Week 10 - Simulating Combustion of Natural Gas.

Aim: To Simulate the combustion of natural gas.

Objective:

• To import the combustion chamber 3D model in space claim, convert it into 2D and 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 chamber species – Co2, H2o, O2, N2, CH4, NOx and soot formation.

 Introduction:

Natural gas colorless highly flammable gaseous hydrocarbon consisting primarily of methane and ethane . Natural gas is one of the major combustion fluid used through out the globe . Natural gas Combustion is an exothermic chemical reaction in which natural gas and oxygen react , producing heat and several chemical by products . By products 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 fuels and air , temp rise, flame propagation , by products properties etc can be predicted . In this study we will study natural gas simulation and final by products fractions with effect of mixture and fuel as inlet , on soot formation.

Theory:

The constant increase in world energy consumption , That represents a crucial challenge for researches and scientist . Mostly fossil fuels under goes some type of combustion and produce energy and by products . Obnoxity of emission is an main concern today because is it raising the global average temperature . NOx is one of the most dangerous by product to limit it   water can be mixed during the combustion of natural gas . In this study we will discuss the effect of water quality at inlet, on mass fraction of NOx and soot formation.

Combustion is  a complex process because it involves large number of chemical reaction , which will have multiple steps in their mechanism . CFD software makes the task easy , it provides several model 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 temp and velocity , which will helps us to visualize the flame propagation and hot spot etc.,

Flames can be divided into two types premixed flame and non-premixed ( diffusion ) flames ,this further divided into laminar and turbulent . In premixed fuel air mixed homogeneously before combustion. In non-premixed fuel  air meets during fuel combustion , when a self-sustainable flame propagates at subsonic velocity then flame is called as laminar flame. Turbulent flame is just opposite to that , most of the combustion devices produces turbulent flame . In this study we are assuming that turbulent flame will raise during natural gas combustion. Also there is  no energy source which incites combustion Burning is starting by turbulence

In CFD Modelling several combustion 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 reaction mix at a 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 , The model as been widely applied in the prediction of industrial flames.

The finite chemistry model :

The finite - rate chemical 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 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 Flame-let Model:

The flame let model can be provide information on minor species and radicals such as Co and OH , and accounts for turbulent fluctuation 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 system and requires a chemistry library as input. The model can be used only for non- premixed system.

Burning velocity Model and the extended coherent flame model:

The burning velocity model and the coherent flame model are the propagation of a premixed or partially premixed flame by solving a scalar transport.The equation for the reaction progress. The BVM use the algebraic  correlation for modelling the turbulent burning velocity . When the ECFM , is the turbulent velocity is closed by solving an additional equation for the flame surface density.

 Natural gas combustion reaction:

 

This equation will  be used to calculate mass fraction and mole fraction species.

Procedure:

In this simulation two different cases are studied

Case1:Focusing on species fraction in combustion

Case 2: Focusing on the effect of  water with fuel as inlet on soot formation.

The following procedures was followed during simulation.

Geometry:

We created two chambered combustor in a CAD software and it was brought in into space claim and converted  to 2D Model because solver takes a lot of time to solve 3D Model.

3D Model:-

 

There is slot which is an indication of separation of fuel and air at the beginning of combustion posses. We choose half of 2D model because it is symmetric . Simulation results will be same for both parts.

2D model:- 

Meshing:

Model was imported from space claim 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:-

Generate Mesh:-

Solver setup:-

Started with general setting we proceeded with pressure based state solver , because fluid flow velocity is very low. In 2D space axis symmetric was chosen due to symmetrically.

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 results in analysis of fluid flow near wall.

Solver Model:

Viscous:

 

Species model was be enabled , species transport was selected , because it solves conservation equation for all species. Which predict the transport and their products 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 the setup combustion chamber does not have any ignition source , combustion is raising due to diffusion turbulence of fuel and air . Methane -  air - 2 step selected as mixture material which allows water addition. We proceed 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:0.5m/s

fuel inlet velocity:80 m/s

Boundary conditions For O2is 0.23 and CH4 is 1 by mass fraction.

These derived from equation from natural gas combustion.

Total mole fraction of air=  9.52 moles ( 2moles O2+2*3.76 mole 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 also enables because we wanted to analysis NOx and soot formation.

Inlet Boundary conditions;

Air velocity:

 

Fuel Inlet:

Nox Model:

Soot Model:

Results:

Temperature contour was created to get insights of flame front and temperature hot spot are five probe lines were created at distance of 0.1X, 0.2X, 0.3X,0.4X,0.5X 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:-

* H20 mass fraction plot:-

* O2 mass fraction plot:-

* N2 mass fraction plot:-

* CH4 mass fraction plot:-

* Nox mass fraction plot:-

 

* soot mass fraction plot:-

 

Case2:

CH4 -95% and water -5%

* Residual:-

* 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:-

   CH4 -90% and water -10%

* Residual :-

* 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:-

 

CH4 -85% and water -15%

* Residual :-

* 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:-

 

 

CH4 -80% and water -20%

* Residual:-

* 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:-

 

CH4 -75% and water -25%

* Residual :-

* 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:-

 

CH4 -70% and water -30%

*Residual :-

* 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:-

Conclusion:-

This study investigates the mass fraction of product produced during combustion of natural gases 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 cannot 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.