Combustion Analysis On Air Methane Mixture

Based on Mixing:

• 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.

• Premixed Combustion:

In premixed combustion, fuel and oxidizer are mixed at the molecular level prior to ignition. Combustion occurs as a flame front propagating into the unburnt reactants. Examples of premixed combustion include aspirated internal combustion engines, lean- premixed gas turbine com- bustors, and gas-leak explosions.

• Partially Combustion:

In this combustion takes place in between the premixed and Non_premixed combustion.

Based on Phase:

• Fluid Phase: The combustion takes palce in the fluid phase/region ,such as Direct injection combustion in C.I engines etc .(Volumetric reactions)
• Walls(Surface reactions): The reaction take place on the walls and the surfaces Ex:Evaporation
• Particles(Surface reactions): Reactions take place at the particle surfaces levels such as atomization .
• Porous region: The reactions take place with the pressure drop across the domain such as after treatment reactions in catalytic convertor etc

The Combustion Process:

The basic combustion process can be described by the fuel (the hydrocarbon) plus oxydizer (air or oxygen) called the Reactants,which undergo a chemical process while releasing heat to form the Products of combustion such that the mass is conserved.In the simplest combustion process known as Stiochiometric Combustion,all the carbon in the fuel forms carbondioxide (CO₂) and all the hyderogen forms water(H₂O) in the products,thus we can write the chemical reaction as follows

Note that this reaction yields five unknowns: z, a, b, c, d, thus we need five equations to solve. Stoichiometric combustion assumes that no excess oxygen exists in the products, thus d = 0. We obtain the other four equations from balancing the number of atoms of each element in the reactants (carbon, hydrogen, oxygen and nitrogen) with the number of atoms of those elements in the products. This means that no atoms are destroyed or lost in a combustion reaction.

Note that the water formed could be in the vapour or liquid phase,depending on the temperature and pressure of the combustion products.

As an example consider the Stoichiometric combustion of methane(CH₄) in atomespheric air.Equating the molar coefficients of the reactants and the products we obtain:

Now for 1 mole of CH4 we need 2 moles of Oxygen (O2) and 7.52 moles of Nitrogen (N2) to balance the reaction.Air mixture implies the 9.52 the mole fraction of the O2 is 2/9.52 =0.21 and the mole fraction of the N2 is 7.52/9.52 =0.79 .

The mass fractions for O2 is 0.23 and mass fraction of N2 is 0.77 which is used as inputs for our solution.

Geometry:

We import the STEP file of the combustor model into the spaceclaim.This shows in the below image.

Once the 3D model is imported into the spaceclaim then by using the split body command we can split the body by creating the planes.Then the splitted body is copied and pasted into new design as a 2D body.Then the combine tool command is used to combine all the different layers of the geometry.Below images shows the process of converting the body into 3D to 2D.

2D geometry:

This is the required 2D geometry which can be used for the simulation.

Meshing:

After creating the 2D geometry we can import into the Mesh and assign the necessary boundaries name for the geometry.Here the naming of the 2D geometry is shown below .

After completion of assign the naming we will generate the meshing with a 1mm element size and by the use of triangular method to mesh the geometry and we are enabled the capture small proximity with the number of cells is 3.

Element Size:1 mm

Element Type:Triangular Method

Mesh Details:

Number of elemets :34864

Number of nodes:17953

SET-UP:

• Solver Time:Steady state
• Type: Pressure based
• 2D Space Type: Axissymmetric
• Energy equation is enabled
• Viscous Model: k-epsilon with standard and with Standard wall functions,because the flow is turbulent

Species Model:

• Model Name:Species Transport
• Reaction Type:Volumetric
• Options:Turned on the Inlet Diffusion and Diffusion Energy sources
• Mixture Material:x methane-air-2step
• Turbulence-Chemical Interaction Name: Eddy_Disipation

we have to add another 2 species from the models they are Nox and Soot species.

Turn on the Nox model

• Pathways:turn on the Thermal Nox and Prompt Nox
• Fuel streams:Number of fuel streams =1, then select the Fuel Species as CH4.

Turn on the Soot Model:

• Model name:One -step
• keept all the remaining values are same then click ok.

Boundary conditions:

For Air:

• For Air Inlet velocity=0.5 m/sec
• Inlet Temperature=300 k
• Species Mass fraction at the air inlet=O2 is 0.23

For Fuel:

• For fuel Inlet velocity=80 m/sec
• Inlet Temperature=300 k
• Species Mass fraction at the fuel inlet=CH4 is 1

• Now,we initialisize the solution as the Hybrid initialisization scheme
• Run the simulation for number of iterations=1000

Results:

Residuals:

Tempearture Countour:

Temperture Simulation:

Mass fraction of CH4:

Below plot shows the different probes of lines are which are palced at different locations.

Mass fraction of CO:

Below plot shows the different probes of lines are which are palced at different locations.

Mass fraction of CO2:

Below plot shows the different probes of lines are which are palced at different locations.

Mass fraction of H2O:

Below plot shows the different probes of lines are which are palced at different locations.

Mass fraction of N2:

Below plot shows the different probes of lines are which are palced at different locations.

Mass fraction of O2:

Below plot shows the different probes of lines are which are palced at different locations.

Mass fraction of NOX:

Below plot shows the different probes of lines are which are palced at different locations.

Mass fraction of SOOT:

Below plot shows the different probes of lines are which are palced at different locations.

Case-II

Now the parametric study is performed to analyse the effect of water addition to the fuel inlet.Water must be(5% to 30% of mole fraction ) is added to the fuel side.

Parametric study Table:

For Designpoint I:

Temperature:

Below plot shows the different probes of lines are which are palced at different locations.

Mass fractins of NOX:

Below plot shows the different probes of lines are which are palced at different locations.

Mass fractins of SOOT:

Below plot shows the different probes of lines are which are palced at different locations.

For Designpoint II:

Temperature:

Below plot shows the different probes of lines are which are palced at different locations.

Mass fractins of NOX:

Below plot shows the different probes of lines are which are palced at different locations.

Mass fractins of SOOT:

Below plot shows the different probes of lines are which are palced at different locations.

For Designpoint III:

Temperature:

Below plot shows the different probes of lines are which are palced at different locations.

Mass fractins of NOX:

Below plot shows the different probes of lines are which are palced at different locations.

Mass fractins of SOOT:

Below plot shows the different probes of lines are which are palced at different locations.

Conclusion:

• The mass fraction plot for the fuel inlet is justified because the fuel enters into the combustion chamber through the fuel inlet.This can be clearly observed in the plots and reacts with the oxygen and thus the mass fractions are decreased.
• The experimental investigations of water injection to reduce NO and soot emission in Combustion chamber have been presented. Although there are a lot of water
  injection systems developed already it has to be noted each of them affects on its own way to the pollutant emission reduction depending strongly on the engine combustion type and its operating condition. However three different water Percentages have been presented in this project. All of them showed practically the same influence on the NO emission reduction and poor effect on the soot emission.
• High latent heat absorption of water particle during the combustion will lower the local high temperature resulting in the reduction of NOx.
• Mass fractions of CO,CO2,H2O,O2,NOX,N2O & CH4 is succesfuuly calculated and plotted the required graphs.
• The maximum temperature attained during the simulation is 2171k.

References:

1. https://www.ohio.edu/mechanical/thermo/Applied/Chapt.7_11/Chapter11.html