Week 10 - Simulating Combustion of Natural Gas.

1. COMBUSTION

Combustion is defined as a chemical reaction in which a hydrocarbon reacts with an oxidant to form products, accompanied by the release of energy in the form of heat. 

Combustion manifests as awode domain during the design, analysis, and performance characteristics stage by being an integral part of various engineering applications like internal combustion engines, furnaces and power station combustors. 

2. Combustion model for CFD Analysis

Computational fluid dynamics modeling of combustion calls upon the proper selection and implementation of model suitable to faithfully represent the complex physical and chemical phenomenon associated with any combustion process. 

The model should be competent enough to deliver information related to the species concentration, their volumetric generation or destruction rate and changes in the parameters of the system like enthalpy, temperature and mixture density. 

The model should be capable of solving the general transport equations for fluid flow and heat transfer as well as additional equations of combustion chemistry and chemical kinetics incorporated into that as per the simulating environment desired. 

3. Possible types of Combustion Simulation possible in Ansys Fluent

The simulation of combustion can be divided into two types- 

Combustion based on Mixing

• Non-Premixed Combustion 

• In non-premixed combustion, fuel and oxidizer enter the reaction zone in distinct streams. 
• It occurs during direct injection or late injection of fuel in IC engines. 
• Examples of non-premixed combustion includes 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 flames front propagating into the unburnt reactants. 
• Examples of premixed combustion include aspirated internal combustion engines, lean- premixed gas turbine combustors, and gas-leak explosions. 
• Partially Premixed Combustion 
   
• Partially premixed combustion systems are premixed flames with non-uniform fuel-oxidizer mixtures (equivalence ratios). 
• Such flames include premixed jets discharging into an inert atmosphere, lean premixed combustors with diffusion pilot frames and cooling air jets, and imperfectly mixed inlets. 

Combustion based on Phase

• Fluid Phase (Volumetric Reactions) 

• Combustion takes place in the fluid region. 

• Wall (Surface Reactions) 
   

• Combustion takes place on the wall or a solid surface. 

• Particles (Surface Reactions) 

• Combustion takes place on the Lagrangian particles. 
• Combustion of coal is an example of particle combustion. 

• Porous Region 

• Combustion takes place in a porous region. 
• The reactions in an after-treatment system are an example of combustion in the porous regions.

Combustion Reactions

The general reaction of combustion is given as follows- 

`CH_4+ar(O_2+3.76N_2)→aCO2+bH_2O+cN_2`

Balancing the given stoichiometric equation- 

• a=1
• 2b=4⇒b=2
• 2ar=2a+b⇒ar=2
• c=7.52ar⇒c=15.04

The above values correspond to the number of moles of the given species. 

• Fuel: 
• Air: 2moles of O2+7.52moles ofN2

Therefore, 

• Mole Fractions: 

• The mole fraction of `O_2=2/(2+7.52)=0.21`
  
• The mole fraction of `N_2=7.52/(2+7.52)=0.79`
  

• Mass Fractions: 
   

• The mass fraction of `O_2=(0.21×16)/((0.21×16)+(0.79×14))=0.23O2`
• The mass fraction of `N_2=(0.79×14)/((0.21×16)+(0.79×14))=0.77`
  

 

Geometry 

• Importing 3D model into the spaceclaim for cleanup 
• By assuming combustion properties are axisymmetric, split the domain with different planes in order to use the extracted mid-2D geometry for the analysis.

 

 

Mesh 

• Naming the boundary condition as air inlet, fuel inlet, outlet and wall  
• Element size 5mm 
• Generated Mesh by capturing curvature and proximity 

 

Case setup 

Solver Type- Pressure Based 

Velocity Formulation-Absolute 

2D Space -Axisymmetric 

Time- Steady state 

Viscous model - k-epsilon turbulence model with scalable wall function, Energy-on 

Models- species -Transport-on 

Materials- Air, methane -air -mixture -2step 

Air inlet- velocity inlet (0.5m/s, 300K temperature, species O2=0.21 (mole fraction) 

Fuel inlet- Velocity inlet (velocity 80m/s, temperature 300K) species (CH4 and H2O) 

outlet- Pressure outlet 

Walls- stationary walls 

Axis- Axisymmetric 

Solution-Pressure -Velocity coupled 

Hybrid initialization and set to 500 iterations 

Results

Charts At the Line Probes 

With help of line probes, we can plot the variation of Mass fraction at different locations. 

