Week 10 - Simulating Combustion of Natural Gas.
Non-Pre Mixed Combustion Analysis Report
Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion does not always result in fire, because a flame is only visible when substances undergoing combustion vapourise, but when it does, a flame is a characteristic indicator of the reaction. While the activation energy must be overcome to initiate combustion (e.g., using a lit match to light a fire), the heat from a flame may provide enough energy to make the reaction self-sustaining.
Combustion is often a complicated sequence of elementary radical reactions. Solid fuels, such as wood and coal, first undergo endothermic pyrolysis to produce gaseous fuels whose combustion then supplies the heat required to produce more of them. Combustion is often hot enough that incandescent light in the form of either glowing or a flame is produced. A simple example can be seen in the combustion of hydrogen and oxygen into water vapor, a reaction commonly used to fuel rocket engines. This reaction releases 242 kJ/mol of heat and reduces the enthalpy accordingly (at constant temperature and pressure):
Combustion of an organic fuel in air is always exothermic because the double bond in O2 is much weaker than other double bonds or pairs of single bonds, and therefore the formation of the stronger bonds in the combustion products CO2 and H2O results in the release of energy. The bond energies in the fuel play only a minor role, since they are similar to those in the combustion products; e.g., the sum of the bond energies of CH4 is nearly the same as that of CO2. The heat of combustion is approximately −418 kJ per mole of O2 used up in the combustion reaction, and can be estimated from the elemental composition of the fuel.
Types of Combustion:-
- Complete Combustion
In complete combustion, the reactant burns in oxygen and produces a limited number of products. When a hydrocarbon burns in oxygen, the reaction will primarily yield carbon dioxide and water. When elements are burned, the products are primarily the most common oxides. Carbon will yield carbon dioxide, sulfur will yield sulfur dioxide, and iron will yield iron(III) oxide. Nitrogen is not considered to be a combustible substance when oxygen is the oxidant. Still, small amounts of various nitrogen oxides (commonly designated NOx species) form when the air is the oxidative.
Incomplete combustion
Incomplete combustion will occur when there is not enough oxygen to allow the fuel to react completely to produce carbon dioxide and water. It also happens when the combustion is quenched by a heat sink, such as a solid surface or flame trap. As is the case with complete combustion, water is produced by incomplete combustion; however, carbon, carbon monoxide, and hydroxide are produced instead of carbon dioxide.
For most fuels, such as diesel oil, coal, or wood, pyrolysis occurs before combustion. In incomplete combustion, products of pyrolysis remain unburnt and contaminate the smoke with noxious particulate matter and gases. Partially oxidized compounds are also a concern; partial oxidation of ethanol can produce harmful acetaldehyde, and carbon can produce toxic carbon monoxide.
Incomplete combustion Produced Carbon-monoxide:-
Carbon monoxide is one of the products from incomplete combustion. Carbon is released in the normal incomplete combustion reaction, forming soot and dust. Since carbon monoxide is a poisonous gas, complete combustion is preferable, as carbon monoxide may also lead to respiratory troubles when breathed since it takes the place of oxygen and combines with hemoglobin.
Problems associated with Incomplete Combustion:-
Environmental problems:
These oxides combine with water and oxygen in the atmosphere, creating nitric acid and sulfuric acids, which return to Earth's surface as acid deposition, or "acid rain." Acid deposition harms aquatic organisms and kills trees. Due to its formation of certain nutrients that are less available to plants such as calcium and phosphorus, it reduces the productivity of the ecosystem and farms. An additional problem associated with nitrogen oxides is that they, along with hydrocarbon pollutants, contribute to the formation of ground level ozone, a major component of smog.
Human health problems:
Breathing carbon monoxide causes headache, dizziness, vomiting, and nausea. If carbon monoxide levels are high enough, humans become unconscious or die. Exposure to moderate and high levels of carbon monoxide over long periods is positively correlated with risk of heart disease. People who survive severe carbon monoxide poisoning may suffer long-term health problems.Carbon monoxide from air is absorbed in the lungs which then binds with hemoglobin in human's red blood cells. This would reduce the capacity of red blood cells to carry oxygen throughout the body.
