Week 10 - Simulating Combustion of Natural Gas.
OBJECTIVE: To perform a combustion simulation on the combustor model.
Part I
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, NOx emissions & Soot formation.
Part II
As you must have observed from the above simulation, the Nox and soot is getting formed at the outlet of the combustor. Such formation has harmful effects on the environment and humans. The stringent government norms also demand the least formation of Nox and soot and to satisfy those requirements, you need to check the effect of adding the water in the fuel.
In this part, you need to add the water content in the fuel from 5% to 30% by mole and observe the effect of it on the results. It is necessary to provide line plots and contours to prove your claim.
INTRODUCTION:
Combustion:
- Combustion involves chemical reactions converting fuel and oxygen to combustion products with light and heat energy. It involves fluid flow, consumption and generation of species by chemical kinetics and heat transfer ( convection and radiation).
- Combustion becomes more complex because of phenomena of flame propagation, slow or fast ignition, explosions, slow ignition, correct fuel to oxidizer ratios, phase change for liquid fuel,and so forth depend in subtle ways on the conditions under which combustion takes place.
Application of Combustion Modeling:
- Burners: LPG gas stoves, Furnaces, Boilers, Gasifier etc.
- Engine : Petrol and diesel engines
- Gas turbine combustor: Power plant steam turbine, air-craft gas turbine combustor etc.
- Safety: protection from fires, explosions or blast etc.
- Emission controls: Design and development of low NOx/SOx burner or combustor.
Based on mixing
Premixed combustion
- In the mixing chamber, fuel and oxidizer (air) are already mixed at the molecular level before ignition or combustion
- Cold reactants propagate into hot products
- Rate of propagation (flame speed) depends on the internal flame structure
- Much more difficult to model than non-premixed combustion problems
- Turbulence distorts the laminar
- Examples: SI enginer, LPG gas stove etc.
Non-premixed combustion
- Separate inlet/ streams for Fuel and oxidizer (air) to the combustion chamber
- Convection or diffusion of reactants from either side into a flame sheet
- Turbulent eddies distort the laminar flame shape and enhance mixing
- May be simplified to a mixing problem
- Examples: Burner for boiler /furnaces, coal combustion, candle flame, wood fire etc.
Partially premixed combustion
- In some applications, primary air (oxidizer) is mixed with fuel to form the unburnt mixture before combustion such cases are treated as partially premixed combustion.
- Premixed fuel/oxidizer inlet streams
Based on Phase
Volumetric: Takes place when the fluid is in fluid phase.
Surface: Takes place on the wall.
Porous: Takes place inside pores
Eddy Dissipation:
Most fuels are fast burning and the overall rate of reaction is controlled by turbulence mixing. In the non-premixed flames, turbulence slowly mixes the fuel and oxidizer into the reaction zones where they burn quickly. In premixed flames the turbulence slowly mixes cold reactants and hot products into the reaction zones where reaction occurs rapidly. In such cases the combustion is said to be mixing-limited, and the complex and often unknown chemical kinetics can be safely neglected. In this model, the chemical reaction is governed by large eddy mixing time scale. Combustion initiates whenever there is turbulence present in the flow. It does not need an ignition source to initiate the combustion. This type of model is valid for the non-premixed combustion, but for the premixed flames the reactant is assumed to burn at the moment it enters the computational model , which is a shortcoming of this model as in practice the reactant needs some time to get to the ignition temperature to initiate the combustion.
Natural Gas Combustion Reaction
PROCEDURE:
GEOMETRY:
- Upload the combustor model onto SpaceClaim, since it is a 3D model and we are only interested in the 2D geometry of the combustor.
- We need to split the body by 3/4th and select the plane XY plane region of geometry since we are using the axisymmetric condition.
- After splitting the body, select the 2D Plane normal to the Z-axis and copy that geometry to a new design sheet and delete the old one since we do not need it.
MESHING:
- Generate base mesh with capture proximity settings and set the cell gap to 3 cells or as per your requirement.
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Using the named selection feature of Ansys, we are naming as
1. Axis
2. fuel_inlet
3. Air_inlet
4. walls
5. outlet
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MESH SETTING
1. Element Size: 1 mm
2. Method: All Triangle Method
3. Number of Nodes: 79030
4. Number Of Elements: 165947
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:
RESULTS:
CASE I - For 1 molar fraction of CH4
Residuals
NOx
We can observe the formation of NOx at the initial stage up to 5-10 iteration but after that there is no formation of NOx it is almost zero.
Soot
- Soot is black powder that comes from burning things and collects in chimney.
- here we can observe that soot is being formed up to 5-30 iteration but after that there is no formation of soot. it is almost zero at the outlet at 1000 iteration.
Temperature contour
Mass fraction of NOx and soot at outlet
Mass fraction of CO2 at Line probes
Initially mass fraction of the CO2 at the inlet is Zero but it increases by the length of the burner. but it can be observed that the maximum mass fraction of the CO2 is 0.143 at line 2 then gradually it is decreasing upward.
Fraction of H2O at Line probes
Initially mass fraction of the H2O at the inlet is Zero but it increases by the length of the burner. But it can be observed that the maximum mass fraction of the H2O is 0.120 at line 2 then gradually it is decreasing upward.
