Simulating Combustion of Natural Gas.
SIMULATIPOM OF COMBUSTION ON A COMBUSTER MODEL
l. OBJECTIVES
1. Simulate the process of combustion on a model using a mixture of fuel and air.
2. Plot the variation of the mass fraction of different species in the simulation using line probes at different location of the model.
ll. 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 impliesb=2`
- `2ar=2a+bimpliesar=2`
- `c=7.52arimpliesc=15.04`
The above values corresponds to the number of moles of the given species.
- Fuel:
- Air: 2moles of `O_2+7.52`moles of`N_2`
Therefore,
- Mole Fractions:
- The mole fraction of `O_2=2/(2+7.52)=0.21`
- The mole fraction of `N_2=7.52/(2+7.52)=0.79`
- Mass Fractions:
- The mole fraction of `O_2=(0.21xx 16 )/((0.21xx16)+(0.79xx14))=0.23`
- The mole fraction of `N_2=(0.79xx14)/((0.21xx16)+(0.79xx14))=0.77`
lll. PROBLEM STAEMENT
1. The fuel species is methane and the fluid is air, which contains a mixture of oxygen and nitrogen
2. The velocity of air at the inlet is 0.5 m/s.
3.The velocity of fuel at the inlet is 80 m/s.
4. Part 1 : Combustion simulation to get mass fraction of different species like `CO_2,H_2O,CH_4,N_2,O_2,NO_x and SOOT`formation
5. Part 2: Adding water in fuel from 5% to 30% by mole and observing the results of`NO_x`emission and SOOT formation.
NOTE:
- For Fuel, Use Methane-Air-2step as a mixture (It allows to add waterbin the fuel or air)
- For Soot formation, use one-step Soot Model.
lV. SPACECLAIM MODEL
1. Combustion Model
- This is 3D model, we can't run simulation for entire 3D model, it will cost a lot interms of time.
- So instead of 3D, We will perform simulation on 2D geometry, which is Axis-symmetric.
- For 2D geomerty we have to cut 3D model in two parts, and extract 2D geometry of the quadrant portion
- Then we copy all faces from X-Y plane and paste it in New Design
- Then we paste it only in X-Y plane.
- After pasting, we merge all faces by using 'combine' command and can make it a single surface.
- So the 2D geometry can be ready for further process
V. BOUNDARIES
the boundaries of the geometry are generated using the named selection feature of Ansys
1. Axis
2.fuel_inlet
3.Air_inlet
4.walls
5.outlet
Vl. MESH SETTING
1. Element Size: 1 mm
2. Method: All Triangle Method
3. Number of Nodes: 73670
4. Number Of Elements: 145540
Vll. SIMULATION SETUP
1. Solver: Steady
2. Type: Pressure Based
3. Turbulence Model: Standard k-epsilon with Standard Wall Functions
4. Energy Model: On
5. Species Model:
6: NOx Model:
7: Soot Model:
8. Boundaries:
- Air_inlet
- Velocity: 0.5 m/s
- Thermal Temperature: 300 K
- Species Mass Fraction:
- CH4: 0
- O2: 0.23
- CO2: 0
- H2O: 0
- Fuel_inlet:
- Velocity: 80 m/s
- Thermal Temperature: 300k
- Species Mass Fraction:
- CH4: 1
- O2: 0
- CO2: 0
- H2O: 0
- Outlet
- Pressure: 0 Pa(Gauge Pressure)
8. Solution Method: SIMPLE
9. Initialization:
- Method: Hybrid Initialization
- Number of Iteration: 1200
Vlll. POST PROCESSING
A. Contours
We create contours for Temperature, no,n2o,soot,ch4,o2
B. Charts At the Line Probes
With help of line probes, we can plot the variation of Mass fraction at different locations.
For that we have to create line probe at different locations.
