Week 10 - Simulating Combustion of Natural Gas.

Introduction

Combustion is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products.
When the combustion is incomplete, various pollutants are formed, some pollutants are carbon dioxide, carbon monoxide, lead, nitric oxide.

Being the integral part of various engineering applications like: internal combustion engines, 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.
With the added complexity of chemical kinetics and achieving reacting flow mixture environment, proper modeling physics has to be incorporated during computational fluid dynamic (CFD) simulations of combustion.

Theory

Computational fluid dynamics modeling of combustion calls upon the proper selection and implementation of a model suitable to faithfully represent the complex physical and chemical phenomenon associated with any combustion process.

The model should be competent enough to deliver information related to the species concentration, their volumetric generation or destruction rate and changes in the parameters of the

The model should be capable of solving the general transport equations for fluid flow and heat transfer as well as the additional equations of combustion chemistry and chemical kinetics incorporated into that as per the simulating environment desired

Model Used
The eddy dissipation model, Based on the work of Magnussen and Hjertager, is a turbulent-chemistry reaction model.
Most fuels are fast burning and the overall rate of reaction is controlled by turbulence mixing.

In the non-premixed flames, turbulence slowly mixes the fuel and oxidizer into the reaction zones where they burn quickly.

Combustion initiates whenever there is turbulence present in the flow. It does not need an ignition source to initiate the combustion.

Application

The mathematical models that describe chemical reaction kinetics provide chemists and chemical engineers with tools to better understand and describe chemical processesThese models can also be used in the design or modification of chemical reactors to optimize product yield, more efficiently separate products, and eliminate environmentally harmful by-products. When performing catalytic cracking of heavy hydrocarbons into gasoline and light gas, for example, kinetic models can be used to find the temperature and pressure at which the highest yield of heavy hydrocarbons into gasoline will occur.

Aim

1. Perform a combustion simulation on the combustor model
2. The fuel species is methane and the fluid is air, which contains a mixture of oxygen and nitrogen
   
3. Plot the variation of the mass fraction of the different species’ in the

simulation using line probes at different locations of the combustor.

4. Check the effect of adding the water in the fuel on pollutants such as Soot and NOx emissions
5. Observe the effect of changing water composition in the fuel from 5% to 30%

Geometry

3D geometry

2D geometry

MESH

Details
Element Size : 2 mm
Nodes : 18065
Elements: 36118

Mesh Images

Mesh Quality

SOLVER SETUP

Solver : Pressure Based, Steady State
2D Space: Axisymmetric

Models
Energy Equations: ON
Viscous Model: Standard K-Epsilon with standard wall functions
Species
Model: Species Transport
Mixture Properties: Methane - air 2 step
Reactions: Volumetric
Options: Inlet diffusion, Diffusion energy source
Turbulence Chemistry Interaction: Eddy Dissipation

Nox Model

Soot Model

Boundary Conditions
Air Inlet
Velocity: 0.5 m/s
Temperature: 300K
Species mass fraction: O2=0.23

Fuel Inlet
Velocity: 80 m/s
Temperature: 300K
Species Mole Fraction:
CH4 = Parameter 1 = 1
H20 = Parameter 2 = 0
Parameter 1 decreases by 0.05 with each case
Parameter 2 increases by 0.05 with each case

Initialization : Hybrid
No of iterations : 700

POST PROCESSING

Line Probes are set up at .1x,.2x,.3x,.5x and .7x. Where x is the total length.
They are used to study the variations of the mass fractions of the Reactants, Products and Temperature on these lines