Mass fraction of CH4 at Line probes
The mass fraction of CH4 in the burner at all line probes is zero, because after entering into burner it is being getting combusted, that's why there is no traces of CH4 in burner except inlet.
Mass fraction of N2 at Line probes
77% of Nitrogen present in air. so, it enters the burner with air because we need oxygen for the combustion so we feed air for the source of Oxygen. Since N2 is an inert gas so it does not react with anyone in the burner. so, it is left unreacted that's why we can observe that mass fraction of nitrogen is higher everywhere in the burner except inlet. but a small amount of Nitrogen reacts with oxygen due to higher temperature and form NOx.
Mass fraction of O2 at Line probes
We can observe that oxygen is present out of the flame but not in the flame. because the molecules present in the flame are consumed for the combustion. That's why we can see there is no traces of oxygen in flame.
We can observe that mass fraction of NO is increasing as we move upward in the burner.
Mass fraction of soot at Line probes
We can observe that soot is being formed in flame but there are no traces of soot outside the flame, because it is being converted to carbon dioxide.
CASE II: For 0.95 molar fraction of CH4 & 0.05 molar fraction of H2O
Residuals
NOx
Soot
Temperature contour
Mass fraction of soot at Line probes
Mass fraction of NO at Line probes
Mass fraction of CH4 at Line probes
Mass fraction of CO at Line probes
Mass fraction of CO2 at Line probes
Mass fraction of N2 at Line probes
Mass fraction of O2 at Line probes
CASE III: For 0.90 molar fraction of CH4 and 0.10 molar fraction of H2O
Residuals
NOx
Soot
Temperature contour
Mass fraction of NO at Line probes
Mass fraction of Soot at Line probes
Mass fraction of CH4 at Line probes
Mass fraction of CO at Line probes
Mass fraction of CO2 at Line probes
Mass fraction of H2O at Line probes
Mass fraction of N2 at Line probes
Mass fraction of O2 at Line probes
CASE IV: For 0.85 molar fraction of CH4 & 0.15 molar fraction of H2O
Residuals
NOx
Soot
Temperature contour
Mass fraction of NO at Line probes
Mass fraction of soot at Line probes
Mass fraction of CH4 at Line probes
Mass fraction of CO at Line probes
Mass fraction of CO2 at Line probes
Mass fraction of H2O at Line probes
Mass fraction of N2 at Line probes
Mass fraction of O2 at Line probes
CASE V: For 0.80 molar fraction of CH4 & 0.20 molar fraction of H2O
Residuals
NOx
Soot
Temperature contour
Mass fraction of NO at Line probes
Mass fraction of soot at Line probes
Mass fraction of CH4 at Line probes
Mass fraction of CO at Line probes
Mass fraction of CO2 at Line probes
Mass fraction of H2O at Line probes
Mass fraction of N2 at Line probes
Mass fraction of O2 at Line probes
CASE VI: For 0.75 molar fraction of CH4 & 0.25 molar fraction of H2O
Residuals
NOx
Soot
Temperature contour
Mass fraction of NO at Line probes
Mass fraction of soot at Line probes
Mass fraction of CH4 at Line probes
Mass fraction of CO at Line probes
Mass fraction of CO2 at Line probes
Mass fraction of H2O at Line probes
Mass fraction of N2 at Line probes
Mass fraction of O2 at Line probes
CASE VII: For 0.70 molar fraction of CH4 and 0.30 molar fraction of H2O
Residuals
NOx
Soot
Temperature contour
Mass fraction of NO at Line probes
Mass fraction of soot at Line probes
Mass fraction of CH4 at Line probes
Mass fraction of CO at Line probes
Mass fraction of CO2 at Line probes
Mass fraction of H2O at Line probes
Mass fraction of N2 at Line probes
Mass fraction of O2 at Line probes
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 hydrocarbons, generation of SOOT or Fly Ash happens.
- We can see there are NOx emissions, which are due to combustion of nitrogen present in the air, which is 78%. Due to that we can see N2 is maximum all over the contour
Comparing the NOx and soot levels for different water addition levels:
| Percentage of water addition (%) | The ratio of methane and water mixed (CH4:H2O) | Temperature | Mass fraction of NOx emissions | Mass fraction of soot formations |
| 10 | 0.9:0.1 | 2.302 | 6.552e-03 | 5.66e-02 |
| 20 | 0.8:0.2 | 2.263 | 3.721e-03 | 3.286e-02 |
| 30 | 0.7:0.3 | 2.230 | 2.297e-03 | 2.063e-02 |
- 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 10% to 30%.
- Also we can observe small decrease in NOx emission as we add water from 10% 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
RESULT:
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From the above experiment and the data from the table, it can be concluded,
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NOx emissions and soot formations reduced as there is an increase in the addition of water.
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But the efficiency of combustion reduces as there is reduction incomplete combustion.
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Few predictions as per the research paper about the soot reduction wrt to water addition.
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Reduction in equivalence ratio, or enlistment. Hence, lean mixtures soot less than rich mixtures.
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After checking the mass fraction of soot formation, it’s evident that for all cases results in unbalanced. But the amount of soot formation is significantly lower and is nearer to outlet of burner.