Combustion simulation on a combustor model and plot the variation of the mass fraction of the different species in ANSYS FLUENT.

Objective

• Briefly explain about the possible types of combustion simulations in FLUENT.
• 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.
• Plot for CO2, H2O, CH4, N2, O2 and NOx emissions.

                                                              PART-1

Combustion

Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually, atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion in a fire produces a flame, and the heat produced can make combustion self-sustaining. Combustion is often a complicated sequence of elementary radical reactions. Solid fuels, such as wood and coal, first undergo endothermic pyrolysis to produce gaseous fuels whose combustion then supplies the heat required to produce more of them. Combustion is often hot enough that incandescent light in the form of either glowing or a flame is produced. A simple example can be seen in the combustion of hydrogen and oxygen into water vapor, a reaction commonly used to fuel rocket engines. This reaction releases 242 kJ/mol of heat and reduces the enthalpy accordingly (at constant temperature and pressure)

Combustion of an organic fuel in air is always exothermic because the double bond in O2 is much weaker than other double bonds or pairs of single bonds, and therefore the formation of the stronger bonds in the combustion products CO2 and H2O results in the release of energy. The bond energies in the fuel play only a minor role since they are similar to those in the combustion products; e.g., the sum of the bond energies of CH4 is nearly the same as that of CO2. The heat of combustion is approximately -418 kJ per mole of O2 used up in the combustion reaction and can be estimated from the elemental composition of the fuel.

Combustion models for CFD refer 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 by the release of energy in the form of heat. Being 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. With the added complexity of chemical kinetics and achieving reacting flow mixture environment, proper modeling physics has to be incorporated during computational fluid dynamic (CFD) simulations of combustion. Hence the following discussion presents a general outline of the various adequate models incorporated with the Computational fluid dynamic code to model the process of combustion.

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

Based on phase

• Fluid phase (Volumetric reaction): Combustion takes place when the fuel is in the fluid phase.
• Wall (Surface reaction): Combustion takes place on the wall or on 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

The eddy dissipation model, based on the work of Magnussen and Hjertager, is a turbulent-chemistry reaction model. Most fuels are fast burning and the overall rate of reaction is controlled by turbulence mixing. In the non-premixed flames, turbulence slowly mixes the fuel and oxidizer into the reaction zones where they burn quickly. In premixed flames, the turbulence slowly mixes cold reactants and hot products into the reaction zones where the reaction occurs rapidly. In such cases, the combustion is said to be mixing-limited, and the complex and often unknown chemical kinetics can be safely neglected. In this model, the chemical reaction is governed by large eddy mixing time scale. Combustion initiates whenever there is turbulence present in the flow. It does not need an ignition source to initiate the combustion. This type of model is valid for the non-premixed combustion, but for the premixed flames the reactant is assumed to burn at the moment it enters the computation model, which is a shortcoming of this model as in practice the reactant needs some time to get to the ignition temperature to initiate the combustion.

                                                             PART-2

1. Geometry

Cut section

• This is the geometry of the combustor model. We have to extract the 2D plane from this tree dimensional geometry.
• We can do this by inserting the plane at the origin which will give us the plane through which we can split the geometry.

• After this, we have to select the "split body" option and select the geometry. Then select the plane to split the geometry along that plane.

• Now we have the desired plane. We have to copy this plane and paste it in the new design window of spaceClaim.

• This is the required 2D plane on which we have to perform the combustion simulation in Fluent.

2. Meshing

• Meshing method: Triangles.
• Capture proximity enabled.
• Element size: 0.0015 m.
• Total nodes: 31412.
• Total elements: 61602.

Named selection

Generated mesh

Mesh details

3. Setup

• Solver: steady-state
• Type: Pressure based
• Turbulence model:k epsilon.
• Axisymmetric enabled
• Energy enabled

Species model

• Model: Species transport
• Reactions: Volumetric
• Options: Inlet diffusion, Diffusion energy source
• Mixture material: Methane and air
• Turbulence-Chemistry Interaction: Eddy-Dissipation
• NOx calculation enabled

Boundaries:

• Air inlet type: velocity inlet
• Air inlet velocity: 0.5 m/s
• Air inlet temperature: 300 K
• Species mass fraction at the air inlet: O2 - 0.23
• Fuel inlet type: velocity inlet
• Fuel inlet velocity: 80 m/s
• Fuel inlet temperature: 300 K
• Species mass fraction at the fuel inlet: CH4 - 1
• Outlet: pressure-outlet, Gauge pressure: 0 pascal.
• Initialization method: Hybrid initialization
• Number of iterations: 750

Chemical equation:

• `CH_4 + 2(O_2 + 3.76N_2) -> 2CO_2 + 2H_2O + 7.52N_2`
• O2 and 3.76N2, constitutes the air mixture.
• By stoichiometric calculations, the mass fraction of oxygen and dinitrogen gas are 0.23 and 0.77 respectively.
• Dinitrogen is inert in the above reaction and hence it is excluded from the calculation.

4. Results

Residuals

• The above image is of the residuals and we can see that the NOx calculation is enabled to calculate all the NOx pollutants.

Temperature Contour

CO2 mass fraction contour

H2O mass fraction contour

CH4 mass fraction contour

N2 mass fraction contour

O2 mass fraction contour

In order to calculate the NOx emissions during combustion, we have to enable the option for calculation of NOx in Ansys Fluent.

NO mass fraction contour

N2O mass fraction contour

Temperature contour animation of the combustion

Now we will create five line-probes, equidistance to each other and then plot various charts at these line probes. The distance of each line probe from the origin is given below.

• Line 1 - 0.10 m.
• Line 2 - 0.25 m.
• Line 3 - 0.40 m.
• Line 4 - 0.55 m.
• Line 5 - 0.70 m.

Line probes at different locations:

Variation of Temperature at all the line probes

• The highest temperature in the above plot is at the line probe 3. The maximum temperature reached at probe 3 is around 2400 K.

Variation of CO2 mass fraction at all the line probes

Variation of H2O mass fraction at all the line probes

Variation of CH4 mass fraction at all the line probes

Variation of N2 mass fraction at all the line probes

Variation of O2 mass fraction at all the line probes

Variation of NO mass fraction at all the line probes

Variation of N2O mass fraction at all the line probes

Conclusions

• The combustion simulation was performed in Ansys Fluent. The simulation ran for 750 iterations and we can that the residuals have reached a steady-state at around 400 iterations.
• The maximum temperature reached during the combustion simulation is 2413.16 K.
• Mass fractions of CO2, H2O, CH4, N2, O2, and NOx emissions are calculated and plotted.
• From the plot of NO and N2O, we can see that the NOx emissions are negligible.