Week 10 - Simulating Combustion of Natural Gas.
Simulating Combustion of Natural Gas
AIM:
To perform Simulation of Combustion of Natural Gas in ANSYS Fluent.
OBJECTIVE:
To plot the mass fraction of CO2, H2O, CH4, N2, O2, NOx and soot formation.
To study the NOx and soot formation
To setup the combustion simulation using eddy dissipation model.
To perform the simulation by adding water with fuel in range of 5% to 30% to lower the NOx and soot formation in combustion.
INTRODUCTION
1. COMBUSTION
Combustion is defined as achemical reaction in which a hydrocarbon reacts with an oxident to form products, accompanied by the release of energy in the form of heat.
Combustion manifests as awode domain during the design, analysis, and performance characteristics stage by being an integral part of various engineering applications like internal combustion engines,furnaces and power station conbustors.
2. Conbustion model for CFD Analysis
Comptational fluid dynamics modeling of combustion calls upon the proper selection and implementation of 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, theri 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 additional equations of combustion chemistry and chemical kinetics incorporated into that as per the simulating environment desired.
3. Possible types of Combustion Simulation possible in Ansys Fluent
The simulation of combustion can be divided into two types-
A. Combustion based on Mixing
- Non-Premixed Combustion
- In non-premixed combustion, fuel and oxidizer enter the reaction zone in distinct streams.
- It occours during direct injection or late injection of fuel in IC engines.
- Examples of non-premixedcombustion includes pulvarized coal furnaces, diesel internal-combustion engines and pool fires.
- Premixed Combustion
- In premixed combustion,fuel and oxidizer are mixed at the molecular level prior to ignition.
- Combustion occurs as flames front propagating into the unburnt reactants.
- Examples of premixed combustion include aspirated internal combustion engines, lean- premixed gas turbine combustors, and gas-leak explosions.
- Partially Premixed Combustion
- Partially premixed combustion systems are premixed flames with non-uniform fuel-oxidizer mixtures(equivalence ratios).
- Such falmes include premixed jets discharging into an inert atmosphere, lean premixed combustors with difussion pilot fames and cooling air jets, and imperfectly mixed inlets.
B. Combustion based on Phase
- Fluid Phase (Volumetric Reactions)
- Combustion takes place in the fluid region.
- Wall (Surface Reactions)
- Combustion takes place on the wall or a solid surface.
- Particles ( Surface Reactions)
- Combustion takes place on the Lagrangian particles.
- Combustion of coal is an example of particle combustion.
- Porous Region
- Combustion takes place in a porous region.
- The reactions in an after-treatment system is am example of combustion in the porous regions.
4. Combustion Reactions
The general reaction of combustion is given as follows-
`CH_4+ar(O_2+3.76N_2)rarraCO_2+bH_2O+cN_2`
Balancing the given stoichiometric equation,
a=1
2b=4, implies b=2
2ar=2a+b, implies ar=2
c=7.52ar ,implies c=15.04
The above values corresponds to the number of moles of the given species.
- Fuel: 2 moles of O_2+7.52
- Air: 2 moles of N_2
- Mole Fractions:
- The mole fraction of O_2=2/(2+7.52)=0.
- The mole fraction of N_2=7.52/(2+7.52)=0.7
Mass Fractions:
The mass fraction of
O_2=(0.21xx 16 )/((0.21xx16)+(0.79xx14))=0.23
The mass fraction of
N_2=(0.79xx14)/((0.21xx16)+(0.79xx14))=0.77
MODELLING & SOLVING APPROACH:
MESHING:
| MESH SIZE | 20 MM |
| NODES | 128002 |
| ELEMENTS | 121622 |
SOLVING METHOD,
For Case 1,
| SOLVER INPUTS | |
| Solver Time | Steady |
| Solver Type | Pressure |
| Energy | ON |
| Models | |
| Viscous | K-Epsilon standard |
| Species | Eddy Dissipation Model |
| Material | Methane-air-2step |
| air Inlet velocity | 0.5m/s |
| fuel inlet velocity | 80m/s |
| Species | |
| Mass Fraction of CH4 | 1 |
| Mass Fraction of H20 | 0 |
| Mass Fraaction of 02 | 0.23 |
| Iterations | 300 |
| Nox | off |
| Soot | off |
RESIDUAL PLOT,
CH4 MASS FRACTION PLOT,
CO MASS FRACTION PLOT,
CO2 MASS FRACTION PLOT,
H20 MASS FRACTION PLOT,
N2 MASS FRACTION PLOT,
O2 MASS FRACTION PLOT,
TEMPERATURE PLOT,
SOOT MASS FRACTION PLOT,
CHART OF MASS FRACTION OF POLLUTANT NO,
For Case 2,
| SOLVER INPUTS | |
| Solver Time | Steady |
| Solver Type | Pressure |
| Energy | ON |
| Models | |
| Viscous | K-Epsilon standard |
| Species | Eddy Dissipation Model |
| Material | Methane-air-2step |
| air Inlet velocity | 0.5m/s |
| fuel inlet velocity | 80m/s |
| Species | |
| Mass Fraction of CH4 | 0.95 |
| Mass Fraction of H20 | 0.05 |
| Mass Fraaction of 02 | 0.23 |
| Iterations | 300 |
| Nox | on |
| Soot | on |
RESIDUAL PLOT,
CH4 MASS FRACTION PLOT,
CO MASS FRACTION PLOT,
CO2 MASS FRACTION PLOT,
H20 MASS FRACTION PLOT,
N2 MASS FRACTION PLOT,
O2 MASS FRACTION PLOT,
TEMPERATURE PLOT,
SOOT MASS FRACTION PLOT,
CHART OF MASS FRACTION OF POLLUTANT NO,
For Case 3,
| SOLVER INPUTS | |
| Solver Time | Steady |
| Solver Type | Pressure |
| Energy | ON |
| Models | |
| Viscous | K-Epsilon standard |
| Species | Eddy Dissipation Model |
| Material | Methane-air-2step |
| air Inlet velocity | 0.5m/s |
| fuel inlet velocity | 80m/s |
| Species | |
| Mass Fraction of CH4 | 0.7 |
| Mass Fraction of H20 | 0.3 |
| Mass Fraction of 02 | 0.23 |
| Iterations | 300 |
| Nox | on |
| Soot | on |
RESIDUAL PLOT,
CH4 MASS FRACTION PLOT,
CO MASS FRACTION PLOT,
CO2 MASS FRACTION PLOT,
H20 MASS FRACTION PLOT,
N2 MASS FRACTION PLOT,
O2 MASS FRACTION PLOT,
TEMPERATURE PLOT,
SOOT MASS FRACTION PLOT,
CHART OF MASS FRACTION OF POLLUTANT NO,
RESULTS:
Comparison of mass fraction of pollutant NO for 3 cases,
For mass fraction of CH4 = 1 & mass fraction of H2O = 0
For mass fraction of CH4 = 0.95 & mass fraction of H2O = 0.05
For mass fraction of CH4 = 0.7 & mass fraction of H2O = 0.3
Comparison of Temperature at the Outlet for 3 cases,
CONCLUSION AND DISCUSSION:
From the above results obtained ,it is clearly evident that the pollutant NO at the outlet is less compared to the other two cases, which is achieved by mixing the water with fuel.
Also the temperature at the outlet for the third case is significntly lower than the other two cases, which indicates the reduction in the partial burning of fuel.
Hence the simulation of combustion of natural gas for different cases is studied and results are plotted.