Week 10 - Simulating Combustion of Natural Gas.

AIM - 

To perform simulation combustion of Natural gas.

OBJECTIVE - 

1. To perform combustion simulation of a combustor model and plot the mass fraction of different species.
2. To perfrom analysis for the reduction of pollutants like NOx and soot by adding water in the fuel.

SOLVING AND MODELING APPROACH 

1. Create a desired 2D geometry from the desgin of the 3D combustor model. 
2. Use this 2D geometry to create a mesh with a good quality.
3. load the mesh in the fluent solver and apply a proper setup according to the physics of the combustion process.
4. Here for combustion add a species transport model for a 2 step methane-air mixture and turn on NOx and soot formation in the model.
5. Initialize and Run the simualtion for appropiate number of iterations until convergence and plot the desired results.

 

PRE-PROCESSING - 

GEOMETRY :

Create a new standalone system in workbench.

Import the 3D-combustor model design in SpaceClaim component and open Spaceclaim.

Now first create a single component form the 3 components for that use combine tool in the design tab to combine the first 2 components after that combine the third component with the component to create a single body.

after combine the first two part the design look like as shown below

Now combine the part in Component3 with the part in component1 

now we get a single part in Component1 as shown below

now create 3 planes by clicking at the origin using the create plane tool

now using the split body tool we will cut this body in half along the XY plane and remove the part on the front

now once again using the split body tool cut the bottom half of the body using along the XZ plane and remove the bottom half component

now select the face on the XY plane and copy it , as that face is the desired 2D geometry and so open a new spaceclaim file and paste the copied face using ctrl+v shortcut. 

the new 2D geometry look like a surface as shown below 

now the desired geometry is ready so save it with appropiate name.

Close spaceclaim and reset the geometry component of the workbench system.

now import the new 2d geometry in the geometry component.

once check if proper 2d model is loaded by opening the geometry component.

Now this geometry is loaded in the ansys mesher and is ready for meshing

 

MESHING :

Open the meshing component.

first add named selection to all the parts as shown here.

 

now to create a mesh give body sizing to the body of about 4mm and turn on the proximity with default features

now generate the mesh . the generate mesh is shown below

zoomed near the inlet region we can see the area near the poximity is capture properly.

the mesh has a good element quality of above 0.4

now the mesh is ready for the fluent setup to update the mesh and it will be loaded in the fluent setup component.

 

SETUP :

For setting up the solution a ressure based solver with steady time is used the 2D space is set to axisymmetric.

Turn on the energy equation 

for setting the viscous model select the standard k-epsilon model with standard wall functions.

Now for setting up the combustion reaction select the species setting in the models and turn on the species transport with reactions a volumetric also turn on inlet diffusion and diffusion energy source. here for the mixture materail select the methane-air 2 step form the dropdown as this will help us add water with the fuel at inlet. Eddy dissipation turbulance chemistry is used so that ignition starts when there is appropiate amount of fuel and air in a control volume elements. 

For the NOx model turn on the thermal and Prompt NOx pathways and select the fuel species as ch4.

For the soot model select one step soot model and for the species definition select ch4 for fuel and o2 for oxidant.

Now to set up the boundary conditions for the air-inlet type is set to velocity inlet with inlet velocity of 0.5 m/s and temperature of 300k also from the species tab set the o2 mass fraction as 0.23 

for the fuel inlet set the velocity as 80 m/s and keep the temperature as 300K . also here input parameters are introduced for the mass fraction of species of ch4 and h2o so that a parametric study can be done with adding water so as to reduce the Nox and soot pollutants that are formed during the combustion process.

the ch4 parameter 

h2o inlet parameter

form the above parameters we can see that the value of mole fraction ch4 and h2o are set to 1 and 0 respectively for this case.

to see the fromation of NOx and soot at the outlet we will be adding report definitions of the aera weighted average of the mass fraction of NO and soot respectives and also set then as outlet parameters so it will be easy to observe the change in the pollutants with respesct to change in the 

setting of repoert definition for NOx:

settings for the repoert definition for soot:

now or setup is ready and we will initialize and then run the simualtion for about 500 iterations so that the residuals gets stablised.