General Applications of Combustion:-
- In heating Devices
- In Explosives
- In Internal-Combustion Engines
- In Rocket Propulsion
- In Chemical Reactions
Possible Types of Combustion simulations:
On the Basis of mixing
- Non-Pre mixed Combustion (Direct Injection)
- Premixed Combustion (Carburator)
- Partially Premixed combustion
On the Basis of phase
- Fluid Phase (Volumetriic Reactions)
- Wall (surface reactions)
- Partcles (surface reactions)
- Porous region (after Treatment system)
Basics of Combustion Simulations:
For Combustion simulation, Equations are solved as given below:
- Mass
- Momentum
- Energy
- Species Transport Equations(Source term is computed using Combustion Model)
Non-premixed Combustion:-
In non-premixed combustion, fuel and oxidizer enter the reaction zone in distinct streams. This is in contrast to premixed systems, in which reactants are mixed at the molecular level before burning. Examples of non-premixed combustion include pulverized coal furnaces, diesel internal-combustion engines and pool fires.
Under certain assumptions, the thermochemistry can be reduced to a single parameter: the mixture fraction. The mixture fraction, denoted by , is the mass fraction that originated from the Fuel stream. In other words, it is the local mass fraction of burnt and unburnt fuel stream elements (C, H, etc.) in all the species (CO , H O, O , etc.).
The approach is elegant because atomic elements are conserved in chemical reactions. In turn, the mixture fraction is a conserved scalar quantity, and therefore its governing transport equation does not have a source term. Combustion is simplified to a mixing problem, and the difficulties associated with closing non-linear mean reaction rates are avoided. Once mixed, the chemistry can be modeled as being in chemical equilibrium with the Equilibrium model, being near chemical equilibrium with the Steady Laminar Flamelet model, or significantly departing from chemical equilibrium with the Unsteady Laminar Flamelet model.
Here, we are taking Considerations regarding non-Premixed combustion Simulations, which are as given below:
Diffusion:-
Turbulent Diffusion:
- Diffusion is a combination of Laminar and Turbulent diffusion.
- For Engineering Applications, Turbulent Diffusion is so high, that we can ignore the Laminar Diffusion.
- Typically, for IC Engines, Gas Turbines and Large Burners etc.
Turbulent Diffusion>> Laminar Diffusion:
- So, we use Fick's Law for 'Dilute Diffusion'.
- with the help of 'Dilute Approximations'. We are looking at diffusion coefficients of each and every species. In which how they diffuses into entire mixture, instead of diffusion coeffcient from species to species.
- Generally, when the Flow is Turbulent, we should adopt the 'Dilute Approximation'.
Combustion Source Calculation:
- It is a combination of "Finite Rate Chemistry" and Eddy Dissipation' Model.
- Where "ignition" is going to take place and "ignition Delay" is important, this model would ideally choose "finite Rate chemistry".
- and where Turbulence has major role than reaction rate is going to predicted by "Eddy dissipation" Model. So, we are applying both the Models.
- Based upon the Flow conditions, whichever rate is going to be the slowest, Solver will pick that.
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Analysis:
- Here, In this simulation we will perform on the 2D geometry. For the study we will perform It into two parts.
- Part_1: 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. Plot for CO2, H2O, CH4, N2, O2, NOx emissions & Soot formation.
- Part_2:We will add the water content in the fuel from 5% to 30% by mole and observe the effect of it on the results of NOx emissions & Soot formation.
- For the Part_2 We will do Parametric Study.
Procedure:
Geometry:-
- First we have to import the Geometry in the Fluent Spaceclaim Named as 'Combustion model.stp'.
- This Geometry consists of three different components as given below.
- First we have to combine all the three components into 1 component. Go to Design Click on Combine Commond and then Select the Whole Body and then Combine it.
- So, here is the 3D geometry If we will run for the 3D model, then It will cost a Lot in terms of time. That is why we will use the 2D model for this study, which is Axis-symmetric.
