Challenge

Week 10 - Simulating Combustion of Natural Gas.

SSukhesh Naik K I30 Mar 2023

CFD

Objective :

In this challenge,

Part I - We 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. We will plot for CO2, H2O, CH4, N2, O2, NOx emissions & Soot formation.
Part II -In this part, We add the water content in the fuel from 5% to 30% by mole and observe the effect of it on the results. We provide line plots and contours to prove our claim.We use the parametric study approach for this part.

Introduction :

Let us first have some introduction about chemical kinetics and some basics of combustion simulation in Ansys fluent and then continue to our main challenge part.

Chemical kinetics :

Whenever if you observe any reaction it is seemed to be happened in one step but in reality no reaction takes place in a single step.Let us assume air and fuel are mixing.The entire reaction is maintained at particular operating temperature and pressure.In this conditions when fuel and air reacts it can lead to some inermediate reactions ,for example fuel you can have fuel decompose and and combine with oxygen in the air or any intermediate species.Reaction between these intermediate species is called as the elementary reaction.
Several such reactions takes place and the entire set of reactions and intermediate species is what we refer as reaction mechanism.
The study of this entire reaction mechanism subjected to different operating conditions is what we called as chemical kinetics.
Whenever we use chemical kinetics solver what we do is ,we try to simulate entire set of elementary reactions and the intermediate species and the different paths that the system take to predict the final products.
This reaction path way is a function of operating conditions and time as well meaning the final products will be completely depend upon the operating conditons and how long the combustion actually taking place in real life. So if your reaction mechanisms and pathays have been computed accurately then this process will give you accurate representation of thermodynamic properties after the combustion process.
Actually what happens is this entire reaction mechanism(the date of reactants and products) is imported to the computer solver .These can be known by experimnetal reactions.This entire data is load in to the computer.So that it can simulate all the process ,so that we can calculate thermodynamic properties after combustion(for emmisions).But for our case the data is already inbuit in the fluent solver.
The chemical kinetics solver is important because the reaction pathways determine

Combustiobn start/ignition delay.
Heat release rate(Rate of work done).
Emmisions (How and how much pollutants are produced ).

Basics of combustion simulation :

Possible types of combustion (done using Ansys fluent) :

1)Based on mixing :

Non - premixed (example - Direct injection , late injection)
Pre - mixed (example - Carburetor,burner)
Partially pre - mixed( In between premixed and non - premixed )

2)based upon phase (where the combustion is taking place) :

Fluid phase - If combustion taking place in fluid phase comes under this model and called it as volumetric reactions.For gas-phase reactions, the reaction rate is defined on a volumetric basis and the rate of creation and destruction of chemical species becomes a source term in the species conservation equations
Wall surface -If combustion taking place on wall those type of reactions are called surface reactions. For surface reactions, the rate of adsorption and desorption is governed by both chemical kinetics and diffusion to and from the surface. Wall surface reactions thus create sources and sinks of chemical species in the gas phase, as well as on the reacting surface.
Particle surface - Lagrangian particles on which reaction taking place is called particle surface reaction .Example - coal combustion .
Porous region - Where reaction taking place in a porous region .Example - After treatment systems ,where we have a porous region where there is lot of pressure drop but also on top of it there is also reaction taking place.
ANSYS FLUENT can model the mixing and transport of chemical species by solving conservation equations describing convection, diffusion, and reaction sources for each component species. Multiple simultaneous chemical reactions can be modeled, with reactions occurring in the bulk phase (volumetric reactions) and/or on wall or particle surfaces, and in the porous region. Species transport modeling capabilities, both with and without reactions.

Theory :

In CFD in general we solve equations for mass , momentum and energy and in addition if there is turbulence we have equations for turbulence parameter(k,epsilon,omega).
In addition to this if we have combustion going on we have reactants and products , together we call them as species .We need to solve an another equation called species transport equation for each species.
When you choose to solve conservation equations for chemical species, ANSYS FLUENT predicts the local mass fraction of each species, , through the solution of a convection-diffusion equation for the th species. This conservation equation takes the following general form:

where is the net rate of production of species by chemical reaction,

The reaction rates that appear as source terms in Equation (combustion source calculation ) are computed in ANSYS FLUENT, for turbulent flows, by one of the models:

