Challenge on Combustion

Aim: 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

Objective:

• 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. You need to plot for CO2, H2O, CH4, N2, O2, NOx emissions & Soot formation.
• As you must have observed from the above simulation, the Nox and soot is getting formed at the outlet of the combustor. Such formation has harmful effects on the environment and humans. The stringent government norms also demand the least formation of Nox and soot and to satisfy those requirements, you need to check the effect of adding the water in the fuel. In this part, you need to add the water content in the fuel from 5% to 30% by mole and observe the effect of it on the results. It is necessary to provide line plots and contours to prove your claim. You can use the parametric study approach if required.

Note:-

1. Use methane-air-2step as a mixture material than methane-air. [It allows to add water in the fuel or air]
2. Use a one-step soot model to calculate the soot formation.

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 the combustion process, as the fuel and oxidizer are turned into exhaust products, heat is generated. Interestingly, some source of heat is also necessary to start combustion. Gasoline and air are both present in your automobile fuel tank; but combustion does not occur because there is no source of heat. Since heat is both required to start combustion and is itself a product of combustion, we can see why combustion takes place very rapidly. Also, once combustion gets started, we don't have to provide the heat source because the heat of combustion will keep things going. We don't have to keep lighting a campfire, it just keep burning.

During combustion, new chemical substances are created from the fuel and the oxidizer. These substances are called exhaust. The exhaust includes the water, oxygen, nitrogen, SOx, NOx and soot.

Combustion Modelling in ANSYS Fluent:

Computational Domain Geometry:

Given 3D geometry:

Simplified 2D geometry:

Mesh and Named Selection:

Case setup:

Steady, pressure based, double precision, axisymmetric solver with four processors are used for solution.

Models:

Energy: On

Turbulence: k-e, standard, standard wall function

Species: Species transport

As we wish to simulate the combustion model we need to turn on the turbulence model in order to species and combustion model to work. the models selected from the species are as shown in below figure. here in this model the combustion initiates whenever there is turbulence present in the flow and the ignition source is not required to initiate the combustion.

NOx and Soot models are activated to get the amount of NOx generated in the reaction.

Boundary conditions:

Air-inlet: the air velocity is specified with a value of 0.5 m/s

Fuel-inlet: the fuel velocity is specified with a value of 80 m/s

Outlet: at outlet the gauge pressure is given 0 Pa.

Axis: axis boundary condition.

Wall: no slip wall boundary condition.

Solution initialization:

The solution is then initialized with the hybsrid initialization.

Run Calculation:

The calculation is then solved for 1500 iteration.

Results:

Case1:

Residual:

Mass fraction of CO2 and plot:

Mass fraction of H2O and plot:

Mass fraction of CH4 and plot:

Mass fraction of N2 and plot:

Mass fraction of O2 and plot:

Mass fraction of NOx and plot:

Mass fraction of soot and plot:

Case 2: 5% water in fuel

Residual:

Mass fraction of NOx and plot:

Mass fraction of soot and plot:

Case 3: 10% water in fuel

Residual:

Mass fraction of NOx and plot:

Mass fraction of soot and plot:

Case 4: 15% water in fuel

Residual:

Mass fraction of NOx and plot:

Mass fraction of soot and plot:

Case 5: 20% water in fuel

Residual:

Mass fraction of NOx and plot:

Mass fraction of soot and plot:

Case 6: 25% water in fuel

Residual:

Mass fraction of NOx and plot:

Mass fraction of soot and plot:

Case 7: 30% water in fuel

Residual:

Mass fraction of NOx and plot:

Mass fraction of soot and plot:

Conclusions:

It is observed from the simulations that the species mass fraction depends on the effective combustion as the contours of CO2, temperature and H2O looks similar.

when enough turbulent mixing of fuel and air is present the eddy dissipation model will form a reaction and creates combustion depending on the amount of mixture present in that cell.

NOx concetration is observed to be negligible and very low amount of NOx is generated during combustion.

As in second case we go on increasing the water content in fuel and the drop in level of nitrogen oxides and soot is observed.

Reference :-

1. https://www.sciencedirect.com/science/article/pii/S1876610217337724

2. https://link.springer.com/article/10.1007/s00773-015-0303-8