Line probes at different location from left to right meter can be created as given below:
Line 1: Two point Method (0.05,0) (0.05,0.0852)
Line 2: Two point Method (0.1,0) (0.1,0.0852)
Line 3: Two point Method (0.15,0) (0.15,0.0852)
Line 4: Two point Method (0.20,0) (0.20,0.0852)
Line 5: Two point Method (0.25,0) (0.25,0.0852)
Line 6: Two point Method (0.3,0) (0.3,0.0852)
Line 7: Two point Method (0.35,0) (0.35,0.0852)
Line 8: Two point Method (0.45,0) (0.45,0.0852)
Line 9: Two point Method (0.6,0) (0.6,0.0852)
Line 10: Two point Method (0.7,0) (0.7,0.0852)
Variation Of Mass Fraction Of different species at anove locations can be given by plots
PART 1:
Residual
Temperature Contour:
CO2 Mass Fraction:
H2O Mass Fraction:
CH4 Mass Fraction:
N2 Mass Fraction:
O2 Mass Fraction:
Contours of NOx Mass Fraction:
NO:
N2O:
Contours of SOOT Mass Fraction:
PART 2:
CASE 1: 95 % CH4 and 5% H2O
Residual:
Temperature Contour:
CO2 Mass Fraction:
H2O Mass Fraction:
CH4 Mass Fraction:
N2 Mass Fraction:
O2 Mass Fraction:
Contours of NOx Mass Fraction:
NO:
N2O:
Contours of SOOT Mass Fraction:
CASE 2: 90 % CH4 and 10 % H2O
Residual:
Temperature Contour:
CO2 Mass Fraction:
H2O Mass Fraction:
CH4 Mass Fraction:
N2 Mass Fraction:
O2 Mass Fraction:
Contours of NOx Mass Fraction:
NO:
N2O:
Contour of SOOT Mass Fraction:
CASE 3: 80 % CH4 and 20 % H2O
Residual:
Temperature Contour:
CO2 Mass Fraction:
H2O Mass Fraction:
CH4 Mass Fraction:
N2 Mass Fraction:
O2 Mass Fraction:
Contours of NOx Mass Fraction:
NO:
N2O:
Contours of SOOT Mass Fraction:
Case 4: 70% CH4 and 30% H2O
Residuals:
Temperature:
CO2 Mass Fraction:
H2O Mass Fraction:
CH4 Mass Fraction:
N2 Mass Fraction:
O2 Mass Fraction:
Contour Of NOx Mass Fraction:
NO Mass Fraction:
N2O Mass Fraction:
SOOT Mass Fraction:
lX. OBSERVATION
- We can see that from the begining, CH4 is burning from high at 1 to near 0. But in real there are some unburnt carbon particles always remains.
- Due to this incomplete combustion of zzhydrocarbons, generation of SOOT or Fly Ash happens.
- We can see there are NOx emissions, which are due to combustion of nitrigen present in the air, which is 78%. Due to that we can see N2 is maximum all over the contour
Comparison of temperature & Mass Fraction Of Nox and Soot
| CASE | Temperature | Mass Fraction Of N2O | Mass Fraction Of NO | Mass Fraction Of SOOT |
| Part 1 | 2.31 E+03 | 3.76 E-07 | 7.59 E-03 | 1.08 E-11 |
| Case_1 | 2.30 E+03 | 3.58 E-07 | 6.37 E-03 | 7.03 E-12 |
| Case_2 | 2.29 E+03 | 3.42 E-07 | 5.32 E-03 | 4.13 E-12 |
| Case_3 | 2.26 E+03 | 3.14 E-07 | 3.53 E-03 | 2.86 E-12 |
| Case_4 | 2.23 E+03 | 2.87 E-07 | 2.17 E-03 | 1.49 E-12 |
- We can observe very small decrease in temperature from Case_1 to Case_2, as we have added water in fuel in each case from 5 % to 30%.
- Also we can observe small decrease in NOx emission as we add water from 5% to 30%.
- We can see the significant amount of decrease in SOOT formation from Case_1 to Case_4.
- Also there a is no ignition source for combustion. So, rate of reactions are controlled by turbulance mixing in Eddy dissipation model.
- So with large eddy mixing time scale, combustion starts when turbulance comes in picture.
X. CLAIMS
- As per above observations, we can say that addition of water to the fuel helps indecreasing NOx and SOOT Formation.
- Water addition leads to increase OH radicals that might have a significant impact in SOOT Oxidation and reduce the SOOT formed in Gas Phase.
- Added water dilutes calories generatd by combustion. That decreases combustion temperature. If temperature reduces sufficiently, NOx cannot form in great concentration.
- We can see small decrease in temperature. Reducing combustion temperature means avoiding stoichiometric ratio. If this ratio is low, it generates lower concentration of thermal NOx.
- So, with these or various types of technologies, we can reduce NOx emissions and SOOT formation, which helps us to meet regulatory requirements by government. And we can reduce its harmful effects on environment and humans.
Xl. CONCLUISION
The simulation provides a complete temperature distribution along the geometry. Contours of various species can be studied.
By adding water, the pollutant soot and NOx formation at the outlet is reduced. The plots of the same is also looked into.