Week 10 - Simulating Combustion of Natural Gas.
REPORT ON COMBUSTION SIMULATION ON A COMBUSTOR MODEL
AIM
To Perform a combustion simulation on the combustor model and plot the variation of the mass fraction of the different species. 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.
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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
THEORY
Combustion:
A combustion is the Exothermic chemical reaction of a fuel with an oxidant to form the product. Basically there are two type combustion one is flame mode in which the thin zone of intense chemical reaction called ‘flame’ exist and another one is Non-flame mode in which volumetric combustion process is exited. The combustion simulation is performed to get the predictions of:
- flow fields and mixing characteristics
- Temperature fields
- Species Concentrations
- Particulates and pollutants.
The application of combustion modelling is applying for various kind of reacting flows such as furnaces, boilers, Process heaters, Gas turbines, automobiles and Rocket engines.
Modelling of combustion reaction in a software is difficult and need to take care for species which are taking part in reaction and its products. The combustion CFD model comprises of chemical kinetics and similarly achieving reacting flow mixture environment. Considering chemical kinetics, it is also important to have properly defined physical model with simulation to get the maximum out of process. Computational fluid dynamics of combustion model should be capable of enough to provide information related to species concentration, their volumetric generation or diffusion rate and changes in the system parameters like enthalpy, temperature and mixture density. The similar model should be good enough to solve the general transport equations for fluid flows and heat transfer as well as additional equations of combustion chemistry and chemical kinetics incorporated as per simulation environment details.
Chemical kinetics is part of combustion phenomenon, which is the study of entire reaction mechanism subject to different operating conditions. The whole reaction mechanism is the overall reaction of fuel and air with the elementary reaction. The elementary reaction is consisting of intermediate reaction cause generation of intermediate species, and the species react with fuel and air will cause the elementary reaction. This intermediate reaction caused due to breakup of chain reaction of fuel and air. The chemical kinetics is important as it provide when combustion process is going to start i.e. time period of ignition delay, rate of work done and used to determine emission from the chemical reactions. The elementary reactions in the case any combustion phenomena are mostly of three types which are bimolecular reactions, unimolecular reactions and termolecular reactions. In bimolecular reaction the reactants produce the product and same product can be undergoing to produce the reactant. This reaction is most common in combustion. In unimolecular reaction there are two types one is isomerization in which transformation of composition is occurred and another case is decomposition in which breakdown of composition is occurred.
Based on mixing:
Pre-Mixed 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 engine, LPG gas stove etc.
Non Pre-Mixed 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 Pre-Mixed 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 computation 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.
PROCEDURE
The first step is the generation of the geometry thus CAD file is imported into Ansys Space Claim.
The 3D model will increase the computational cost and time to run the simulation and also in an Academic license there are limitations for mesh generation. So we are extracting the 2D model from the 3D model and doing a simulation on it.
For that we combine the models and then we create the 2D as per the requirement
Then we move to the meshung part where we give the names for the parts and then we create the mesh
Axis:
Fuel Inlet:
Air Inlet:
Outlet:
Wall:
For refining the mesh for this case we are using the all triangle method. We used mesh size of 1 mm.
Now we jump into setting up of physics where we use Steady state , Pressure based Axisymmetric model.
We use k-epsilon model with standard wall treatement and we use species transport as the species model with volumetric reactions
Now we give the fuel species of Nox and soot.
The next step is giving the boundary conditions at air nlet, fuel inlet along with adding the species mass fraction values for oxygen at air (a value of 0.23) inlet and for fuel inlet we give the mass fraction for methane (a value of 1).
Then we initialize the case and run it for 600 iterations and then in the post processing part we create the line probe as asked
Coordinates of Line probes :
Point 1---> x=0.1m y=0m z=0m
Point 2---> x=0.1m y=0.085m z=0m
Line 2 :
Point 1---> x=0.2m y=0m z=0m
Point 2---> x=0.2m y=0.085m z=0m
Line 3 :
Point 1---> x=0.3m y=0m z=0m
Point 2---> x=0.3m y=0.085m z=0m
Line 4 :
Point 1---> x=0.4m y=0m z=0m
Point 2---> x=0.4m y=0.085m z=0m
RESULTS
CASE 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 as shown in Fig. You need to plot for CO2, H2O, CH4, N2, O2, NOx emissions & Soot formation.
