Week 10 - Simulating Combustion of Natural Gas.

Aim: The aim of the challeneg is to carry out combustion simulation and also consider the effect of water added in fuel on emmisions.

Objective: The objective of the challenege is to understand chemical kinetics and its use to simulate the process of combustion using computer program. The reactant and product are discritized in ode that the equations can solve to predict the intermediate and end products.

The challeneg has 2 parts :

part 1) To perform the simulation and plot the mass fraction vs its variation at different location marked by line probes.

Part 2) To consider the effect of water added in the range of 5% to 30% to the fuel, on the amount of NOx and soot emmision.

Geometry:

The geometry file is shared with us which is a complete combustor geometry. To minimixe our effort and time we have made following changes to the geometry and considered only a single section of it.

Geometry:

 

Using split body command, the geometry is modified.

The 3d geometry is converted to 2d plane by copying the surfaces into xy plane of new design and then using combine tool the final geometry is produced as below.

Meshing:

Solution setup:

 

Energy equation is turned on

Viscous model- K epsilon STD (considering turbulent flow) 

Boundary condition:

Air inlet:

Fuel inlet:

Species:

To capture the Nox and soot emission we need to turn on the Nox model (Thermal Nox) and one step soot model to capture soot formation.

Solution:

The simulation is ran using hybrid initialization with 1000 iterations.

Residual Plot:

Part 1:

I have created 4 line probes at interval of 120 mm to capture the variation of mass fraction of species.

Temperature contour plot:

 

Co2 contour plot and chart of its variation over line probe section

H2O contour plot and chart of its variation over line probe section

CH4 contour plot and chart of its variation over line probe section

 

N2 contour plot and chart of its variation over line probe section

  

O2 contour plot and chart of its variation over line probe section

 

NOx contour plot and chart of its variation over line probe section

 

Soot contour plot and chart of its variation over line probe section

 

The above plots and graphs shows that concentration of Nox and soot is high at the end of the combustion chamber, as combustion is completed in this section and soot which is biproduct of incomplete combustion is released. while nitogen which reacts at a particular temperature with oxygen to form NOx and hence concentration of Nox is less at the beginning of combustion chamber and it increases as temperature increases along the length of chamber.

Part 2) In order to reduce the concentration of NOx and Soot it is suggested to add water in the fuel to see its effect on emmision.

 

 1) Water added 10 % (ch4-0.9 H2O-0.1)

Temperature contour plot:

Co2 contour plot and chart of its variation over line probe section

 H2O contour plot and chart of its variation over line probe section

CH4 contour plot and chart of its variation over line probe section

N2 contour plot and chart of its variation over line probe section

O2 contour plot and chart of its variation over line probe section

 

NOx contour plot and chart of its variation over line probe section

Soot contour plot and chart of its variation over line probe section

 

 2) Water added 15 % (ch4-0.85 H2O-0.15)

Temperature contour plot:

 

Co2 contour plot and chart of its variation over line probe section

 

 H2O contour plot and chart of its variation over line probe section

 

CH4 contour plot and chart of its variation over line probe section

 

 

N2 contour plot and chart of its variation over line probe section

 

O2 contour plot and chart of its variation over line probe section

 

 

 

NOx contour plot and chart of its variation over line probe section

Soot contour plot and chart of its variation over line probe section

 

 

 3) Water added 20 % (ch4-0.8 H2O-0.2)

Temperature contour plot:

Co2 contour plot and chart of its variation over line probe section

 

H2O contour plot and chart of its variation over line probe section

 

CH4 contour plot and chart of its variation over line probe section

N2 contour plot and chart of its variation over line probe section

 

O2 contour plot and chart of its variation over line probe section

 

 

NOx contour plot and chart of its variation over line probe section

Soot contour plot and chart of its variation over line probe section

 

 

4) Water added 30 % (ch4-0.7 H2O-0.3)

 

Temperature contour plot:

 

Co2 contour plot and chart of its variation over line probe section

 

H2O contour plot and chart of its variation over line probe section

 

CH4 contour plot and chart of its variation over line probe section

N2 contour plot and chart of its variation over line probe section

 

O2 contour plot and chart of its variation over line probe section

 

NOx contour plot and chart of its variation over line probe section

 

Soot contour plot and chart of its variation over line probe section

 

Result:

Sr.no	Mass fraction of ch4	Mass fraction of H2O	Mass fraction of  NOx	Mass fraction of Soot
1	0.9	0.1	2.630e-04	6.285e-12
2	0.85	0.15	2.030e-04	4.848e-12
3	0.80	0.2	1.530e-04	6.047e-12
4	0.70	0.3	7.768e-5	1.497e-12

 Conclusion:

1. There is significant reduction in emmision of nox and soot with addition of water in fuel.

2. The variation of temperature in sections seperated by line probe shows that the outlet temperature has reduced noticibly and this has add upto incomplete combustion of fuel.

3. The evaporation temperature of water is lower than diesel due to which the water is evporated before ignition. This can explain the reduced flame temperature as water takes away heat before ignition.

4. The concentration of Nox has reduces considerably as compared to soot.

 

Reference:

https://www.tandfonline.com/doi/full/10.1080/10962247.2014.905508?scroll=top&needAccess=true

https://www.jstor.org/stable/44721075?read-now=1&seq=1#page_scan_tab_contents

https://www.afs.enea.it/project/neptunius/docs/fluent/html/ug/node643.htm

https://youtu.be/1ncUfEVcfbc