Laminar finite-rate model: The effect of turbulent fluctuations are ignored, and reaction rates are determined by Arrhenius kinetic expressions.Here using chemical kinetics we basically calculate rate of production of each species ,we solve ODE for each species which is a costly process.Accurate but slow.
Eddy-dissipation model: Reaction rates are assumed to be controlled by the turbulence, so expensive Arrhenius chemical kinetic calculations can be avoided. The model is computationally cheap, but, for realistic results, only one or two step heat-release mechanisms should be used.Here the idea is , it is based upon turbulence quantities, it calculate mixing time scale based on turbulence quantities k and epsilon and combustion happens based upon the time scale. It does not need any type of ignition source ,ignition happens automatically based upon mixing time scale .It leads to ,mixing is burnt problem .It predicts results even where there is no combustion happens in reality. When you want to calculate ignition delay it shows wrong results as here there will be no delay in ignition , As mixture mixes it burns instantly.So even they are very fast it requires user experience .
Finite rate / eddy dissipation model : It is combination of both finite rate chemistry and eddy-dissipation model. Used to prevent instantaneous burning (mixed is burnt problem ) .Here we solve both and models and based upon flow condition which ever rate is going to have low reaction rate , the solver is going to pick that . IN certain areas where ignition is going to take place and ignition delay is important then model would ideally choose finite rate chemistry its much more suited for that and it may predicits low reaction rate and in things where turbulence play a much more major role , your reaction rate predicted by the eddy dissipation model is smaller and it picks that and let you know how reactants are getting converted to products .
Eddy-dissipation-concept (EDC) model: Detailed Arrhenius chemical kinetics can be incorporated in turbulent flames. Note that detailed chemical kinetic calculations are computationally expensive.

And is the rate of creation by addition from the dispersed phase plus any user-defined sources. An equation of this form will be solved for species where is the total number of fluid phase
chemical species present in the system. Since the mass fraction of the species must sum to unity, the th mass fraction is determined as one minus the sum of the solved mass fractions. To minimize numerical error, the th species should be selected as that species with the overall largest mass fraction, such as N when the oxidizer is air.
In Equation J_i is the diffusion flux of species i , which arises due to gradients of concentration and temperature.
Diffusion is a combination of laminar and turbulent diffusion.
By default, ANSYS FLUENT uses the dilute approximation (also called Fick's law) to model mass diffusion due to concentration gradients.Here in dilute approximation we are looking at diffusion coefficient for each and every species how that species diffuses into entire mixture.
For certain laminar flows, the dilute approximation may not be acceptable, and full multicomponent diffusion is required(Here we see diffusion coefficient of how each species react wih other species). In such cases, the Maxwell-Stefan equations can be solved.
To explain shortly what happens is in each and every cell first the combustion model(finite ratechemistry/eddy dissipation model )is solved then based on that output (mole fractions etc.) obtained the species transport is solved..!

Simulation part :

Part I -

Here we 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. We will plot for CO2, H2O, CH4, N2, O2, NOx emissions & Soot formation.

Solution procedure :

We wil simulate with the steady state.We will not simulate the 3D case because of the limitation af academic license.So, we are going to simulate 2D part . For solving we follow the below procedure :

1)Geometry setup

2)Meshing

3) Problem setup

4)Solving

5)Post processing

1) Geometry setup :

In fluent module we have to open spaceClaim and then import the 3D geometry.The below image is the 3D view of gear box.

If you take the entire geometry and simulate then , this is going to be much more expensive to simulate as we are going to have large number of cells.So we will perform 2D axisymmetric simulation .
Now we have to split this body into half to extract the 2D geometry right in the middle plane.
First create planes by selecting plane in design and click on origin ,which creates 2 planes X,Y and Z planes.
We use the split body command in spaceclaim which is under design to split the geometry it into two parts by selecting the geometry and plane and delete the other part.
Now we can see the interior sectional view.Now if you select the top part of the remaining geometry you can see the yellow top face part,which is what we want .Inorder to extract that area we need to select that face and copy and open new design and then paste it and save it.
Now reset the setup and import the 2D saved geometry and open the meshing module.
The below image is the final 2D geometry that we simulate.

Then we have to cobine all the three parts using combine in design, then we get a single geometry .The below image is the final 2D geometry that we simulate.

2) Meshing :
For meshing we open Meshing,where we mesh our model so that we can use it to solve our model.
In general it is always good to name the surfaces before meshing. So let us do that.
We have to give the name for air inlet , fuel inlet , walls , outlet , axis.
We first select the axis (which is the X-axis line , to perform axisymmetric simulation the axis should be along X-axis ).using the edge selection so that we can provide name for our entity selection.
steps : select the axis by using edge selection -> then press N -> name the selection as axis.
In the similar way we name the air inlet , fuel inlet , walls , outlet.
Then next thing to do is to mesh.We click the mesh option in the side then give element size as 0.0009 m and also select capture proximity to on and generate mesh.
The below image is the final view of mesh .

