Week 10 - Simulating Combustion of Natural Gas.
Aim - Perform a combustion simulation on the combustor model and plot the variation of the mass fraction of the different species’ in the simulation using line probes at different locations of the combustor. Need to plot for CO2, H2O, CH4, N2, O2, NOx emissions & Soot formation.
Introduction –
The combustion analysis help to predict the important aspects like fuel species behaviour inside the cylinder, how chemical kinetic reactions happening inside the cylinder , temprature variation inside the cylinder , exhaust emmission study etc.
It plays major role to check the fuel injection timming which shows whether it is early injected or late injected inside the cylinder. because of these only we come to know how mixture is behaving inside the cylinder and we can estimate the factor that can effect the temprature , power , pressure and other factors.
Species study is important beacuse residual of same will lead to strignent emmission on exhaust side.As we know Nox generates due to high temprature inside cylinder , if in simulation we track the issue we can solve it in design stage only. Beacuse of these the combustion analysis is important.
Procedure –
Geometry –
Load geometry in space claim.
As we are having 3d geometry we will convert it to 2D . In step 1 we will cut the geometry in half with help of split body tool tab. In step 2 the quater geomtery will extracted with help of split body command again. Then select the faces and paste it in new design. With the 2D geomtery we will do study.
Mesh –
Load the geometry in the meshing tab. Generate the Mesh with element size of the 10mm. Proximity has to be on in sizing tab for better refienment near the fuel inlet .
Setup
Solver Type - Pressure based
velocity formation – Absolute
Time - Steady
2D - Axisymmetric
Model -
In this we will choose k epsilon standard for study .
Spieces - model has to be active for study of species . Select the mixture properties as methane - air -2 step .
NOX Model - Activate the NOX model for nox study during combustion.
Soot model - Activate the Soot model
Boundary Condition -
Inlet velocity – 0.5 m/s , Species o2 - 0.23 moles
Fuel velocity – 80 m/S , Species CH4 -1Mole
Initialization – Hybrid
No of iteration - 500
Outputs –
Residual Plots
Temprature contour
NOx emissions contour
Soot formation contour
CH4 contour
o2 contour
C02 contour
N2 contour
H2O contour
CO2 Plot
H2O Plot
CH4 Plot
N2 Plot
O2 Plot
Case 2
As we know nox basically happens due to high cylinder temprature, and there is techinque in which water is injected directly to cylinder or in air intake to reduce the cylinder temprature , so nox content will reduce. In our study we change the mole Proportion in fuel species to check the effect of water on nox formation during combustion .
water content in the fuel 10%
Fuel inlet - Species Ch4 - 0.9 Mole
H20 - 0.1 Mole
Outputs –
Residual plot
Temprature contour
NOx emissions contour
Soot formation contour
CO2 Plot
H2O Plot
CH4 Plot
N2 Plot
O2 Plot
Case 3 water content in the fuel 20%
Species - Ch4 - 0.8 Mole
H20 - 0.2 Mole
Outputs –
Residual plots
Temprature contour
NOx emissions contour
Soot formation contour
CO2 Plot
H2O Plot
CH4 Plot
N2 Plot
O2 Plot
Case 4
water content in the fuel 30%
Species Ch4 - 0.7 Mole
H20 - 0.3 Mole
Outputs –
Temprature contour
NOx emissions contour
Soot formation contour
CO2 Plot
H2O Plot
CH4 Plot
N2 Plot
O2 Plot
Remarks -
1. In case 2 the presence of ch4 and nox is higher than case 3 and case 4.
2. It is observed from the temprature conotur , the cylinder temprature is reduced from case 2 case 4 as we increased the water content in fuel . As the temprature reduces the nox formation also reduces.
3. With the nox mass fraction it is observed that the co2 farction is also dropped from the case 2 to case 4.
Conclusion -
It is always necessary to check the all aspects because if we go with too lean mixture it will lead to the formation of nox. and with rich mixture will encounter with hc and more soot particle. after prefect combustion analysis only we can make out is it necessary to deploy the aftertretment method for particular combustion case or it get rectify in design point only.