Week 10 - Simulating Combustion of Natural Gas.
1. Title – Simulating combustion of natural gas.
2. Objective –
- To import the combustor 3D model in Spaceclaim, convert it into 2D ad save it as new model.
- To mesh the combustor model with suitable mesh setting.
- To setup the solver setting with proper solver model and its parameter.
- To generate plots of mass fraction of in combustion species -CO2, H2O, O2, N2, CH4, NOx and soot formation.
3. Introduction –
Natural gas, colourless highly flammable gaseous hydrocarbon consisting primarily of methane and ethane. Natural gas is one of the major combustion fuels used throughout the globe. Natural gas combustion is an exothermic chemical reaction in which natural gas and oxygen react, producing heat and several chemical byproducts. Byproducts of combustion are a concern, albeit It is certainly cleaner than burning many fossil fuels. To study combustion of natural gas CFD is used, by simulating combustion mixing of fuel and air, temperature rise, flame propagation, byproduct properties etc can be predicted. In this study we will simulate natural gas combustion and find byproduct fractions with effect of mixture of water and fuel as inlet, on soot formation.
4. Theory –
The constant increase of world energy consumption, that represents a crucial challenge for researchers and scientists. Mostly, fossil fuel under goes some type of combustion and produce energy and byproducts. Obnoxity of emission is a main concern today because is it raising the global average temperature. Nox is one of the most dangerous byproduct, to limit it, water can be mixed during the combustion of natural gas. In this study we will study the effect of water quantity at inlet, on mass fraction of NOx and soot formation.
Combustion is a very complex process because it involves large number of chemical reactions, which will have multiple steps in their mechanism. CFD software makes this task easy, it provides several models like eddy dissipation, finite rate chemistry, finite rate eddy dissipation model etc. to find source of combustion in no time and as a result it allows you to plot mass fraction plots, contours of temperature and velocity, which is very helpful to visualize flame propagation, hot spot etc.
Flames can be divided into two types premixed flame and non-premixed(diffusion) flames. These further divided into laminar and turbulent.in premixed fuel and air mixed homogenously before combustion. In non-premixed fuel and air meet during combustion. When a self-sustainable flame propagates at subsonic velocities then flame is called as laminar flame. Turbulent flame is just opposite to that, most of the combustion device produces turbulent flames. In this study we are assuming that turbulent flame will rise during natural gas combustion. Also, there is no energy source which incites combustion, Burning is starting by turbulence.
In CFD modelling several combustion models are available –
Eddy dissipation model - The eddy dissipation model is based on the concept that chemical reaction is fast relative to the transport processes in the flow. When reactants mix at the molecular level, they instantaneously form products. The model assumes that the reaction rate may be related directly to the time required to mix reactants at the molecular level.
The eddy dissipation model was developed for use in a wide range of turbulent reacting flows covering premixed and diffusion flames. Because of its simplicity and robust performance in predicting turbulent reacting flows, this model has been widely applied in the prediction of industrial flames.
The finite chemistry model –
The finite-rate chemistry model allows the computation of reaction rates described by the molecular interaction between the components in the fluid. It can be combined with the eddy dissipation model for flames where chemical reaction rates might be slow compared with the reactant mixing rates. The finite-rate chemistry model is best applied to situations where the chemical time scale is rate-limiting. This model can be used in conjunction with both laminar and turbulent flow.
The flamelet model –
The flamelet model can provide information on minor species and radicals, such as CO and OH, and accounts for turbulent fluctuations in temperature and local extinction at high scalar dissipation rates, for the cost of solving only two transport equations. The model is only applicable for two-feed systems and requires a chemistry library as input. The model can be used only for non-premixed systems.
Burning velocity model and the extended coherent flame model –
The burning velocity model (BVM) and the extended coherent flame model (ECFM) model the propagation of a premixed or partially premixed flame by solving a scalar transport equation for the reaction progress. The BVM uses an algebraic correlation for modelling the turbulent burning velocity. When using the ECFM, the turbulent burning velocity is closed by solving an additional transport equation for the flame surface density.
Natural gas combustion reaction –
This equation will be used to calculate mass fraction and mole fraction species.
5. Procedure –
In this simulation two different cases are studied, case 1 was focusing on species fraction in combustion and case 2 was focusing on the effect of water with fuel as inlet, on soot formation. The following procedure was followed during the simulation –
Geometry –
We created two chambered combustor in a CAD software and it was brought it into Spaceclaim and converted to 2D model because solver takes a lot of time to solve 3D model.