RESULTS

CASE 1
CH4- 100% , Water(H2O) – 0%
Residuals

Temperature
Contour

Plot

CO2 Mass Fraction
Contour

Plot

H20 Mass Fraction
Contour

Plot

CH4 Mass Fraction
Contour

Plot

N2 Mass Fraction
Contour

Plot

O2 Mass Fraction
Contour

Plot

NOx Mass Fraction(No)
Contour


Plot

NOx(N20)
Contour

Plot

Soot Mass Fraction
Contour

Plot

CASE 2
CH4- 95% , Water(H2O) – 5%

Residuals

Temperature
Contour

Plot

CO2 Mass Fraction
Contour

Plot

H20 Mass Fraction
Contour

Plot

CH4 Mass Fraction
Contour

Plot

N2 Mass Fraction
Contour

Plot

O2 Mass Fraction
Contour

Plot

NOx Mass Fraction(NO)
Contour

Plot

NOx Mass Fraction(N2O)
Contour

Plot

Soot Mass Fraction
Contour

Plot

CASE 3

CH4- 90% , Water(H2O) – 10%

Residuals

Temperature
Contour

Plot

CO2 Mass Fraction
Contour

Plot

H20 Mass Fraction
Contour

Plot

CH4 Mass Fraction
Contour

Plot

N2 Mass Fraction
Contour

Plot

O2 Mass Fraction
Contour

Plot

NOx Mass Fraction (NO)
Contour

Plot

NOx Mass Fraction(N2O)
Contour

Plot

Soot Mass Fraction
Contour

Plot

CASE 4
CH4- 85% , Water(H2O) – 15%

Residuals

Temperature
Contour

Plot

CO2 Mass Fraction
Contour

Plot

H20 Mass Fraction
Contour

Plot

CH4 Mass Fraction
Contour

Plot

N2 Mass Fraction
Contour

Plot

O2 Mass Fraction
Contour

Plot

NOx Mass Fraction(NO)
Contour

Plot

NOx Mass Fraction (N2O)
Contour

Plot

Soot Mass Fraction
Contour

Plot

CASE 5
CH4- 80% , Water(H2O) – 20%

Residuals

Temperature
Contour

Plot

CO2 Mass Fraction
Contour

Plot

H20 Mass Fraction
Contour

Plot

CH4 Mass Fraction
Contour

Plot

N2 Mass Fraction
Contour

Plot

O2 Mass Fraction
Contour

Plot

NOx Mass Fraction (NO)
Contour

Plot

NOx Mass Fraction (N2O)
Contour

Plot

Soot Mass Fraction
Contour

Plot

CASE 6
CH4- 75%, Water (H2O) – 25%

Residuals

Temperature
Contour

Plot

CO2 Mass Fraction
Contour

Plot

H20 Mass Fraction
Contour

Plot

CH4 Mass Fraction
Contour

Plot

N2 Mass Fraction
Contour

Plot

O2 Mass Fraction
Contour

Plot

NOx Mass Fraction (NO)
Contour

Plot

NOx Mass Fraction (N2O)
Contour


Plot

Soot Mass Fraction
Contour

Plot

CASE 7
CH4- 70%, Water (H2O) – 30%

Residuals

Temperature
Contour

Plot

CO2 Mass Fraction
Contour

Plot

H20 Mass Fraction
Contour

Plot

CH4 Mass Fraction
Contour

Plot

N2 Mass Fraction
Contour

Plot

O2 Mass Fraction
Contour

Plot

NOx Mass Fraction (NO)
Contour

Plot

NOx Mass Fraction (N2O)
Contour

Plot

Soot Mass Fraction
Contour

Plot

OBSERVATIONS

Case	Ratio of Methane and water	Temp	Mass Fraction
			CH4	H20	CO2	O2	N2	NO	N2O	Soot
1	1:00	2038.8	1	0.120	0.143	0.23	0.77	7.76E-03	3.78E-07	1.54E-11
2	.95 : .05	2027.28	0.944	0.124	0.142	0.23	0.77	6.45E-03	3.58E-07	9.50E-12
3	.90 : .10	2014.13	0.889	0.128	0.142	0.24	0.77	5.30E-03	3.40E-07	6.79E-12
4	.85 : .15	2000.19	0.834	0.165	0.14	0.23	0.77	4.31E-03	3.25E-07	4.11E-12
5	.80 : .20	1984.05	0.78	0.219	0.14	0.23	0.77	3.42E-03	3.10E-07	2.16E-12
6	.75 : .25	1966.48	0.728	0.272	0.139	0.23	0.77	2.66E-03	2.95E-07	1.11E-12
7	.70 : .30	1946.73	0.675	0.325	0.138	0.23	0.77	2.00E-03	2.80E-07	1.43E-12

INFERENCES
The addition of water to the fuel mix reduces the maximum temperature of combustion from 2038.8 C to 1946.73 C as the percentage of water increases from 5% to 30%
The mass fractions of oxides of nitrogen reduce as the percentage of water increases
The mass fraction of soot is higher for a 100% methane case but reduces the percentage of water increases, however after the percentage of water crosses 25% the mass fraction of soot increases.