Now we have to check the quality of the mesh.
steps : mesh ->quality-> mesh metric-> Element quality
Here our minimum quality is around 48 %,so we can move further for our solution setup.

3) Problem Setup :

To set up our problem we open Fluent and setup inital conditions , boundary conditions material etc.,
We first start with checking the mesh quality using perform mesh check under domain.If no error is seen in console window then we can proceed further.
Then we move move to setting up physics where we select the solver,viscous model etc.
We use a steady state pressure based solver to solve our model.

Physics :

General :

Solver type : pressure based

2D space : Axisymmetric

time : steady state

velocity formulation : absolute

Energy : Enable

models :

viscous model : K-epsilon standard model with standard wall functions.

Species model :

model : species transport

Reactions : select volumetric

options : select inlet diffusion and diffusion energy source

In mixture properties ,

mixture material : methane- air-2step

For, turbulence chemistry interaction : select eddy - dissipation

We also select one-step soot model to calculate the soot formation and also select NOx model and enable thermal NOx in NOx model to calculate the NOx formation.

Zones :

Cell zone : In cell zones you will see fluid volume,it is of type fluid .

Boundary conditions :We will basically give our boundary conditions here such as inlet and outlet conditions.

we give,

air inlet :

momentum : velocity magnitude = 0.5 m/sec

thermal : temperature = 300 k

species : under species mass fractions give values ,for O2 = 0.23 and for remaining all species give zero (0) .

fuel inlet :

momentum : velocity magnitude = 80 m/sec

thermal : temperature = 300 k

species : under species mass fractions give values ,for CH4 = 1 and for remaining all species give zero (0) .

Outlet :

pressure outlet : gauge pressure = 0 pa

For wall keep it as wall and foraxis keep it as axis .
In real life if you have burners like this ,the air and fuel only burn if you have an ignition source but in our case since we are using eddy dissipation model the fuel burning rate is not governed by the presence of energy source , it is governed by the presence of turbulence .If your k and epsilon are high enough its going to give a reasonable turbulence mixing time scale , based on which ansys fluent automatically calculates a time for combustion.
Now we are almost ready to run the simulation ,but as the solution is being solved we want to moniter flow quantities while the solution is being generated.
Here we want to moniter the temperature and emmisions( NOx and soot ) at the outlet .
NOx - Nitrogen Oxides are a family of poisonous, highly reactive gases which causes pollution. These gases form when fuel is burned at high temperatures .
Soot - A deep black powdery or flaky substance consisting largely of amorphous carbon, produced by the incomplete burning of organic matter which causes pollution .
steps to create report for the temperature and emmisions ,

steps : solution -> under definitions -> select new -> select the surface report ->area weighted average-> and select the filed variable as temepreature and at surfaces we select outlet-> check report file and report plot ->then okay.

In the similar way we also create report for NOx and soot formation at the outlet.
we also want to create temperature contour and mass fraction contour for CO2, H2O, CH4, N2, O2 , NOx and soot . But before that we have to initialize the domain.We use the Hybrid initialization , this provides a very good initial condition .
Then we create a temperature contour on the domain.

For contour -> go to solutions tab -> graphics -> contour -> in contour of : select temperature under that select static temperature -> select all the surfaces and press save /display.

Then you can see the initial values for temperature which is obtained by initialization which is not the final one.
In the similar way we also create contour for mass fraction of CO2, H2O, CH4, N2, O2 , NOx and soot.
we also want to create amination so that as the solution is changing as the function of time we can see the physics behind it.
Then we create animation so that we can see how the temperature is varying when problem is being solved.

For that,

steps : Activities -> solution animation -> select temperature contour ,which we have created -> preview -> set the required view and use active -> and name it as temperature animation.

In solution method we use Coupled .

4) Solving :

Give iterations such that your solution is converged.
Run the simulation until the steady state is reached.Lets run for 500 iterations , we can find the solution is converged.
Then we can see that in the residual plot that residuals dropping and reach a steady state after 275 iterations.
Then let us post process the results.

5)Post processing :

Residuals plot :

You can observe from the below plot the residuals has converged after 275 iterations.

Area weighted average of temperature at the outlet plot :

To know the area weighted average of temperature at the outlet you can directly view in the console window or by opening the report definitions and clicking on compute.You will observe the value of temperature at the outlet as 1610.877K .

Area weighted average of mass fraction of pollutant NO at the outlet plot :

To know the area weighted average of mass fraction of pollutant NO at the outlet you can directly view in the console window or by opening the report definitions and clicking on compute.You will observe the value of mass fraction of pollutant NO at outlet as 0.0001021756 .