- For the 2D geometry We have to first cut the it into two parts With the help of planes by using the 'Split body' command shown in the Following figures below.
- After that we have to copy the this 3/4 portion into another design file and Make sure It will copy in the X-Y plane.
- We can see the small inlet that is further used for the Fuel inlet and the Air inlet must be Larger one.
- Save this Geometry as '2D combustion' and used for the Further Processes.
Meshing:-
- First we have to Create the Named Selection as
Air_inlet
Fuel-inlet
Axis
Outlet
- Create the Appropiate Mesh Here, are the Mesh Details:-
- We can use triangular Mesh for the more accurate Results.
Mesh:-
- Mesh Quality:-
Setup:-
- General Model Setup: Analysis Type => Steady State.
- Solver=> Pressure-based.
- 2D space => Axis symmetric.
- Energy mode must be ON to see the Temperature Variation
- Viscous Model Setup: K-epsilon Model.
- k-epsilon Model => Realizable
- Near-wall treatment => Standerd Wall Function.
- Species Model: In this model, we can set paramters related to calculations of species transport and combustion.
- Model: Species Transport. It enables calculations of multi-species transport.
- Reactions: Volumetric. It contains options related to modelling of reacting flow and we can enable calculation of the reacting flow with finite rate Formation.
- Options: 1. Inlet Diffusion which includes diffusion Flux of species at the Inlet.
- 2. Diffusion Energy It includes the effect of enthalpy transport fue to the Species diffusion in the Energy equation.
- Mixture Material: methane-air-2step
- Turbulence-Chemistry Interaction: Eddy dissipation
- Species Transport Reactions: Nox and soot Formation
- Nox Model: turn on the Pathways Thermal Nox, Prompt Nox and Fuel Nox. Select CH4 to react with O2 and Combustion takes Place.
- Soot Model : One-step
- Boundary Conditions: For this Study We set the Boundary conditions indivisual to the Fuel- inlet for the Paramteric study.
- Air-Inlet Boundary Conditions for Part_1 and Part_2 setup will be same.
- Air-inlet: Velocity-inlet. Velocity Magnitude=> 0.5 m/s
- Temperature:300K. Species Mass fraction=> 0.23
- For the Fuel-Inlet Boundary conditions must be different for the Part_1 and Part_2. The difference only in the Species setup.
- Fuel-Inlet: Velocity-inlet. Velocity Magnitude=> 80m/s
- Temperature:300K
- For the Part_1: Species Mass Fraction CH4 =>1
- For the Part_2: Species Mass Fraction CH4 and H2O set up the Parameters. We can give the initial Values for CH4 = 0.9 and H2O = 0.1.
- We can get results for different Percentage changes in H2O % amount in Fuel.
- After that we initialize with Hybrid Initialization.
- Then create report defination for Temperature contours, Mass fraction of Nox and Soot formation.
- Make sure that 'Create Output Parameter' must be enabled.
- Then Hit and Calculate upto 1200 Iterations. After Completion of simulation, Open the Parameter section and give the different Percentage of value for CH4 and H2O to add the water in Fuel and see the Results.
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Results for Part_1
Residuals:-
Temperature:-
Results in CFP Post:
Temperture Contours:-
CO2 Contours Mass Fraction:-
CH4 Contours Mass Fraction:-
CO Contours Mass Fraction:-
H2O Contours Mass Fraction:-
N2 Contours Mass Fraction:-
O2 Contours Mass Fraction:-
Nox Contours:-
Soot Contours:-
Now, we will see the Variation of Mass fraction of different species at different Locations on combustor.
