Combustion simulation on the combustor model

1Q. Briefly explain about the possible types of combustion simulations in FLUENT.

A: 

  • Combustion or burning is a high temperature exothermic chemical reaction between a fuel and an oxidant accompanied by the production of heat, light and unburnt gases in the form of either a glow or flames.
  • Combustion is an integral part of various engineering applications like Internal combustion engines, aircraft engines, rocket engines, furnaces and power station combustors. 
  • Combustion manifests itself as a wide domain during the design, analysis and performance characteristics stages of the above mentioned applications. 

Possible types of combustion simulations in Fluent:

  1. Based on mixing of air and fuel

           i) Non-premixed combustion: In this type of combustion simulation, the combustion takes place inside a cylinder/chamber after injecting the fuel. But the air and fuel is not properly mixed before combustion because of insufficient time for mixing. It is also called as Direct injection or late injection, it mostly happens in diesel engine combustion.

          ii) Premixed combustion: In this type of combustion simulation, the mixing of air and fuel takes place before combustion chamber, It mostly happens in petrol engine combustion, where the air and fuel is mixed in carburetor.

         iii) Partially premixed: In this type of combustion, the air and fuel is partially mixed before entering the cylinder/chamber.

   2.  Based on Phase of mixture

         i) Fluid phase: In this type, the combustion takes place within the fluid phase and these are volumetric reactions. Example is a direct injection.

         ii) Wall: In this type, the combustion takes at the wall and these are surface reactions. Example is evaporation

         iii) Particles: In this type, the combustion takes place between the lagragian particles  and these are surface reactions. Example is burning of coal.

         iv) Porous region: In this type, the combustion takes place above the porous region, where the reaction takes place and it is a high-pressure drop region. Example is a After treatment system.

2Q. 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 as shown in Fig. You need to plot for CO2, H2O, CH4, N2, O2 and NOx emissions. If you do not obtain the mass fraction of any of the species, give an explanation as to why you did not and see if you can obtain it any other way. The line probes can be plotted equidistant from each other as shown in the figures

Objectives: 

  • To Perform a combustion simulation on the combustor model and plot the variation of the mass fraction of the different species using line probes at different locations.
  • To plot for CO2, H2O, CH4, N2, O2 and NOx emissions.

3D Geometry:

2D Axisymmetry Geometry of combustion model:

Enlarged 2D Axisymmetry Geometry of combustion model:

2D Mesh Model:

Element size = 1 mm

Number of nodes = 73389

Number of elements = 144941

SIMULATION SETUP DETAILS:

I. SETUP PHYSICS

SOLVER

     Type --> pressure based

     velocity Formulation --> Absolute

     Time --> Steady

     2D Space --> Axisymmetric

MODELS

 Energy - Tick the check box

 Click on viscous

 Model --> k-epsilon

 k-epsilon Model --> Standard

 Near-wall Treatment --> Standard Wall Functions

 press Ok.

 Click on Species Model

 Model --> Species Transport 

 Reactions --> Tick Volumetric 

 Chemistry Solver --> None - Explicit Source

 Options --> Tick Inlet Diffusion

             --> Tick Diffusion Energy Source

Mixture Properties

Mixture Material --> methane-air

Turbulence-Chemistry Interaction --> Eddy-Dissipation

press Ok.

Boundary conditions:

air-inlet -->Edit

         Momentum 

               Velocity magnitude(m/s) = 0.5

         Thermal 

               Temperature(K) = 300

         Species

               Species Mass Fraction

               ch4 - 0

               o2 - 0.23

               co2 - 0

               h20 - 0

fuel-inlet -->Edit

         Momentum 

               Velocity magnitude(m/s) = 80

         Thermal 

               Temperature(K) = 300

         Species

               Species Mass Fraction

               ch4 - 1

               o2 - 0

               co2 - 0

               h20 - 0                

II SOLUTION 

INITIALIZATION

Method --> Hybrid

Click on Initialize 

III RESULTS

GRAPHICS

 Contour --> New 

     Contour Name - Temperature contour

     options --Filled

     contours of -->Temperature

     surfaces - select all surfaces

     save/Display

     close

IV SOLUTION

ACTIVITIES 

Create --> solutions Animation

        Animation object --> Temperature Contour

        click on preview

        Ok

RUN CALCULATON

Time scale Factor = 1

No. of Iterations = 1500

Scaled Residuals: The solution is converged at 171 iterations.

Contours of static temperature:

Contours of velocity :

Line Probes along the length of the domain:

Seven different line probes are mentioned at a distance of 0.05, 0.15, 0.25, 0.35, 0.45, 0.55, 0.65 m from the inlet to observe the variation of the mass fraction of different species along y direction.

Mass Fraction of Methane(Ch4):

Mass Fraction of Carbondioxide(Co2):

Mass Fraction of Hydrogen(H2o):

 

Mass Fraction of Nitrogen(N2):

Mass Fraction of Nitrogen(O2):

The mass fraction of Nox was not obtained, because the nitrogen has high ionization enthalpy due to which it does not react with other species, and therefore is considered to be an inert gas. Any compounds formed of it are present in a very small, negligible amount.

To calculate the mass fraction of Nox, Nox model is enabled in Models from Fluent setup, and appropriate changes are made in Nox Models and again run the simulation for for finding out the mass fraction of Nox emissions.  

Mass Fraction of Nox:

Static temperature of Combustion Simulation Animation:

Conclusion: 

  • The combustion simulation using Eddy-Dissipation Model was simulated, mass fraction of all species at different length of the combustor domain are obtained.
  • The Nox emission was comparatively low than the other combustion products.

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