Challenge on Combustion
Aim:
To Perform a combustion simulation on the combustor model.
Objective:
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.
add the water content in the fuel from 5% to 30% by mole and observe the effect of it on the results.
Theory:
Combustion: Combustion is a process of a chemical reaction, in which a fuel reacts with atmospheric air to generate Heat and Light in the form of flame.
In our case Fuel is Methane (CH4).
Methane is used as a fuel for ovens, homes, water heaters, kilns, automobiles, turbines, and other things. Activated carbon is used to store methane. Refined liquid methane is used as a rocket fuel, when combined with liquid oxygen, as in the BE-4 and Raptor engines.
As the major constituent of natural gas, methane is important for electricity generation by burning it as a fuel in a gas turbine or steam generator. Compared to other hydrocarbon fuels, methane produces less carbon dioxide for each unit of heat released. At about 891 kJ/mol, methane's heat of combustion is lower than that of any other hydrocarbon. However, it produces more heat per mass (55.7 kJ/g) than any other organic molecule due to its relatively large content of hydrogen, which accounts for 55% of the heat of combustion but contributes only 25% of the molecular mass of methane. In many cities, methane is piped into homes for domestic heating and cooking. In this context it is usually known as natural gas, which is considered to have an energy content of 39 megajoules per cubic meter, or 1,000 BTU per standard cubic foot. Liquefied natural gas (LNG) is predominantly methane (CH4) converted into liquid form for ease of storage or transport.
As a rocket fuel, methane offers the advantage over kerosene of depositing less carbon on the internal parts of rocket motors, reducing the difficulty of re-use of boosters.
Combustion of Methane:
Methane when burnt in atmospheric air undergoes the following reaction in the combustor:
In addition, the combustion also produces pollutants like Oxides of Nitrogen (NOx) and Soot.
The goal of the simulation is to simulate the mass fraction of the pollutants occurred when the combustion takes place.
And also, how addition of water to the incoming fuel would affect the mass fraction of the harmful pollutants resulted due to the reaction.
Procedure:
This simulation is performed in two cases:
Part 1: The simulation for the mass fraction of the pollutants.
Part 2: How the addition of water to the fuel affects the pollutants.
Given Geomentry(3D model):
Converted 2D geomentry:
By using Split command to its particular plane.
Meshing:
Meshing methods: All Triangles method
Capture promximity enabled
element size: 1 mm
Setup:
General:
Energy enabled
Viscous model:
Species model:
Nox model:
Soot model:
Boundaries:
Air inlet velocity: 0.5 m/s
Air inlet temperature: 300 k
Species mass fraction at air inlet: O2= 0.23
Fuel inlet velocity: 80 m/s
Fuel inlet temperature : 300 k
Species mass fraction at Fuel inlet: CH4 = 1
Initialization method : hybrid
Number of iterations = 1000
Part: 1:
plot the variation of the mass fraction of the different species’ in the simulation using line probes at different locations of the combusto
Residual plot:
Average mass of nox :
Average mass of soot:
Temperature contour:
Mass fraction of CH4:
Mass fraction of CO2:
Mass fraction of O2:
Mass fraction of N2:
Mass fraction of H2O:
Mass fraction of Pollutant NOx:
Mass fraction of Pollutant soot:
Line probes at different locations of the combustor:
Mass fraction of the species at different lines:
Video file :-
Conclusion:
- The temperature contours display a flame envelope, within the temperature initially attains a maximum value following which it achieves a steady state.
- The mass fraction of methane decreses to zero as it gets completely consumed to generate heat and by products.
- The mass fraction of oxygen is very low in region of combustion that is , the initial lower region of the domain as it gets consumed to generate heat .
- The mass fraction of CO2 and water is high in regions of high temperatures as it generate along the heat.
Part-2:
To study the effect of adding water with fuel a parametric study is conducted, The mole fraction of CH4 and mole fraction of water at fuel inlet are the input parameters.
Here Parameter 1 is Mole fraction of CH4
Parameter 2 is mole fraction of water
DP 0 :
Contour plots:
NOx:
Soot:
Variation of Soot mass fraction in all line probes:
Variation of nox mass fraction in all line probes:
DP 1:
Contour plots:
NOx:
Soot:
Variation of Soot mass fraction in all line probes:
Variation of nox mass fraction in all line probes:
DP 2 :
Contour plots:
NOx:
Soot:
Variation of Soot mass fraction in all line probes:
Variation of nox mass fraction in all line probes:
DP 3 :
Contour plots:
NOx:
Soot:
Variation of Soot mass fraction in all line probes:
Variation of nox mass fraction in all line probes:
DP 4 :
Contour plots:
NOx:
Soot:
Variation of Soot mass fraction in all line probes:
Variation of nox mass fraction in all line probes:
DP 5 :
Contour plots:
NOx:
Soot:
Variation of Soot mass fraction in all line probes:
Variation of nox mass fraction in all line probes:
Conclusion:
- Supplying Water with fuels to minimize Temperature peaks during the combustion.
- Hence the NOx concentrated in the exhaust gas, can be done by means of supplying water into the cylinders , so called wet technologies.
- The emulsion supplied into the cylinder causes the increase of self-ignition delay which affects the flammable mixture becoming more homogeneous in the whole volume of the combustion chamber.
- The increased self ignition delay extends the time necessary to spread the injected fuel which results in fewer zones with not enough oxygen. that decreases the intensity of soot and co formation.
- Emulsifying the mixture reduces local zones with temperature peaks which leads to decreasing nitrogen activity and NOx concentration.