- With the Help of Line Probes, we will Plot the variation of Mass Fraction at different Locations
- Line Probes at different locations created as given below:
Line_1 = 0.1m
Line_2 = 0.2m
Line_3 = 0.3m
Line_4 = 0.4m
Line_5 = 0.5m
Line_6 = 0.6m
Line_7 = 0.7m
Variation of Mass Fraction at different species at above locations as:
Variation of Temperature:-
Variation of Mass fraction of CH4:-
Variation of Mass fraction of Co:-
Variation of Mass fraction of CO2:-
Variation of Mass fraction of H2O:-
Variation of Mass fraction of N2:-
Variation of Mass fraction of O2:-
Variation of Mass fraction of NOx:-
Variation of Mass fraction of soot:-
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Results For Part_2:-
After run upto 350 Iterations open the Paramter set and give different percentage value of Ch4 and H2o to add the water and see the Results as shown in below Image:-
Parametric Study:-
- After Input the values as per percentage of ch4 into Water make sure 'Retained Data' option must be enabled.
- After that click on 'Update all Design Points'.
- Then we will get the Results according to the input Parameters and here are the Results as shown below:-
Case1 : Dp0
Residuals:-
Temperature:-
Nox Report:-
Soot Report:-
Temperature Contours:-
CH4 Contours Mass Fraction:-
Co Contours Mass Fraction:-
CO2 Contours Mass Fraction:-
H2O Contours Mass Fraction:-
N2 Contours Mass Fraction:-
O2 Contours Mass Fraction:-
Nox Contours Mass Fraction:-
Soot Contours Mass Fraction:-
Variation of Mass fraction of Temperature:-
Variation of Mass fraction of CH4:-
Variation of Mass fraction of Co:-
Variation of Mass fraction of Co2:-
Variation of Mass fraction of H2O:-
Variation of Mass fraction of N2:-
Variation of Mass fraction of O2:-
Variation of Mass fraction of NOx:-
Variation of Mass fraction of soot:-
Case 2: Dp1
Residuals:-
Temperature:-
Nox Report:-
Soot Report:-
Temperature Contours Mass Fraction:-
Ch4 Contours Mass Fraction:-
Co Contours Mass Fraction:-
Co2 Contours Mass Fraction:-
H2O Contours Mass Fraction:-
N2 Contours Mass Fraction:-
O2 Contours Mass Fraction:-
Nox Contours Mass Fraction:-
Soot Contours Mass Fraction:-
Variation of Mass fraction of Temperature:-
Variation of Mass fraction of CH4:-
Variation of Mass fraction of Co:-
Variation of Mass fraction of CO2:-
Variation of Mass fraction of H2o:-
Variation of Mass fraction of N2:-
Variation of Mass fraction of O2:-
Variation of Mass fraction of Nox:-
Variation of Mass fraction of Soot:-
Case 3: Dp2
Residuals:-
Temperture:-
Nox Report:-
Soot Report:-
Temperature contours:-
Ch4 Contours Mass Fraction:-
Co Contours Mass Fraction:-
CO2 Contours Mass Fraction:-
H2o Contours Mass Fraction:-
N2 Contours Mass Fraction:-
O2 Contours Mass Fraction:-
Nox Contours Mass Fraction:-
Soot Contours Mass Fraction:-
Variation of Mass fraction of Temperature:-
Variation of Mass fraction of Ch4:-
Variation of Mass fraction of Co:-
Variation of Mass fraction of CO2:-
Variation of Mass fraction of H2o:-
Variation of Mass fraction of N2:-
Variation of Mass fraction of O2:-
Variation of Mass fraction of Nox:-
Variation of Mass fraction of Soot:-
Case 4: Dp3
Residuals:-
Temperature:-
Nox Report:-
Soot Report:-
Temperature contours:-
Ch4 Contours Mass Fraction:-
Co Contours Mass Fraction:-
CO2 Contours Mass Fraction:-
H2O Contours Mass Fraction:-
N2 Contours Mass Fraction:-
O2 Contours Mass Fraction:-
Nox Contours Mass Fraction:-
Soot Contours Mass Fraction:-
Variation of Mass fraction of Temperature:-
Variation of Mass fraction of Ch4:-
Variation of Mass fraction of CO:-
Variation of Mass fraction of CO2:-
