Challenge on Combustion
Aim - To perform a combustion simulation on the combustor model and plot the variation of the mass fraction of the different species and check the effect of adding the water in the fuel by adding the water content in the fuel from 5% to 30% by mole.
Theory -
Combustion or burning is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually, atmospheric oxygen.
Complete combustion is stoichiometric concerning the fuel, where there is no remaining fuel, and ideally, no residual oxidant. Thermodynamically, the chemical equilibrium of combustion in air is overwhelmingly on the side of the products. However, complete combustion is almost impossible to achieve, since the chemical equilibrium is not necessarily reached, or may contain unburnt products such as carbon monoxide, hydrogen and even carbon (soot or ash). Any combustion at high temperatures in atmospheric air, which is 78 per cent nitrogen, will also create small amounts of several nitrogen oxides, commonly referred to as NOx. Thus, the produced smoke is usually toxic and contains unburned or partially oxidized products.
In this project, we will observe the burning of methane in the air inside a cylindrical combustor using the eddy-dissipation model in ANSYS. Here, the source term was modelled using the Eddy dissipation model where the beginning of the ignition is calculated using the turbulent time scale. This means that the ignition will start when and where there is turbulence. This is a crude approximation but helps us to visualize the steady-state temperature distribution in the combustion chamber at a faster rate. This type of mixing based model is called a Turbulence Chemistry interaction (TCI) model.
Geometry -
A small nozzle in the centre of the combustor
Only 1/4th part of model is used for analysis . Body split tool is used for spliting bodies. 2D front face is use for analysis by copy-paste tool.
Meshing -
Mesh size: 2mm
Note: Named selections must be created to assign proper boundary conditions
Nodes - 17596
Element - 18604
Setup & Solution:
A small nozzle in the centre of the combustor introduces methane at 80 m/s and ambient air enters the combustor coaxially at 0.5 m/s
Solver: Pressure based
Time: Steady
2D space: axisymmetric
Check Energy
Turbulence model: K-epsilon – Standard
Species: Species transport > Volumetric
Check inlet diffusion as we are not interested to calculate at inlet
Mixture Material: methane-air-2step
Turbulence chemical interaction: Eddy-dissipation
Turn on NOx (prompt and thermal - Ch4) and soot (one step)
Part I -
Boundary conditions: Inlet-1
velocity(air) – 0.5m/s
Species mass fraction: O2 =0.23
Inlet-2
velocity(fuel) – 80m/s
Species mass fraction: Ch4 =1
Species mass fraction: H2O =0
Iterations: 800
Result -
Residual plot
Temperature plot
Part II -
(A) Boundary conditions: Inlet-1
velocity(air) – 0.5m/s
Species mass fraction: O2 =0.23
Inlet-2
velocity(fuel) – 80m/s
Species mass fraction: Ch4 = 0.95
Species mass fraction: H2O = 0.05
Residual plot
Temperature plot
(B)Boundary conditions: Inlet-1
velocity(air) – 0.5m/s
Species mass fraction: O2 =0.23
Inlet-2
velocity(fuel) – 80m/s
Species mass fraction: Ch4 = 0.85
Species mass fraction: H2O =0.15
Residual plot
Temperature plot
(C)Boundary conditions: Inlet-1
velocity(air) – 0.5m/s
Species mass fraction: O2 =0.23
Inlet-2
velocity(fuel) – 80m/s
Species mass fraction: Ch4 = 0.7
Species mass fraction: H2O =0.3
Residual plot
Temperature plot
Conclusion -
- From table and graphical representations , mass fraction of CH4 decreases from 1 to 0 along the y-axis. Far from the inlets it burnt very low but not completely burnt
- Value of H2O and C2O remain very much similar near outlet in all cases.
- We can see that the value of mass fraction of NOx decreases with increase in addition of water (H2O) in simulation which is great as according to government norms with reduction of NOx formation.
- We can also observe that mass fraction of soot increases with increase in addition of water.