Area weighted average of mass fraction of soot at the outlet plot :

To know the area weighted average of mass fraction of soot at the outlet you can directly view in the console window or by opening the report definitions and clicking on compute.You will observe the value of mass fraction of soot at outlet as 1.8160752e-05

Temperature contour :

In the below contour you can see the temperature distribution in the combustor.

Mass fraction contour of CO2 :

In the below contour you can see the mass fraction contour of CO2 in the combustor.

Chart showing variation of the mass fraction of CO2 :

Chart showing variation of the mass fraction of CO2 along Y-direction at different locations (along X-axis) of the combustor.

Mass fraction contour of H2O :

In the below contour you can see mass fraction contour of H2O in the combustor.

Chart showing variation of the mass fraction of H2O :

Chart showing variation of the mass fraction of H2O along Y-direction at different locations (along X-axis) of the combustor.

Mass fraction contour of CH4 :

In the below contour you can see mass fraction contour of CH4 in the combustor.

Chart showing variation of the mass fraction of CH4 :

Chart showing variation of the mass fraction of CH4 along Y-direction at different locations (along X-axis) of the combustor.

Mass fraction contour of N2 :

In the below contour you can see mass fraction contour of N2 in the combustor.

Chart showing variation of the mass fraction of N2 :

Chart showing variation of the mass fraction of N2 along Y-direction at different locations (along X-axis) of the combustor.

Mass fraction contour of O2 :

In the below contour you can see mass fraction contour of O2 in the combustor.

Chart showing variation of the mass fraction of O2 :

Chart showing variation of the mass fraction of O2 along Y-direction at different locations (along X-axis) of the combustor.

Mass fraction contour of NOx :

In the below contour you can see mass fraction contour of NOx in the combustor.

Chart showing variation of the mass fraction of NOx :

Chart showing variation of the mass fraction of NOx along Y-direction at different locations (along X-axis) of the combustor.

Mass fraction contour of soot :

In the below contour you can see mass fraction contour of soot in the combustor.

Chart showing variation of the mass fraction of soot :

Chart showing variation of the mass fraction of soot along Y-direction at different locations (along X-axis) of the combustor.

Part II -

In this part, We add the water content in the fuel from 5% to 30% by mole and observe the effect of it on the results. We provide line plots and contours to prove our claim.We use the parametric study approach for this part.

As you must have observed from the above simulation, the Nox and soot is getting formed at the outlet of the combustor. Such formation has harmful effects on the environment and humans. The stringent government norms also demand the least formation of Nox and soot and to satisfy those requirements,we need to check the effect of adding the water in the fuel.
In this part, we will add the water content in the fuel from 5% to 30% by mole and observe the effect of it on the results and we also provide line plots and contours to prove your claim. We use the parametric study approach for this part.So let us start by setting up simulation.
Everything is same as previous case from the point of geometry and mesh.
Comming to set up the change comes here .As we are performing parametric study we need to set input parameters and output parameters to perform the parametric study.
We need to keep water and CH4 mass fraction as input parameters and want to moniter the temperature and emmisions ( NOx and soot ) at the outlet .So , we keep area weighted average of them as output parameters.
To parametrize the mass fractions of water(H2O) and CH4 input parameters we have to follow below steps,

steps : while giving bounday conditions to fuel inlet , under species mass fractions instead of giving values for mass fractions ,click on the side options of CH4 and select new input parameter->then name it -> click ok .

In the same way set mass fractions of H2O as new input parameter and for remaining all species give zero (0) .
To parametrize temperature and emmisions ( NOx and soot ) as output parameter we have to follow below steps,

In the similar way we also create report for NOx and soot formation at the outlet and select/mark the output parameter.This creates area weighted average of NOx and soot formation as output parameter.
In the same way you can also parametrize other quantities if needed.
Remaining everything is same as previous case.
Now if you close the fluent and go back to workbench you will observe a new option has added ie., parameters.
If you double click on it you will find a new tab is opened as shown in the below image.

In the left side tab you will find the input and output parameters for current design point and in the right side tab you will see a table consisting of design point with input and output parameters .
There you can also observe a empty row to create new design point.There we can give input mass fractions for water(H2O) and CH4.
We create 4 design point with water content in the fuel from 5% to 30%(mass fractions for water(H2O)as 0 , 0.05 ,0.15, 0.25, 0.3) as there is already one design point with water percent 0 .
To view the updated geometry ,meshing ,solution data ,animations ,contours etc. of newly created design point you need to click on the retain data.
Now we need to update all the design point by clicking on the update all design points.
After updating the design points you will observe the values of temperature and emmisions ( NOx and soot ) in the output column for all the created design points as shown in the below image.

