Combustion Analysis on Methane-Air Mixture

OBJECTIVE :

- To Perform a combustion simulation on the combustor model .

- To Plot the variation of the mass fraction of the different species’ in the simulation using line probes at different locations of the combustor .

- To check the effect of adding the water in the fuel by adding the water content in the fuel from ‘ 5% to 10% ’ by mole.

SOFTWARE USED:

ANSYS, SpaceClaim - To prepare FV model.

ANSYS, Meshing - To mesh the given model.

ANSYS, FLUNET - To setup the boundary conditions and to solve the given model.

ANSYS, CFD Post - To perform Post Processing.

THEORY :

Possible Types of Combustion Simulation :

The combustion simulation that can be performed using Ansys Fluent are given below :

1) Based on MIXING :

a) Non – Premixed Combustion :

This model is used when fuel is directly injected at the time of combustion .

Eg : Diesel engine , Late injection

b) Premixed Combustion :

In Spark-Ignition (SI) engine , fuel and air are mixed way before the combustion process in the carburetor.

c) Partially premixed :

In SI engine , where the fuel is injected in the intake manifold just before the combustion is an example of partially premixed which is in between non- premixed combustion and premixed combustion.

2) Based On PHASE :

a) Fluid Phase (Volumetric reactions) : Combustion takes place when the fuel is in fluid phase .

b) Wall (Surface reaction) : Combustion takes place on wall or the surface .

c )Particle (Surface Reaction) : Combustion takes place on the surface of the particles.

d) Porous Regions : Combustion takes place inside the pores (After treatment System).

Equations solved in Combustion Simulation :

Mass Equation

Momentum equations

Species Transport Equation

Species Transport Equation :

When combustion takes place , Source terms is computed using the combustion model.

Motion of species because of velocity field .

Diffusion is combination of Laminar and Turbulent . For the engineering application like IC engine , Gas turbine , large burners ; the turbulent diffusion is very high (Turbulent Diffusion >> Laminar Diffusion) so laminar term is neglected in the species transport equation . This is also known as dilute approximation ( Fick’s law ) .

Combustion Source calculation methods :

Finite Rate Chemistry

Mixing based Models

Finite Rate Eddy Dissipation Model

PROCEDURE :

STEP 1 : GEOMETRY USED

1.Importing the 3D model:

The first step is to import the 3D model to the space claim.

For finding the result using the 3D model is increase the computational cost and time to run the simulation and also we are having the Academic license for some limitations for mesh generation . So we are extracting 2D model from 3D model and done simulation on it.

We have many separate layers . So, we have to combine all layers to form a single layer model to run the simulation.

2. 2D Axis Symmetric Geometry :

STEP 2 : MESHING

Naming Parts :

Named selections must be created in order to assign proper boundary conditions in the setup solution.

Mesh Quality :

STEP 3 : SET UP & SOLUTION

- Solver type : Pressure based

- Steady state solver

- Check the Energy equation

- Turblent Model : K-Epsilon (standard)-standard wall functions

- ** 2D space : Axis symmetric **

- Species : Species transport a volumetric

Check inlet diffusion (since we not calculating at inlet )

Check the Diffusion energy Source

Mixture Material : methane-air-2step

Turbulent chemical interaction : Eddy-dissipation

• After applying above conditions , you will be able see NOx and soot under species tab

Turn on NOx : prompt and thermal – CH₄

Turn on Soot : One-step

- Boundary Conditions :

- Inlet 1 (Air) :

Velocity = 0.5 m/s

Species mass fraction : O₂ = 0.23

- Inlet 2 (Fuel) :

Velocity =80 m/s

Species a mass fraction : CH₄ = 1 (parameter -1)

Species a mass fraction : H₂O = 0 (Parameter -2)

For further cases : Parameter -1 decreases by 5 % (-0.05)

Parameter -2 Increases by 5 % (+0.05)

- Intialization : Hybrid

- Iterations done up to : 600

STEP 4 : RESULTS

Parametric Case Study :

The following are the readings of Area-weighted-Average in the combustion challenge :

PART -1 : (WITHOUT adding WATERto the FUEL) @0%H2O

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.

Residual Plot :

Temperature Contour :

CH4 Contour :

O2 Contour :

CO2 Contour :

H20 contour :

N2 Contour :

NOx Contour :

Soot Contour :

LINE Probs :

The line Probes plotted equidistant from each other as shown below in the figure to represent the mass fraction of Species at different locations.

- Taken as reference points for line 1 are as follows :

Point 1 – (0.1 , 0 , 0)

Point 2 – (0.1,0.0852,0)

Each line differs by 0.1 m

PART - 2 :

In this part, we need to add the water content in the fuel from 5% to 30% by mole and observe the effect of it on the results.

Readings were taken for water content in the fuel for 5 , 10 , 15 , 20 , 25 , 30 %

Contours and line plots were posted inorder to avoid complexity and confusion for 5 , 15 , 30 %

Comparision of contours :

Line Probs (Charts) at different locations :

TABULATION :

This table shows only the maximum value of respective species during the combustion simulation.

Graphical Analysis :

Observations :

- As the water percentage is increasing in the fuel , NOx emission level is reduced .

- Similarly , Soot emission level get dropped.

- Temperature gradually decreased with the increase of water content in the fuel.

- Also higher the addition of water there will more chances of incomplete combustion which will result in the unburnt hydrocarbons.

- So addition of water to the fuel should be like it reduce the soot and NOx rather than resulting an incomplete combustion.

References :

1. https://www.sciencedirect.com/science/article/pii/S1876610217337724

2. https://link.springer.com/article/10.1007/s00773-015-0303-8