05 to 0.3)

Outlet - 0 Pa. 300 K atmospheric temperature.

6. NOx and Soot calculation models turned on. Soot calculation using one step model and NOx calculation with Thermal and Fuel prompts turned on.

7. Hybrid Initilization and calculation .

• Case I : 5% Water

CH4 :

CO2 :

H2O :

NOx :

N2 :

O2 :

Soot :

Case II : 10% Water

CH4 :

CO2 :

H2O :

NOx :

N2 :

O2 :

Soot :

Case III : 20% Water

CH4 :

CO2 :

H2O :

NOx :

N2 :

O2 :

Soot :

• Case IV : 30% Water

CH4 :

CO2 :

H2O :

NOx :

N2 :

O2 :

Soot :

• Observations and Conclusion :

1. CO2 production reduces as we increase amount of water % in fuel.
2. There is significant reduction in NOx as we increase water % in fuel.
3. N2 production more or less remains the same.
4. Soot production is minimal at 15%(plots not included) water and maximum at 30% water.
5. Temperature range is almost same for all cases.
6. We can concur that at adding water upto 15% can reduce the production of soot effectively while not affecting other parameters significantly.