Then after that you can see the individual solution date by right click on the name of design point and selecting set as current.Then you can go to workbench and view the result of the required design point.
- Now let us plot the contour of temperature distribution , mass fraction of NOx and soot and also Chart showing variation of the mass fraction of soot and NOx along Y-direction at different locations (along X-axis) of the combustor for all the cases and then conclude by observing the effect of addition of water on results.
Case -1 : Mass fraction of CH4 = 1 and water(H2O) = 0 :-

Temperature contour :
In the below contour you can see the temperature distribution in the combustor.
Mass fraction contour of NOx :
In the below contour you can see mass fraction contour of NOx in the combustor.
Chart showing variation of the mass fraction of NOx :
Chart showing variation of the mass fraction of NOx along Y-direction at different locations (along X-axis) of the combustor.
Mass fraction contour of soot :
In the below contour you can see mass fraction contour of soot in the combustor.
Chart showing variation of the mass fraction of soot :
Chart showing variation of the mass fraction of soot along Y-direction at different locations (along X-axis) of the combustor.
Case -2 : Mass fraction of CH4 = 0.95 and water(H2O) = 0.05 :-

Temperature contour :
In the below contour you can see the temperature distribution in the combustor.
Mass fraction contour of NOx :
In the below contour you can see mass fraction contour of NOx in the combustor.
Chart showing variation of the mass fraction of NOx :
Chart showing variation of the mass fraction of NOx along Y-direction at different locations (along X-axis) of the combustor.
Mass fraction contour of soot :
In the below contour you can see mass fraction contour of soot in the combustor.
Chart showing variation of the mass fraction of soot :
Chart showing variation of the mass fraction of soot along Y-direction at different locations (along X-axis) of the combustor.

Case -3 : Mass fractions of CH4 = 0.85 and water(H2O) = 0.15 :-

Temperature contour :
In the below contour you can see the temperature distribution in the combustor.

Mass fraction contour of NOx :
In the below contour you can see mass fraction contour of NOx in the combustor.
Chart showing variation of the mass fraction of NOx :
Chart showing variation of the mass fraction of NOx along Y-direction at different locations (along X-axis) of the combustor.
Mass fraction contour of soot :
In the below contour you can see mass fraction contour of soot in the combustor.

Chart showing variation of the mass fraction of soot :

Chart showing variation of the mass fraction of soot along Y-direction at different locations (along X-axis) of the combustor.

Case -4 : Mass fractions of CH4 = 0.75 and water(H2O) = 0.25 :-

Temperature contour :
In the below contour you can see the temperature distribution in the combustor.
Mass fraction contour of NOx :
In the below contour you can see mass fraction contour of NOx in the combustor.
Chart showing variation of the mass fraction of NOx :
Chart showing variation of the mass fraction of NOx along Y-direction at different locations (along X-axis) of the combustor.
Mass fraction contour of soot :
In the below contour you can see mass fraction contour of soot in the combustor.
Chart showing variation of the mass fraction of soot :
Chart showing variation of the mass fraction of soot along Y-direction at different locations (along X-axis) of the combustor.

Case -5 : Mass fractions of CH4 = 0.7 and water(H2O) = 0.3 :-

Temperature contour :
In the below contour you can see the temperature distribution in the combustor.
Mass fraction contour of NOx :
In the below contour you can see mass fraction contour of NOx in the combustor.
Chart showing variation of the mass fraction of NOx :
Chart showing variation of the mass fraction of NOx along Y-direction at different locations (along X-axis) of the combustor.
Mass fraction contour of soot :
In the below contour you can see mass fraction contour of soot in the combustor.
Chart showing variation of the mass fraction of soot :
Chart showing variation of the mass fraction of soot along Y-direction at different locations (along X-axis) of the combustor.
Conclusion:
Effect of adding the water in the fuel :
We can finally conclude that from the parametric chart or by above contours and charts that on adding the water to the fuel the emissions (formation of Nox and soot )have been the least .In case -1 you can observe that the emmisons are quite high than when compared with case -5 ,where the amount of addition of water have been increased. So the more addition of water the less the emmisions , but if you also observe the maximum temperature at the outlet is decreasing which leads to the decrease of efficiency(if it is a burner). So we can finally conclude that adding the water to the fuel the emissions have been controlled , but if you add more water the combustion would not be that much proper .So , the addition of water quantity should be increased such that the emmisions are controlled as per the required government norms .

Week 10 - Simulating Combustion of Natural Gas.

Ship work like this
in your own course.

Talk to a career counsellor

Ship work like thisin your own course.

Talk to a career counsellor

Ship work like this
in your own course.