Residuals
Temperature Contour
Contour and plot of co2 mass fraction:
Contour and plot of CH4 mass fraction:
Contour and plot of H20 mass fraction:
Contour and plot of N2 mass fraction:
Contour and plot of 02 mass fraction:
Contour and plot of N0x mass fraction:
Contour and plot of soot mass fraction:
CASE 2 : Add the water content in the fuel from 5% to 30% by mole
1 Methane 0.95 and h2o 0.05
RESIDUAL AND TEMPERATURE CONTOUR
Contour and plot of ch4 mass fraction:
Contour and plot of co2 mass fraction:
Contour and plot of h20 mass fraction:
Contour and plot of n2 mass fraction:
Contour and plot of o2 mass fraction:
Contour and plot of Noxs fraction:
Contour and plot of soot mass fraction:
REPORT DEFINITIONS
At 90% Methane and 10% water
RESIDUALS AND TEMPERATURE CONTOUR
Contour and plot of ch4 mass fraction:
Contour and plot of co2 mass fraction:
Contour and plot of h2o mass fraction:
Contour and plot of n2 mass fraction:
Contour and plot of o2 mass fraction:
Contour and plot of soot mass fraction:
Contour and plot of NO mass fraction:
REPORT DEFINITIONS
At 85% Methane and 15% water
RESIDUALS AND TEMPERATURE CONTOUR
Contour and plot of ch4 mass fraction:
Contour and plot of co2 mass fraction:
Contour and plot of h2o mass fraction:
Contour and plot of n2 mass fraction:
Contour and plot of o2 mass fraction:
Contour and plot of soot mass fraction:
Contour and plot of NO mass fraction:
REPORT DEFINITIONS
At 80% Methane and 20% water
RESIDUALS AND TEMPERATURE CONTOUR
Contour and plot of ch4 mass fraction:
Contour and plot of co2 mass fraction:
Contour and plot of h20 mass fraction:
Contour and plot of N2 mass fraction:
Contour and plot of O2 mass fraction:
Contour and plot of soot mass fraction:
Contour and plot of NO mass fraction:
REPORT DEFINITIONS
At 75% Methane and 25% water
RESIDUALS AND TEMPERATURE CONTOUR
Contour and plot of ch4 mass fraction:
Contour and plot of co2 mass fraction:
Contour and plot of h2o mass fraction:
Contour and plot of n2 mass fraction:
Contour and plot of o2 mass fraction:
Contour and plot of soot mass fraction:
Contour and plot of NO mass fraction:
REPORT DEFINITIONS
At 70% Methane and 30% water
RESIDUALS AND TEMPERATURE CONTOUR
Contour and plot of ch4 mass fraction:
Contour and plot of co2 mass fraction:
Contour and plot of h2o mass fraction:
Contour and plot of N2 mass fraction:
Contour and plot of o2 mass fraction:
Contour and plot of soot mass fraction:
Contour and plot of NO mass fraction:
REPORT DEFINITIONS
| Percentage of water addition (%) | The ratio of methane and water mixed (CH4:H2O) | Mass fraction of NOx emissions | Mass fraction of soot formations |
| 5 | 0.95:0.05 | 5.325e-5 | 1.32e-3 |
| 10 | 0.90:0.10 | 4.169e-5 | 1.29e-3 |
| 15 | 0.85:0.15 | 3.196e-5 | 1.27e-3 |
| 20 | 0.80:0.20 | 2.375e-5 | 1.08e-3 |
| 25 | 0.75:0.25 | 1.703e-5 | 3.13e-4 |
| 30 | 0.70:0.30 | 1.175e-5 | 2.02e-4 |
CONCLUSIONS
From the above experiment and the data from the table, it can be concluded,
1. NOx emissions and soot formations reduced as there is an increase in the addition of water.
2. But the efficiency of combustion reduces as there is reduction incomplete combustion.
3. Few predictions as per the research paper about the soot reduction wrt to water addition.
Reduction in equivalence ratio, or enlistment. Hence, lean mixtures soot less than rich mixtures.
Increase in unburned gas temperature or, more directly increase in flame temperature above values of 1800 K. Hence, increased temperature above typical values will reduce soot.
Decrease in flame temperature below values of 1600 K. Hence, very-low combustion temperatures will reduce soot