Challenge on Combustion

AIM

The aim of this challenge is to simulate the combustion process in a combustor

OBJECTIVES

The objectives of this simulation are

• Emission modelling for various reaction mechanisms
• Variation of mass fractions of different species for combustion model
• Effect of adding water content at the fuel inlet

COMBUSTION

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. The fuel can be a solid, liquid, or gas, although for airplane propulsion the fuel is usually a liquid. The oxidizer, likewise, could be solid liquid or gas, but is usually a gas (air) for airplanes. For model rockets, a solid fuel and oxidizer is used.

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.

COMBUSTION SIMULATION

The combustion process invloves the following stages

• Geometry
• Mesh
• Setup
• Solution
• Results

Geometry

The first stage of the combustion process is the combustor 3D model from which the 2D model is extracted from the above model. First the planes were created and then split body command was used to cut the model into half and then pasted onto another design window.

Here we see that the radial direction is aligned along the x-axis and we are performing an axisymmetric flow and combustion simulation. There is a small inlet for the fuel and then air enters through the large intake above the fuel inlet.

Mesh

In the mesh module we create all triangular mesh elements by inserting method option. Here the size of the element is taken as 0.001m and the elements count goes upto 145540 elements.

Setup

For this simulation the setup was as follows

1. solver - pressure solver
2. solver state - steady state
3. 2D space - Axisymmetric
4. tturbulence model - k - epsilon model
5. Chemistry model - Eddy Dissipation
6. Mixture - methane air mixture
7. Additional model - Species transport equations
8. Boundary conditions
9. Inlet velocity for air = 0.5 m/s
10. mole fraction for oxygen = 0.23 moles
11. Inlet velocity for air = 80 m/s
12. Mole fraction for methane - 1
13. Soot model - on with one step model
14. Nox model - on with prompt and thermal checked

Residual plot

The simulation was run for 500 iterations and the solution has converged after 250 iterations.

Results

Temperature contour

Carbondioxide contour

Water contour

Methane contour

Nitrogen contour

Oxygen contour

Nitrous Oxide contour

Soot contour

Line probs

Line probes are created for the concentration variation of different species across the combustor

Temperature line plot

Carbondioxide line plot

Water line plot

Methane line plot

Nitrogen line chart

Oxygen line plot

Nitrogen oxide line plot

Soot line plot

EFFECT OF WATER VAPOUR ON EMISSIONS

In this part of the simulation the water vapour is added in the fuel from 5% to 30% by mole percentage and observe the contour and line plots

Temperature Contour

Carbondioxide contour

Water Contour

Methane contour

Nitrogen contour

Oxygen contour

Nitrogen oxide contour

Soot contour

Temperature line plot

Carbondioxide line plot

Water line plot

Methane line plot

Nitrogen line plot

Oxygen line plot

NOX line plot

Soot line plot

CONCLUSION

1. There is a drop in the level of nitrogen oxides as water content in the fuel is increased
2. Similarly, soot levels also decreases
3. Temperature also increases as water content is added to the fuel
4. Unburnt hydrocarbons forms at much faster rate as water is added
5. Incomplete combustion could be a result of having water
6. The various species of the combustion in the reaction mechanism is seen
7. The species transport and the eddy dissipation is applied in this simulation
8. Thus it is concluded that the combustion simulation helps to see the detailed chemical kinetics study