COMBUSTION SIMULATION OF METHANE-AIR MIXTURE USING ANSYS FLUENT

OBJECTIVE:

The primary objective of the project is to simulate a species-transported combustion model with methane being the fuel using Ansys Fluent. Also to find the mass fractions of different species involved in the combustion process at different locations of the combustor.

TYPES OF COMBUSTION SIMULATIONS IN FLUENT:

1. Species transport equation:

In this model, the conservation equation is solve every particular species in the reaction to predict the consumption/production of the species. It is widely used for non premixed combustion types. The equation is as follows

2. Non premixed combustion:

In non-premixed combustion, fuel and oxidizer enter the reaction zone in distinct streams. This is in contrast to premixed systems, in which reactants are mixed at the molecular level before burning. Examples of non-premixed combustion include pulverized coal furnaces, diesel internal-combustion engines and pool fires. Under certain assumptions, the thermochemistry can be reduced to a single parameter: the mixture fraction. The mixture fraction, denoted by , is the mass fraction that originated from the fuel stream. In other words, it is the local mass fraction of burnt and unburnt fuel stream elements (C, H, etc.) in all the species (CO , H O, O , etc.). The approach is elegant because atomic elements are conserved in chemical reactions. In turn, the mixture fraction is a conserved scalar quantity, and therefore its governing transport equation does not have a source term. Combustion is simplified to a mixing problem, and the difficulties associated with closing non-linear mean reaction rates are avoided. Once mixed, the chemistry can be modeled as being in chemical equilibrium with the Equilibrium model, being near chemical equilibrium with the Steady Laminar Flamelet model, or significantly departing from chemical equilibrium with the Unsteady Laminar Flamelet model.

3. Premixed combustion:

In premixed combustion, fuel and oxidizer are mixed at the molecular level prior to ignition. Combustion occurs as a flame front propagating into the unburnt reactants. Examples of premixed combustion include aspirated internal combustion engines, lean-premixed gas turbine combustors, and gas-leak explosions. Premixed combustion is much more difficult to model than non-premixed combustion. The reason for this is that premixed combustion usually occurs as a thin flame that is stretched and contorted by turbulence. The overall rate of propagation of the flame is determined by both the laminar flame speed and the turbulent eddies. The laminar flame speed is determined by the rate that species and heat diffuse upstream into the reactants and burn. To capture the laminar flame speed, the internal flame structure would need to be resolved, as well as the detailed chemical kinetics and molecular diffusion processes. Since practical laminar flame thicknesses are of the order of millimetres or smaller, resolution requirements are usually unaffordable.

4.Partially premixed combustion:

The partially premixed model solves a transport equation for the mean reaction progress variable, (to determine the position of the flame front), as well as the mean mixture fraction, f, and the mixture fraction variance, f\'^2. Ahead of the flame ( c=0), the fuel and oxidizer are mixed but unburnt, and behind the flame ( c=1), the mixture is burnt.

5. Composition PDF transport combustion:

The composition PDF transport model, like the EDC model should be used when you are interested in simulating finite-rate chemical kinetic effects in turbulent reacting flows. With an appropriate chemical mechanism, kinetically-controlled species such as CO and NOx, as well as flame extinction and ignition, can be predicted. PDF transport simulations are computationally expensive, and it is recommended that you start your modelling with small grids, and preferably in 2D. A limitation that applies to the composition PDF transport model is that you must use the segregated solver. The composition PDF transport model is not available with either of the coupled solvers

SIMULATION PROCEDURE:

1. Modelling the combustor:

The 3D model of the combustor is imported into spaceclaim. Then the model is splitted up. After splitting, only the mid plane which is a 2D geometry is extracted which will be the domain of simulation in this process.

Fig-1:3D CAD model of the combustor

Fig-2:Cut section view of the combustor

Fig-3:2D geometry of the combustor

This is done in order to reduce the computational cost and time by greater extent.

2. Meshing and creating named selections:

The 2D model is now meshed with the element size being 5 mm and the element type is quads. Also capture proximity is enabled in order to attain a refined mesh at the fuel inlet region. After meshing, the edges are named by creating named selections.

Fig-4:Mesh model

Fig-5:Mesh model(2)

Fig-6:Named selections

3. Setting up physics and solving:

After meshing, the model is loaded in Fluent and physics settings are being setup. The setup parameters are as follows.

a) Solver : Pressure based steady state solver (Axisymmetric)

b) Energy : Enabled

c) Viscous model : Standard k-epsilon with standard wall functions

d) Species

• Model : Species tranport
• Reactions : Volumetric
• Mixture : Methane-air
• Turbulence-chemistry interaction : Eddy dissipation
• Nox model : Enabled with ch4 as fuel stream

e) Solution method : SIMPLE

f) Boundary conditions

• Air inlet : Velocity inlet at 0.5 m/s, with o2 at a mass fraction of 0.23
• Fuel inlet : Velocity inlet at 80 m/s, with ch4 at a mass fraction of 1
• Outlet : Pressure outlet at 0 pascal gauge pressure

g) Initialization : Hybrid

h) No.of iterations : 1000

Fig-7:Residuals of the combustion simulation

Fig-8:Temperature contour

The temperature contour shows that the temperature at the outlet is uniform at the end of simulation. From the below animation, it is evident that the flow becomes stable after around 700 iterations.

Temperature contour animation

4. Results:

The post-processing is done in CFD-Post. Line probes are provided at different locations equidistant along the X-axis. Then the mass fractions of different species are plotted in those locations.

Fig-9:Location of line probes

Chart-1

Chart-2

Chart-3

Chart-4

Chart-5

Chart-6

Chart-7

INFERENCE FROM THE CHARTS:

1. Near the inlets, the mass fraction of O2 increases from 0 to 0.23 as gone higher along the Y-axis. But after X=0.7 m, it becomes completely zero.

2. This shows that all the O2 molecules are completely burnt over there. For CH4, things are a bit different. Since the fuel inlet is at the origin, the mass fraction of CH4 decreases from 1 to 0 along the Y-axis. Far from the inlets, it becomes very low but not completely burnt.

3. Both products CO2 and H2O have a graph similar to a normal distribution near the inlets. This is because their mass will be the highest at the center of the flame and gradually decreases on both sides. But moving along the X-axis, the distribution becomes flat, since the temperature becomes uniform.

4. Mass fraction of the pollutant N2O is very low in the order of e-07. But NO is a bit higher than N2O. It\'s mass fraction increases towards the outlet.

5. Mass fractions of the pollutants HCN and NH3 are zero throughout the reaction.

CONCLUSION:

This particular reaction involving methane-air mixture is almost a complete reaction without leaving any unburned reactants. If ran for few more iterations, the very least amount of methane will also be burnt. Compared with the other mass fractions, the mass fraction of the pollutants are very low.