3D model -
There is a slot which is an indication of separation of fuel and air at the beginning of combustion prosses. We chose half of 2D model because it is symmetric. Simulation results will be same for both parts.
2D model -
Meshing –
Model was imported from Spaceclaim and tia mesh was deployed with element size 1mm. Named selection was created like fuel inlet, air inlet, wall etc, these used for boundary conditions in solver setup.
Mesh parameters -
Generated Mesh -
Solver setup –
Started with general setting we proceeded with pressure based steady state solver, because fluid flow velocity is very low. In 2D space axisymmetric was chosen due to symmetricity.
General Setting -
Realizable K-epsilon with standard wall functions chosen as solver model, for this simulation flow near wall was not major concern. K-omega produces best result in analysis of fluid flow near wall.
Solver Model -
Species model was enabled. Species transport was selected, because it solves conservation equation for all species, which predicts the transport and their production in the combustion zone. In fluid phase volumetric reaction is considered always, so it was selected. Inlet diffusion and diffusion energy source was selected in this setup combustion chamber does not have any ignition source, combustion is arising due to diffusion turbulence of fuel and air. Methane-air-2step selected as mixture material which allows water addition. We proceeded with eddy dissipation model due to its robustness and it takes less time to solve model compare to other models.
Species Model -
Air inlet velocity was 0.5m/s and fuel inlet velocity were given 80m/s in the boundary condition with 0.23 O2 and 1 CH4 by mass fraction. These derived from equation of natural gas combustion.
Total mole fraction OF air = 9.52 moles (2moles O2+2*3.76 moles N2)
Mole fraction of O2 = moles of O2/total moles = 2/9.52 = 0.21
Mass fraction of O2 in air = 0.23
Moles of CH4 = 1
Mole fraction or mass fraction can be given to specify species. Nox model and soot model was also enable because we wanted to analyse Nox and soot formation.
Inlet Boundary conditions -
Air inlet -
Fuel Inlet -
NOx model -
Soot model -
Results –
Temperature contour was created to get insights of flame front and temperature hotspots. Five probe lines were created at distance 0.1x, 0.2x, 0.3x, 0.5x, 0.7x x is the total distance, to inspect mass fraction of six species CO2, H2O, O2, N2, CH4 and NOx. Chart of soot formation was also plotted. In case 2 fuel is added into chamber with water from 5% to 30% to investigate the effect of water on soot formation.
Case 1 -
Residuals -
Temperature Contour -
CO2 mass fraction plot
H2O mass fraction plot
O2 mass fraction plot
N2 mass fraction plot
CH4 mass fraction plot
Nox mass fraction plot
Soot mass fraction plot
Case 2 -
CH4 - 95% and water 5%
Temperatue Contour
CO2 mass fraction plot
H2O mass fraction plot
O2 mass fraction plot
N2 mass fraction plot
CH4 mass fraction plot
Nox mass fraction plot
Soot mass fraction plot
CH4 - 90% and water 10%
Temperatue Contour
CO2 mass fraction plot
H2O mass fraction plot
O2 mass fraction plot
N2 mass fraction plot
CH4 mass fraction plot
Nox mass fraction plot
Soot mass fraction plot
CH4 - 85% and water 15%
Temperatue Contour
CO2 mass fraction plot
H2O mass fraction plot
O2 mass fraction plot
N2 mass fraction plot
CH4 mass fraction plot
Nox mass fraction plot
Soot mass fraction plot
CH4 - 80% and water 20%
Temperatue Contour
CO2 mass fraction plot
H2O mass fraction plot
O2 mass fraction plot
N2 mass fraction plot
CH4 mass fraction plot
Nox mass fraction plot
Soot mass fraction plot
CH4 - 75% and water 25%
Temperatue Contour
CO2 mass fraction plot
H2O mass fraction plot
O2 mass fraction plot
N2 mass fraction plot
CH4 mass fraction plot
Nox mass fraction plot
Soot mass fraction plot
CH4 - 70% and water 30%
Temperatue Contour
CO2 mass fraction plot
H2O mass fraction plot
O2 mass fraction plot
N2 mass fraction plot
CH4 mass fraction plot
Nox mass fraction plot
Soot mass fraction plot
6. Conclusion –
This study investigates the mass fraction of product produced during combustion of natural gas and soot formation. Soot and Nox has adverse effect on environment although soot can be used as black pigment. Soot can be reduced by after treatment methods. In this study we added water into fuel and simulated. Water inlet is affecting soot formation but we can’t say much about it and NOx mass fraction is decreasing as water content is increasing. Methane is decreasing as we are proceeding towards outlet, because burning is arising somewhere middle of the combustor.