Challenge on Combustion

Objective:

To simulate combustion in a cylindrical combustor using the axisymmetric formulation.

Introduction:

The rapid chemical reaction between fuel and oxidant and involves fluid flow. Combustion simulation is carried out to increase power generation and to decrease polutants.

Types of Combustion Modelling:

• Non-Premixed Combustion:

A non-premixed flame occurs when the fuel and oxidizer are not mixed prior to reacting.

Spark Ignition Petrol Engine

Lean burn gas turbines

Household burners

Bunsen Burner

• Premixed Combustion:

A premixed flame results when fuel and oxidizer are mixed prior to the passage of the reactions zone.

Diesel Engines

Aircraft Engines

Furnaces

• Partially premixed Combustion

Direct Injection Petrol Engines

Combustion can be modelled using

1. Finite Rate Chemistry
2. Mixing based models
3. Finite Rate Eddy Dissipation

In mixing based models, k and epsilon values are used for combustion model. In addition to the mass, momentum and energy equations, the species diffusion equation is also solved for combustion simulations.

Combustion of Methane is chosen to be simulated in the current case.

The equation describing the combustion of Methane can be seen below:

`CH_4+2(O_2+3.78N_2) rightarrow CO_2+ 2H_2O+7.56N_2`

Mass Fraction Calculation:

Air Inlet:

Component	Moles	M. W	Mass	Mass Fraction
Oxygen	2	32	64	0.232
Nitrogen	7.56	28	211.68	0.767
Total	9.56		275.68	1

Fuel Inlet:

Since Methane is the only species entering through the fuel inlet the mass fraction of methane is 1.

Water is then added into the fual inlet along with the fuel and the fuel and water is defined in terms of mole fraction.

Geometry:

The given geometry can be seen in the figure below.

model is 3D and symmetric about the axis and this a 2D simulation using the axisymmetric approach can be used. The geometry needs to be prepared accordingly.

The body is split using the split body option and only the quarter of the cylinder is kept. It is ensured that the axis about which the symmetry is observed should be aligned with the x axis.

The 2D face is then extracted from the quarter.

The next step is to combine the faces into a single face. This is done using the combine body option in Space Claim. The final geometry used for analysis can be seen in the figure below.

Meshing:

The meshing is done and the proximity and curvature option is activated with gap between no of calls set as 3 in order to refine the mesh near the fuel inlet.

The names selections made can be seen in the figure below.

Case Setup:

Simulation Type: Steady

Turbulence: k-epsilon

Species Transport: Volumetric with Inlet Diffusion and diffusion energy source enabled

Material: Methane air

Turbulence Chemistry Interaction: Eddy Dissipation

Boundary Conditions:

Air_Inlet: Velocity Inlet with 0.5m/s

Fuel Inlet: Velocity Inlet with 80m/s

The mass fractions of air are entered as calculated previously.

At the fuel Inlet , water is added and four cases are run with no water addition , 5%, 15%, 20% and 30% water by mole fraction.

The One step soot model and the NOx model are employed.

Outlet: Pressure Outlet

Initialization: Hybrid

Results:

The simulation is carried out and the resulta are post processed using CFD post. The temperature profiles obtained can be seen below.

The temperature is maximum at the core of the combuster ane lower at the outer surface. The flame can be seen to be spreading from the centre near the fuel inlet to the entire combustor.

Line plots are introduced in order to monitor various parameters at different points.

The line plots of the temperature can be seen in the below plot.

The temperature is maximum near the axis and lowers along the radius but increases along the axis due to the flame.

The mass fraction of oxygen, nitrogen and methane can be seen in the below contours followed by the line plots. Contours of O2:

The oxygen concentration is maximum at the air inlet and it then mixes with the fuel and as the combustion occurs the concentration of the oxygen can be seen to be changing.

From the line plot below it can be seen that the oxygen concentratio is maxumum at the inlet. This is attributed to the fact that oxygen is present as a reactant and is not a product of combustion.

Contours of Nitrogen:

It can be observed that the nitrogen concentration is maximum at the air inlet.

The line plots show that nitrogen being a non reacting componenet the concentration does not change much.

Contours of Methane.

Methane entry is visible only at the fuel inlet and it then mixes with air and burns off.

The contour and line plots of mathane show the depletion in concentration of methane as the combustion reaction takes place.

The products of combustion are analysed as follows:

The following images show the contour and line plots of CO2:

The formation of carbon dioxide is maximum at the centre of the combustor.

Contours of NO and Soot produced:

The pollutants formed can be seen. Now water is added along with the fuel through the fuel inlet and the soot and NO2 obtained are shown in the contours below:

For 5% water:

For 15% water:

For 25% water:

For 30% water:

Below is the line plot showing the soot content for various amounts of water at the fuel inlet:

The formation of soot reduces with addition of water along with the fuel at the fuel inlet.

Conclusion:

• An axisymmetric profile of a cylindrical combustor is generated in order to reduce the size of the compytational domain.
• Combustion using the mixed is burnt principle with the help of k epsilon turbulence is performed.
• The temperature and species concentrations at various locations are monitored.
• Water is added along with the fuel in the fuel inlet and the effect on the soot formation is noted.