 

 

RESULTS :

PART 1:

Residuals:

Report plot for the mass fraction of NOx at outlet:

Report plot of the mass fraction of Soot:

From the above plots we can see that the residuals and the report definitions are stabilized so we consider our solution to be converged.

Various contours and plots for our simulation are shown below.

Temperature contour:

NOx contour:

Soot contour:

 

Plots:

Mass Fraction of NOx:

Mass fraction of Soot:

Mass fraction of ch4:

Mass fraction of co:

Mass Fraction of co2:

Mass Fraction of h2o:

Mass Fraction of n2:

Mass Fraction of o2:

From the above plots we can see that:

Maximum level of pollutant NO and soot froms midway inside the combustor then near the outlet it reduces.

The mass fraction of ch4, co , n2 and o2 reduces over the length of the combustor.

The mass fraction of h2o and co2 increases over the length of the combustor.

 

 

PART 2:

Now the results of the Part 2 where water is added along with the ch4 in the fuel inlet are discussed.

For this analysis parametric study was performed and the level h2o in the fuel was increased from 5% to 30% by mole.

Case 1: For 5% h2o:

Temperature contour:

NOx contour:

Soot contour:

 

Plots:

Mass Fraction of NOx:

Mass fraction of Soot:

Mass fraction of ch4:

Mass fraction of co:

Mass Fraction of co2:

Mass Fraction of h2o:

Mass Fraction of n2:

Mass Fraction of o2:

 

 

Case 2 : For 10% h2o:

Temperature contour:

NOx contour:

Soot contour:

 

Plots:

Mass Fraction of NOx:

Mass fraction of Soot:

Mass fraction of ch4:

Mass fraction of co:

Mass Fraction of co2:

Mass Fraction of h2o:

Mass Fraction of n2:

Mass Fraction of o2:

 

 

Case 3 : For 15% h2o:

Temperature contour:

NOx contour:

Soot contour:

 

Plots:

Mass Fraction of NOx:

Mass fraction of Soot:

Mass fraction of ch4:

Mass fraction of co:

Mass Fraction of co2:

Mass Fraction of h2o:

Mass Fraction of n2:

Mass Fraction of o2:

 

 

Case 4 : For 20% h2o:

Temperature contour:

NOx contour:

Soot contour:

 

Plots:

Mass Fraction of NOx:

Mass fraction of Soot:

Mass fraction of ch4:

Mass fraction of co:

Mass Fraction of co2:

Mass Fraction of h2o:

Mass Fraction of n2:

Mass Fraction of o2:

 

 

Case 5 : For 25% h2o:

Temperature contour:

NOx contour:

Soot contour:

 

Plots:

Mass Fraction of NOx:

Mass fraction of Soot:

Mass fraction of ch4:

Mass fraction of co:

Mass Fraction of co2:

Mass Fraction of h2o:

Mass Fraction of n2:

Mass Fraction of o2:

 

 

Case 6 : For 30% h2o:

Temperature contour:

NOx contour:

Soot contour:

 

Plots:

Mass Fraction of NOx:

Mass fraction of Soot:

Mass fraction of ch4:

Mass fraction of co:

Mass Fraction of co2:

Mass Fraction of h2o:

Mass Fraction of n2:

Mass Fraction of o2:

 

 

Also from the following image of the input and output parameters it is clear that as the water content in the fuel increases the pollution in the from of NOx and soot decreases.

 

following charts show the nature of the reduction for the pollutant NO and soot

 

 

CONCLUSION :

1. By create a combustion simulation in fluent, it is earier to analyse a combustion process.
2. Combustion reaction involves various species and mass fraction of various species can be calculated at eact loaction in the combustor.
3. From the methane-air 2 step combustion reaction we can get the idea of how a combustion inside a combustor takes place.
4. To reduce the pollution that is generate in the methane air combustion reaction we add water and by adding the water content in the fuel we can see that the amount of pollutants generated due to the reaction decreases.
5. By increasing the water content in fuel the maximum temperature generated inside the combustor decreases.
6. NOx and soot are harmful to human health and should be minimised, it is advisable to have some water content of around 25-30% by mole fraction in the fuel so as to get the least amount of pollutants.