Challenge on Combustion

Aim:- Perform a combustion simulation on the combustor model and plot the variation of the mass fraction of the different species.

Introduction

Combustion models for CFD refers to combustion models for computational fluid dynamic. Combustion is defined as a chemical reaction in which a hydrocarbon fuel reacts with an oxidant to form products, accompanied with the release of energy in the form of heat. Being the integral part of various engineering applications like: internal combustion engine, aircraft engines, rocket engines, furnaces, and power station combustors, combustion manifests itself as a wide domain during the design, analysis and performance characteristics stages of the above-mentioned applications.

Types of combustion simulation

Based on mixing

• Non-premixed combustion(Direct injection, Late injection)
• Premixed combustion(Carburetor)
• Partially premixed

Based on Phase

• Fluid phase (Volumetric reaction)
• Wall (Surface reactions)
• Particles (Surface reaction)
• Porous region (Example After treatment system)

Combustion can be modelled using

• Finite rate chemistry
• Mixing based model
• Finite rate eddy dissipation

In mixing based model k-epsilon are used for combustion model. Mass ,momentum, energy and species diffusion equation is used for combustion simulation. For combustion methane is chosen for the simulation.

Equation of combustion of Methane

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

Moles and mass fraction calculation

Air Inlet

Component	Molecular weight	Moles	Mole fraction	Mass fraction
Oxygen	32	2	0.21	0.23
Nitrogen	28	7.52	0.79	0.77
Total	60	9.52

Fuel inlet

Methane is the only species entrying through the fuel inlet therefore mass fraction of methane is 1.

Add the water content in the fuel from 5% to 30% by mole and observe the effect of it on the results.

Geometry

Import the given geometry in ansys fluent

The body is split in to 2 parts by split body option and kept one fourth of the body. It is ensures that the axis about which the symmetry is obsorved should be aligned with the x-axis. Combine the faces using single faces by combine option.

Extract the 2D geometry from 3D geometry quarter.

Setup

Simulation type- Steady

Model- K-epsilon

Species Transport- Volumetric with inlet diffusion and diffusion energy source enabled

Material -Methane-air

Turbulance chemistry interation-Eddy dissipation

Bounary condition

Air-inlet velocity -0.5 m/s

Fuel inlet velocity-80 m/s

One step soot model

Hybrid initilization

Result

Case-1

Fuel inlet with 0% water by mole fraction and 100% `CH_4`

Residual plot

From above residual plot we can say that solution is converged at90 iteration

Temperature contour

`CH_4` contour

`CO_2` contour

`O_2`Contou`

`N_2` Contou`

Mass fraction of pollutant number contour

Mass fraction of soot contour

For ploting the graph of different species we choose the line at different location 0.02m,0.1m,0.2m,0.3m,0.4m along X-axis from the origin.

Temperature plot

The above plot represent the variation of temperate at different position away from the inlet of the fuel. Series 1 shows the variation of temperature along Y-axis at 0.02 m distance from the fuel inlet. From the above plot we analyze that when move from left to right side of the fuel inlet temperature increases up to certain point there after decreases and then constant.

`CH_4` plot

Above plot represent the variation of mass fraction of `CH_4`at specific location of X- axis along Y-axis

`CO_2`plot

When methane burns , releases carbon dioxide gas plot shows the variation of mass fraction of `CO_2` along the Y-axis at different location of X-axis .

`O_2` plot

`N_2` Plot

Mass fraction of pollutant number plot

Mass fraction of soot plot

Part-2

When water is mixed with the fuel through the fuel inlet Variation of different parameter shown below

Temperature plot

`CH_4` plot

`CO_2`plot

Mass fraction of pollutant number plot

Mass fraction of soot plot

When 25% water is added in the fuel by mole fraction

Temperature plot

`CH_4` plot

`CO_2` plot

Mass fraction of pollutant number plot

Mass fraction of soot plot

Conclusion

1.From the above table we can say that when water is added in the fuel then average temperature decreases when water percentage increases from 0% to 30%.

2. When water percentage increases percentage of `CO_2` decreases

3. Nitrogen remains constant it is indepenent of the water percentage.

4. Mass fraction of pollutant number decreases when water percentage increases in the fuel.

5. When water percentage increases percentage of soot first increases then decreases due to incomplete combution of the fuel.