Non Premixed Combustion in Combustor Model
OBJECTIVE:
1. Possible types of Combustion simulation in FLUENT.
2. 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.
Types of Combustion Simulation
Possible types of combustion simulation
- Based on mixing
- Based on Phase
Based on mixing
- Non-premixed combustion (Direct injection, Late injection)
- Premixed combustion
- Partially premixed
Based on Phase
- Fluid Phase (Volumetric reaction)
- Wall (surface reaction)
- Particles (surface reaction)
- Porous region (After treatment system)
Combustion Models provided by Fluent can be Categorized into 2 sections:
1. Finite Rate (Slow) Chemistry Models
2. Fast Chemistry Model
1. Finite Rate (Slow) Chemistry Models
- Laminar Finite rate Model
- EDC
- Composition PDF
2. Fast Chemistry Model
- Eddy Dissipation Model
- Premixed Model
- Equilibrium Model
- Steady Laminar Flamelet Model
- Flamelet Generated MAnifold model
- Partially premixed model
Combustor Model
Cross Section
Here the innermost slot (circle) is the fuel inlet and the outer area is an air inlet.
The 2D X-Y axis model is extracted out of full 360deg combustor
Here the inlet below the slot is fuel inlet and inlet above the slot is Air inlet.
We are doing a 2D Axisymmetry geometry and flow is axisymmetric.
MESH
A very finer mesh is used with element size 0.55mm and 469974 total elements as student version allow up to half a million cells.
Capture proximity was enabled with 3 Num cells across the gap.
A steady-state, Pressure based solver was used. RNG k-epsilon Model with Standard Wall Functions Near Wall Treatment Model was used.
In Species Model, Species Transport model was selected with Volumetric reaction. Inlet Diffusion and Diffusion Energy Source options were enabled. A methane-air mixture material was selected and an Eddy Dissipation model was selected for Turbulence-Chemistry Interaction.
For air inlet, 0.5 m/s velocity of magnitude was given with 5% turbulent intensity. The temperature was kept constant at 300k and species mass fraction of O2 0.23
For fuel inlet, 80 m/s velocity of magnitude was given with 5% turbulent intensity. The temperature was kept constant at 300k and species mass fraction of CH4 0.23
Residual Plots
Average Temperature plot w.r.t no. of iterations
After the solution getting converged, we get a constant temperature of about 1200k
Temperature Contour
In temperature contour, we clearly see the maximum temperature attained is 2150k at the innermost region near axisymmetry and as we go towards the wall the temperature reduces and we can refer it with the legend scale displayed along the side of the image.
Below, a closeup look is displayed and hence clearly see the combustion and its temperature along with the clear separation of different temperature regions.
Line probes were plotted equidistant from each other within the combustor to capture the change in the mass fraction at different locations for CO2, O2, CH4, N2, H2O and NOx emissions. Here, there are 5 equidistant line probe starting from left to right and the plots for the mass fractions are displayed below in this project page.
Contours for CO2, H2O, CH4, N2, O2 Mass Fraction:
Below contours gives us the information in terms of local temperature with its concentration and behaviour of that particular species.
CO2
Overall maximum mass fraction of CO2 reaches up to 1.434e-01. Most CO2 is present at the core centre and reduces as we go away from the axisymmetry.
H2O
H2O and CO2 kind of shows a similar pattern. Here as well the H2O max (1.174e-01) is present in the centre of the combustor and fades away as we move towards the liner.
CH4
In the case of CH4, max is 1 as given by us. It stretches up to a very small distance of about 0.06m and the mass fraction reduces in outer direction.
N2
The maximum mass fraction for N2 attend is 7.712e - 01 which is at the outermost most flame. But Nitrogen doesn't reach up to the liner and stays in the core region of the combustor.
O2
The maximum mass fraction for O2 attend is 2.314e - 01 which is at the outermost most flame.
Plots of Mass Fraction at Line1, Line2, Line3, Line4, Line5 for CO2, H2O, CH4, N2, O2
CO2 Plot
At all the lines as we move up towards the liner (y-axis) of the combustor mass fraction for CO2 goes on decreasing which justifies the results we got above through contour.
As we move from line1 to line5, the start of the graph suggest that the mass fraction in the axisymmetry region increases as we go along the x-axis i.e. away from the fuel intake in case of CO2.
H2O Plot
H2O behaves same as that of CO2 and the plot validates the result obtained through contours.
CH4 Plot
The mass fraction for CH4 decreases as we move from line1 to line 5. It also decreases as we move up along the y-axis through the different line probes. The concentration is more at line 2 and then decreases up to line 5 ultimately reaching approx. 0
N2 Plot
As we go along the axisymmetry the mass fraction of N2 increases, also as we move along the probe lines in an upward direction, the mass fraction increases and becomes constant at 0.77 till line 5. This again validates our results that N2 is most present in the outer layer of the flame.
O2 Plot
O2 remains constant at 0 along the axisymmetry but as we move upwards in y-axis direction the mass fraction increases because of the huge air inlet.
Again Simulation was performed for NOx emissions
Data for NOx emissions were obtained by editing the 'ANSYS SetUp' and enabling the NOx model.
Residuals
NOx Emissions Mass fraction
As we see in the plot the values for NOX emissions decreases as we move from line1 to line5. The maximum values for NOx are obtained at line2. NOx emission values don't show a particular pattern as we move along the axisymmetry except the final value along the y-axis is greater than the initial value for every line probe drawn.
Combustor Animation Temperature: