Combustion simulation on a combustor model using FLUENT

Objective :-

• Explain about the possible types of combustion simulations in FLUENT.
• To perform a combustion simulation on the combustor model.
• Plot the variation of the mass fraction of the different species (CO2, H2O, CH4, N2, O2 and NOx emissions).

Introduction :-

Combustion models for CFD refers to combustion models for computational fluid dynamics. Combustion is defined as a chemical reaction in which a hydrocarbon fuel reacts with an oxidant to form products, accompanied with the release of energy in the form of heat. Being the integral part of various engineering applications like: internal combustion engines, aircraft engines, rocket engines, and power stations combustors, combustion manifests itself as a wide domain during the design, analysis and performance characteristics stages of the above-mentioned applications. With the added complexity of chemical kinetics and achieving reacting flow mixture environment, proper modeling physics has to be incorporated during computational fluid dynamics CFD simulations of combustion.

Possible types of combustion simulations in FLUENT.

Computational fluid dynamics modeling of combustion calls upon the proper selection and implementation of a model suitable to faithfully represent the complex physical and chemical phenomenon associated with any combustion process. The model should be competent enough to deliver information related to the species concentration, their volumetric generation or destruction rate and changes in the parameters of the system like enthalpy, temperature and mixture density. The model should be capable of solving the general transport equations for fluid flow and heat transfer as well as the additional equations of combustion chemistry and chemical kinetics incorporated into that as per the simulating environment desired.

BASED ON MIXING

• Non-Premixed Combustion : In this type of combustion, fuel and oxidizer enters the reaction zone in distinct streams. Mixing takes place at the time of combustion.
• Pre-Mixed Combustion : In this type of combustion, fuel and oxidizer are mixed at molecular level before combustion.
• Partially Mixed Combustion : In this type of combustion, the fuel and oxidizer are partially pre-mixed at non uniform ratios before combustion.

BASED ON PHASE

• Fluid Phase (Volumertic Reaction): Combustion takes place when the fuel is in fluid phase.
• Wall (Surface Reaction): Combustion takes place on wall or the surface.
• Particle (Surface Reaction): Combustion takes place on the surface of the particles.
• POROUS REGIONS: Combustion takes place inside the pores.

Eddy Dissipation Model : 

    This model is used when turbulent mixing of the constituents has to be taken into consideration. The k/Ɛ turbulent time scale is used to calculate the reaction rate. A comparison between the turbulent dissipation rates of the fuel, oxidant and products is done and the minimum amongst all is taken as the rate of the reaction. The transport equations for the mass fractions of the constituents are solved using this rate of reaction. Apart from this a mean enthalpy equation is also solved and temperature, density and viscosity are calculated accordingly. The model can also be implemented when finite rate kinetically controlled reaction is to be simulated. In such situation while deciding the rate of the reaction the Arrhenius kinetic rate expression is also taken into account and the rate of reaction is taken as minimum amongst the turbulent dissipation rates of all the constituents and the Arrhenius kinetic rate expression. Since turbulent mixing governs the characteristics of this model, there exists a limit to the quality of the combustion simulation depending upon the type of the turbulent model implemented to represent the flow. The model can also be modified to account for mixing of fine structures during the turbulent reaction. This modification of the model results in the eddy dissipation model which consider the mass fraction of fine structures in its calculations.

Combustion simulation on the combustor model

Geometry :- 3D Model 

Fig : Cross sectional view of 3D model

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

As shown in above fig we have many seperate layers so, we have to combine all layer to form a single layer model to run the simulation.

Meshing :-

For refining the mesh for this case we are using all triangle method

• Element size = 2.5 mm
• Capture proximity ON
• Proximity minimum size = 0.025 mm
• Number of cells across gap = 4
• Number of Nodes = 11927
• Number of Elements = 23071

Simulation Setup :

Setting up the physics for Combustion in ANSYS fluent.

Solver :

• For this case we are using pressure-based Steady state solver
• Velocity formulation - Absolute
• 2D space - Axisymmetric

Models :

• Follow the Energy Equations
•  Viscous Model
  • model - k-epsilon
  • k-epsilon model - Standard
  • Near wall treatment - Standard wall function
•  Species
  • Model - Species Transport
  • Reactions - Volumetric
  • Chemistry solver - None-Explicit source
  • Options - Inlet diffusion, Diffusion Energy Source
  • Mixture Material - methane-air
  • Turbulence-Chemistry Interaction - Eddy-Dissipation

Boundary Conditions :

Air-Inlet

• Velocity = 0.5 m/s
• Temperatures = 300 K
• Species mass fraction = O2 - 0.23

Fuel-Inlet

• Velocity = 80 m/s
• Temperatures = 300 K
• Species mass fraction = CH4 - 1

Outlet

• Pressure-outlet = 0 pascal (gauge pressure)

Results and Plot :-

Fig : Residuals plots

So, by the above residuals plot we can say that our result are converged after 300 iterations.

Fig : Temperature contour

So, by the above temperature contour we observe that the maximum temperature in the combustion goes up to 2310 K.

Fig : Mass fraction of CH4

Fig : Mass fraction of CO2

Fig : Mass fraction of H2O

Fig : Mass fraction of O2

Fig : Mass fraction of N2

The line probes plotted equidistant from each other as shown in the figures to repersent the mass fraction of Species at different location.

Line location from the inlet in X-direction

• 0.1 m
• 0.2 m
• 0.3 m
• 0.4 m
• 0.5 m

Fig : Variation in Mass fraction of CH4 at different location

Fig : Variation in Mass fraction of CO2 at different location

Fig : Variation in Mass fraction of H2O at different location

Fig : Variation in Mass fraction of N2 at different location

Fig : Variation in Mass fraction of O2 at different location

So, by the above simulation we didn\'t get any mass fraction for the NOx emission. Air, mainly composed of oxygen and nitrogen, is initially drawn into the combustion chamber. Then the fuel is injected at very high velocity directly into air in the combustion chamber. The fuel is burned, and the heat is released. Normally in this process, the nitrogen in the air does not react with oxygen in the combustion chamber and it is emitted identically out. However, high temperatures above 1,600 °C in the cylinders cause the nitrogen to react with oxygen and generate NO_x emissions. So, it will not be wrong to say that the major influences of the formation of NO_x are the temperature and concentration of oxygen in the combustion. Most of the emitted NO_x is formed early in the combustion process. This is when the flame temperature is the highest. Increasing the temperature of combustion increases the amount of NO_x.

We have method to find NOx in the ANSYS as shown below.

For this we have to go in setup and done some changes as discussed below

Setting up the physics for calculating the NOx emission

For this go to Models - More - Species - Species Transport, Reactions and double click on NOx

Fig : Mass fraction of NO

Fig : Variation in Mass fraction of NO at different location

Conclusion : 

• Simulation takes around 300 iterations to converge the solution and reach to steady state residuals.
• The mass fraction for all the species CO2, H2O, CH4, N2, O2 and NOx emissions are found from the simulation.
• The maximum temperature by the flame is 2310 K.
• The emission NOx produced by the above combution is very low or negligible because N2 is an inert gas here.