Challenge on Combustion

Title: Challenge on Combustion.

Objective:

1. 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 given combustor.
2. To perform a study on how adding water of 5% – 30% into fuel will affect the mass fraction of the pollutants from the reaction.

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:

Case 1: The simulation for the mass fraction of the pollutants.

Case 2: How the addition of water to the fuel affects the pollutants.

Step 1: Geometry:

In order to perform the simulation in acceptable time span, the given 3D geometry is converted into 2D geometry as it reduces simulation time with the same result.

3D geometry:

2D geometry:

Step 2: Meshing:

Meshing is done using element size of 2 mm.

And given named selections as required.

Step 3: Setup & Solution:

Solver type: Pressure based.

Time: Steady.

2D space: Axisymmetric.

Turbulence model: k-epsilon model.

Species transport is enabled.

Mixture material: Methane-air (2-step).

Turbulence chemistry interaction: Eddy dissipation.

NOx model is enabled.

Soot model: 1-step.

Boundary conditions:

Air inlet: Inlet velocity = 0.5m/s.

Mass fraction of O2 = 0.23.

Fuel inlet: Inlet velocity = 80m/s.

Case 1: Mass fraction of CH4 = 1.

Case 2: Mass fraction of CH4 = 0.95, 0.85 and 0.7.

Mass fraction of H2O = 0.05, 0.15 and 0.3 respectively.

No. of iterations: 1500.

Step 4: Results.

Case 1:

Residual plot:

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:

Mass fraction of the species at different lines:

Case 2(a): Mass fraction of CH4 = 0.95 and H2O = 0.05.

Residual plot:

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:

Mass fraction of the species at different lines:

Case 2(b): Mass fraction of CH4 = 0.85 and H2O = 0.15.

Residual plot:

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:

Mass fraction of the species at different lines:

Case 2(c): Mass fraction of CH4 = 0.7 and H2O = 0.3.

Residual plot:

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:

Mass fraction of the species at different lines:

From the above results the following table is plotted:

Case	Temperature (K)	Mass fraction[NOx]	Mass fraction[soot]
Case 1	2309	2.77e-03	1.47e-11
Case 2(a)	2299	2.31e-03	1.05e-11
Case 2(b)	2275	1.53e-03	3.71e-12
Case 2(c)	2228	6.60e-04	9.06e-13

Conclusion:

1. From the above results, it can be said that addition of water in to the fuel brings down the peak temperature. Thus, reducing the amount of NOx generated due to high reaction temperature in the combustor.
2. Addition of water also helps in delaying self-ignition due to which the fuel air mixture is more homogeneous thus decreasing the soot formation gradually.
3. Fewer areas of low oxygen content caused by addition of water also reduces the risk of generating harmful CO formation.