Challenge on Combustion

OBJECTIVES:

1) To Perform a combustion simulation on the combustor model and plot the variation of the mass fraction of the different species’ in the simulation and also plot for CO2, H2O, CH4, N2, O2, NOx emissions & Soot formation. 

2) To Observe the effect of adding the water content in the fuel.

Introduction:

Combustion: It 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. The fuel can be a solid, liquid, or gas, although for airplane propulsion the fuel is usually a liquid.

Combustion is often hot enough that incandescent light in the form of either glowing or a flame is produced. A simple example can be seen in the combustion of hydrogen and oxygen into the water vapor, a reaction commonly used to fuel the rocket engines. This reaction releases 242 KJ/MOL of heat and reduces the enthalpy accordingly (at constant temperature and pressure):

`2H_2(g)+O_2(g) → 2H_2O(g)`

Combustion Models:

1) Based on Combustion

a) Non-Premixed: In NP-combustion fuel and oxidizer enter the reaction zone in separate streams and are not mixed before combustion. (ex: diesel internal combustion Engine)

b) Premixed Combustion: In P-combustion fuel and oxidizer mixed before burning. and mixing done at the molecular level. Here the flame front propagates into the unburnt reactions as the combustion takes place. (ex: Gas Leak combustion explosions, pre-mixed gas turbine combustors)

c) Partially premixed Combustions: It is a model combustion process intended to be used in internal combustion engines of motorized vehicles and other automobiles. Here fuel and oxidizer are primarily mixed before combustion. They are premixed flames with non-uniform fuel-oxidizer mixtures. Its high specific power, high fuel efficiency, and low exhaust pollutions made it a promising technology.

2) Based on Phases

a) Volumetric Phase/ Gas phase

b) Fluid Phase

Simulation: 

In the present system, Ch4 and Air produce from the fuel and air inlets. and there will be combustion present due to the mixing of CH4 and air. There is a small inlet of fuel as compared to the air inlet and when they mixed up, they produce different pollutants such as CH4, H2O, CO2, N2, O2, NOx emissions & soot formations at the outlet. We also use the water with the Ch4 at fuel inlet by mole fraction to observe its effect.

1) Create the Model: 

2d sketch is prepared using ANSYS Spaceclaim.

Fuel Inlet: 10 MM

Air Inlet: 285 mm

2D Model

2) Meshing:

Generate the Mesh using Ansys Meshing.

 

Proximity Capture: On

Provide the name selections:

Set Up for Physics and Simulation:

Solver: Steady State

Type: Pressure Based Solver

Velocity Formulation: Absolute

2D Space: Axisymmetric 

Energy Equation: Enabled

Viscous Model: K-epsilon (k-epsilon model-standard with standard wall functions)

Species Model: Species Transport

Reactions: Volumetric

Mixture Material: Methane-air

Turbulence-Chemistry Interaction: Eddy -Dissipation

NOx Model: Thermal NOx

Soot Model: One-Step

Species Definition: Fuel-Ch4

                            Oxidant-O2

Boundary Conditions: 

            Air Inlet: Velocity- 0.5 m/s, Temperature- 300 K

            Species mass fraction at air inlet: O2-0.23

            Fuel Inlet: Velocity-80 m/s, Temperature-300 K

            Species Mass fraction at fuel Inlet: CH4= 1 

Initialization: Hybrid

Solution: After initializing the setup, run the simulation for 500 iterations.

Part 1:  Only Ch4 mass fraction is used as a fuel inlet.

Contours: 

                                           Ch4 Mass Fraction

                                              CO2 Mass Fraction

                                                  H2O Mass Fraction

                                                      N2 Mass Fraction

                                                         O2 Mass Fraction

                                                           Pollutant No Mass Fraction

                                                          Soot Contour Mass Fraction

                                                    Line Plot of Ch4 Mass Fraction

                                                         Line Plot of CO2 Mass Fraction

                                                           Line Plot of H2O Mass Fraction

                                                        Line Plot of N2 Mass Fraction

                                                      Line Plot of O2 Mass Fraction

                                                       Line Plot of Pollutant Mass Fraction

                                                       Line Plot of Soot Mass Fraction

Part 2:  H2O is mixed here with the inlet fuel in term s of mole fraction from 0% to 30 % to determine the effects on the number of pollutants and soot formation at Outlet.

a) 5 % H2O mixed with Inlet Fuel

Ch4-0.95

H2O-0.5

Contours: 

                                           Ch4 Mass Fraction

                                              CO2 Mass Fraction

                                                  H2O Mass Fraction

                                                      N2 Mass Fraction

                                                         O2 Mass Fraction

                                                           Pollutant No Mass Fraction

                                                          Soot Contour Mass Fraction

                                                    Temperature Volume Rendering

 

                                                       Line Plot of Pollutant Mass Fraction

                                                       Line Plot of Soot Mass Fraction

b) 15 % H2O mixed with Inlet Fuel

Ch4-0.85

H2O-0.15

Contours: 

                                           Ch4 Mass Fraction

                                              CO2 Mass Fraction

                                                  H2O Mass Fraction

                                                      N2 Mass Fraction

                                                         O2 Mass Fraction

                                                           Pollutant No Mass Fraction

                                                          Soot Contour Mass Fraction

                                                    Temperature Volume Rendering

 

                                                       Line Plot of Pollutant Mass Fraction

                                                       Line Plot of Soot Mass Fraction

c) 30 % H2O mixed with Inlet Fuel

Ch4-0.70

H2O-0.30

Contours: 

                                           Ch4 Mass Fraction

                                              CO2 Mass Fraction

                                                  H2O Mass Fraction

                                                      N2 Mass Fraction

                                                         O2 Mass Fraction

                                                           Pollutant No Mass Fraction

                                                          Soot Contour Mass Fraction

                                                    Temperature Volume Rendering

 

                                                       Line Plot of Pollutant Mass Fraction

                                                       Line Plot of Soot Mass Fraction

S. N.	Mole Fraction of H2O in fuel at Inlet (%)	Ch4  emissions	CO2  emissions	H2O  emissions	N2  emissions	O2  emissions	Pollutant emissions	soot emissions	Temperature at Outlet (K)	Max Temperature (K)
1	0	0.0363777	0.144102	0.117976	0.701536	8.5037e-06	0.000204321	0.0860365	2103.89	2272.72
2	5	0.0325334	0.144008	0.122374	0.701078	7.49728e-06	0.000197676	0.0790256	2111.05	2267.88
3	15	0.0244131	0.143862	0.131344	0.70037	1.12726e-05	0.000192044	0.0722096	2127.81	2251.97
4	30	0.0119365	0.143617	0.145134	0.699276	3.69293e-05	0.000190487	0.0601092	2153.89	2218.69

  

Conclusions: 

1. Combustion occurs due to the turbulent mixing of Ch4 and air at the inlet and produces pollutants and soot at the outlet.

2. As we add H2O with the inlet fuel, it will delay ignition and because of this, two fuels get more time for mixing. 

3. Mass fraction of Pollutant No (NOx) decreases with an increase in addition to H20 at the fuel inlet that is good as per government norms.

4. Mass fraction of soot increases with an increase in addition to H20 at the fuel inlet.

5. Temperature also reduces with an increase in addition to H20.