CFD Combustion Study
Combustion
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 combustion, new chemical substances are created from the fuel and the oxidizer. These substances are called exhaust. Most of the exhaust comes from chemical combinations of fuel and oxygen. When a hydrogen-carbon-based fuel (like methanol) burns, the exhaust includes water (hydrogen + oxygen) and carbon dioxide (carbon + oxygen). But the exhaust can also include chemical combinations from the oxidizer alone. If the gasoline is burned in air
which contains 21% oxygen and 78% nitrogen, the exhaust can also include nitrous oxides (NOX, nitrogen + oxygen).
The temperature of the exhaust is high because of the heat that is transferred to the exhaust during combustion. Because of the high temperatures, exhaust usually occurs as a gas, but there can be liquid or solid exhaust products as well. Soot, for example, is a form of solid exhaust that occurs in some combustion processes.
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 keeps burning.
To summarize, for combustion to occur three things must be present: a fuel to be burned, a source of oxygen, and a source of heat. As a result of combustion, exhausts are created and heat is released. You can control or stop the combustion process by controlling the amount of the fuel available, the amount of oxygen available, or the source of heat.
Stoichiometric
is also called Theoretical Combustion is the ideal combustion process where fuel is burned completely.
Complete combustion is a process burning all the carbon (C) to (CO2), all the hydrogen (H) to (H2O) and all the sulphur (S) to (SO2).
With unburned components in the exhaust gas such as C, H2, CO, the combustion process is uncompleted and not stoichiometric.
The combustion process can be expressed as
`C_xH_y+zO_2+3.77zN_2rarrxCO_2+y/2H_2O+3.77zN_2`
`z=x+1/4y`
for example in our case for complete combustion of fuel methane(`CH_4`) & oxidizer air
`x=1,y=4,z=(1+8/4)=3`
`CH_4+3O_2+11.31N_2rarrCO_2+2H_2O+11.31N_2`
the above for complete combustion we need to obtain stoichiometric combustion to neglate pollutant
Investigation study on combustion of methane
base model
cross-section view
The extracted study area for analysis
symmetry on the axis so the volume extracted quarter portion for analysis along x-y plane
Boundary condition asign
Mesh
setup
steady-state
k-e-Turbulent
combustion input as
species-Transport
methane-air-2step
thermal
turn on
no pollutant-thermal
soot pollutant-one step
Boundary condition
Part I
case1
fuel `CH_4` & air `(0_2+3.76N_2)`
case 1
fuel inlet
v=80m/s
species
Mass molar fraction
`CH_4=1`
air inlet
v=0.5 m/s
`0_2=0.21`
Part II
case2,3,4 parametric study
fuel (methane `(CH_4)`+water `(H_2O)` ) & automizer (air `(0_2+3.76N_2)`)
fuel with water ranges from 8,16,24 % mass fraction
clear all;
close all;
clc;
%mass fraction for fuel
air=(8:8:24)./100;
fuel=1.-air;
% mole fraction for air
t_a =2+7.52
O_2=2/t_a;
N_2=7.52/t_a;
% mass fraction for air
WO_2=2/t_a;
WN_2=7.52/t_a;
MO_2=WO_2*O_2;
MN_2=WN_2*N_2;fuel inlet set parameter study as variable species as `CH_4 & H_2 0`
Result
case 1
Monitors
Residuals
Area weighted average of Nox at the outlet
Area weighted average of soot at the outlet
Area weighted average of Temperature at the outlet
Animation
Temperature contour
Contours
Temperature
CO2
H2O
CH4
N2
O2
NOx
soot
Turbulent Eddy Dissipation
which clearly define the region where Turbulent Eddy Dissipation region where combustion start automatically
graph
Case 1
Temperature
CO2
H2O
CH4
N2
O2
NOx
soot
PART II
case 2
Monitors
Residuals
Area weighted average of Nox at the outlet
Area weighted average of soot at the outlet
Area weighted average of Temperature at the outlet
Contours
Temperature
CO2
H2O
CH4
N2
O2
NOx
soot
Graph
Temperature
CO2
H2O
CH4
N2
O2
NOx
soot
case 3
Monitors
Residuals
Area weighted average of Nox at the outlet
Area weighted average of soot at the outlet
Area weighted average of Temperature at the outlet
Contours
Temperature
CO2
H2O
CH4
N2
O2
NOx
soot
Graph
Temperature
CO2
H2O
CH4
N2
O2
NOx
soot
case 4
Monitors
Residuals
Area weighted average of Nox at the outlet
Area weighted average of soot at the outlet
Area weighted average of Temperature at the outlet
Contours
Temperature
CO2
H2O
CH4
N2
O2
NOx
soot
Graph
Temperature
CO2
H2O
CH4
N2
O2
NOx
soot
Area weighted average of T, Nox, soot at the outlet for all cases
code
clear all;
close all;
clc;
% Read output result
% nodal point reading
re=readtable(\'compalloutput.csv\');
D=re.Design;
Ta=re.P7;
Noxa=re.P1;
soota=re.P6;
subplot(2,2,[1,2]);
plot(D,Ta,\'color\',\'r\',\'Marker\',\'*\',\'LineWidth\',1);
title(\'Cases vs Temperature\');
xlabel(\'Design cases\');
ylabel(\'Temperature\');
subplot(2,2,3);
plot(D,Noxa,\'color\',\'m\',\'Marker\',\'*\',\'LineWidth\',1);
title(\'Cases vs Nox\');
xlabel(\'Design cases\');
ylabel(\'Nox in ppm\');
subplot(2,2,4);
plot(D,soota,\'color\',\'b\',\'Marker\',\'*\',\'LineWidth\',1);
title(\'Cases vs soot\');
xlabel(\'Design cases\');
ylabel(\'soot in ppm\');Graph
conclusion
where case 4 gives less temperature, Nox, the output after combustion (design point3);
where soot is more than case 2
note
graph taken value at different value equally which is shown in figure
Source
https://www.grc.nasa.gov/WWW/K-12/airplane/combst1.html
https://en.wikipedia.org/wiki/Combustion
https://www.engr.colostate.edu/CBE101/topics/exercise_solutions/exercise_mole_fraction_to_mass_fraction.html