Variation of Mass fraction of H2O:-
Variation of Mass fraction of N2:-
Variation of Mass fraction of O2:-
Variation of Mass fraction of Nox:-
Variation of Mass fraction of Soot:-
Case 5: DP4
Residuals:-
Temperature:-
Nox Report:-
Soot Report:-
Temperature Contours:-
Ch4 Contours Mass Fraction:-
CO2 Contours Mass Fraction:-
CO Contours Mass Fraction:-
H2O Contours Mass Fraction:-
N2 Contours Mass Fraction:-
O2 Contours Mass Fraction:-
Nox Contours Mass Fraction:-
Soot Contours Mass Fraction:-
Variation of Mass fraction of Temperature:-
Variation of Mass fraction of Ch4:-
Variation of Mass fraction of CO2-
Variation of Mass fraction of CO:-
Variation of Mass fraction of H2O:-
Variation of Mass fraction of N2:-
Variation of Mass fraction of O2:-
Variation of Mass fraction of Nox:-
Variation of Mass fraction of Soot:-
Case 6: Dp5:
Residuals:-
Temperature:-
Nox Report:-
Soot Report:-
Temperature contours:-
Ch4 Contours Mass Fraction:-
CO Contours Mass Fraction:-
CO2 Contours Mass Fraction:-
H2O Contours Mass Fraction:-
N2 Contours Mass Fraction:-
O2 Contours Mass Fraction:-
Nox Contours Mass Fraction:-
Soot Contours Mass Fraction:-
Variation of Mass fraction of Temperature:-
Variation of Mass fraction of Ch4:-
Variation of Mass fraction of Co:-
Variation of Mass fraction of CO2:-
Variation of Mass fraction of H2O:-
Variation of Mass fraction of N2:-
Variation of Mass fraction of O2:-
Variation of Mass fraction of Nox:-
Variation of Mass fraction of Soot:-
Comparison from all Data Temperature, Mass Fraction of Nox and Mass Fraction of Soot When Water is added gradually:-
| Cases | Percentage of Water added | Temperature (K) | Mass fraction of Nox | Mass fraction of soot |
| Part1 | 0% | 2315.0 | 8.935E-3 | 5.488E-32 |
| Part2 | 5% | 1137.7 | 5.1192E-5 | 0.00090236 |
| Part2 | 10% | 1107.1 | 4.0083E-5 | 0.00095879 |
| Part2 | 15% | 1075.1 | 3.063E-5 | 0.00049077 |
| Part2 | 20% | 1041.6 | 2.2901E-5 | 0.00067552 |
| Part2 | 25% | 1006.8 | 1.621E-5 | 0.00028565 |
| Part2 | 30% | 970.43 | 1.1256E-5 | 0.000143 |
- From the above data we can conclude that their is small Decreament in the Temperature when you added the water into the Fuel from 5% to 30%.
- The Mass fraction of Nox and Decreases when the amount Percentage (%) of water is added into the Fuel.
- The Mass Fraction of soot increases with increament in the water into Fuel species.
- Also, there is no ignition source for Combustion. So, rate of reactions are controlled by Turbulence mixing in Eddy dissipation Model.
- So, with Large Eddy mixing time scale, combustion starts when turbulence comes into the Picture.
Comparison Between Mass fraction of Nox vs Water Percentage added:-
Comparison Between Mass fraction of Soot vs Water Percentage added:-
Conclusion:-
- From the Above study we can see that the value of Mass fraction of Nox decreases with increase in addition of water in Simulation which is great as according to the Government norms with Reduction of Nox Formation.
- Water Addition leads to increase the OH radicals that might have a significant impact in Soot Oxidation and reduce the Soot Formationin the Gas phase.
- From the Addtion of Water Combustion Temperature is also Decreases that is why Nox Formation is reduced significantly.
- As we can see there is small decreament in the Temperature. Reducing Combustion Temperature means avoiding stochiometric ratio. If this ratio is low, it generates lower concentration of thermal NOx.
- So, we can reduce the Nox emissions and soot formation, which is great as per Government Norms and It also reduces harmful effects on Environment and Humans.