For that we have to create line probes at different locations. 

Line probes at different locations from left to right meter can be created as given below: 

Line 1:              Two-point Method      (0.05,0)          (0.05,0.0852) 

Line 2:              Two-point Method        (0.1,0)            (0.1,0.0852) 

Line 3:              Two-point Method        (0.2,0)          (0.2,0.0852) 

Line 4:              Two-point Method          (0.3,0)          (0.3,0.0852) 

Line 5:              Two-point Method          (0.4,0)          (0.4,0.0852) 

Line 6:              Two-point Method          (0.5,0)            (0.5,0.0852) 

Line 7:              Two-point Method          (0.6,0)          (0.6,0.0852) 

Line 8:              Two-point Method          (0.7,0)          (0.7,0.0852) 

 

Residual: 

Temperature Contour:

CH4 Mass Fraction: 

CO Mass Fraction: 

CO2 Mass Fraction: 

H2O Mass Fraction: 

N2 Mass Fraction: 

O2 Mass Fraction: 

SOOT Mass Fraction: 

POLLUTANT NO Mass Fraction: 

CASE 1:  95 % CH4  and 5% H2O

 

Residual: 

Temperature Contour: 

CH4 Mass Fraction: 

CO Mass Fraction: 

CO2 Mass Fraction: 

H2O Mass Fraction: 

N2 Mass Fraction: 

O2 Mass Fraction: 

SOOT Mass Fraction: 

POLLUTANT NO Mass Fraction: 

CASE 2: 90 % CH4 and 10 % H2O 

Residual:

 

Temperature Contour: 

CH4 Mass Fraction: 

CO Mass Fraction: 

CO2 Mass Fraction: 

H2O Mass Fraction: 

N2 Mass Fraction:

O2 Mass Fraction: 

SOOT Mass Fraction: 

POLLUTANT NO Mass Fraction: 

 

CASE 3: 80 % CH4 and 20 % H2O 

Residual: 

Temperature Contour: 

CH4 Mass Fraction: 

CO Mass Fraction: 

CO2 Mass Fraction: 

H2O Mass Fraction: 

N2 Mass Fraction:

 

O2 Mass Fraction: 

SOOT Mass Fraction: 

POLLUTANT NO Mass Fraction:

 

 

Case 4: 70% CH4 and 30% H2O 

Residual: 

Temperature Contour: 

CH4 Mass Fraction: 

CO Mass Fraction: 

CO2 Mass Fraction: 

H2O Mass Fraction: 

N2 Mass Fraction:

 

O2 Mass Fraction: 

SOOT Mass Fraction: 

POLLUTANT NO Mass Fraction: 

 

OBSERVATION 

We can see that from the beginning, CH4 is burning from high at 1 to near 0. But in real there are some unburnt carbon particles that always remain. 

Due to this incomplete combustion of ZZ hydrocarbons, generation of SOOT or Fly Ash happens. 

We can see there are NOx emissions, which are due to combustion of nitrogen present in the air, which is 78%. Due to that we can see N2 is maximum all over the contour 

We can observe a very small decrease in temperature from Case_1 to Case_2, as we have added water in fuel in each case from 5 % to 30%. 

Also, we can observe a small decrease in NOx emission as we add water from 5% to 30%. 

We can see the significant amount of decrease in SOOT formation from Case_1 to Case_4. 

Also, there is no ignition source for combustion. So, the rate of reactions is controlled by turbulence mixing in Eddy dissipation model. 

So, with large eddy mixing time scale, combustion starts when turbulence comes in picture. 

  

CLAIMS 

• As per the above observations, we can say that addition of water to the fuel helps in decreasing NOx and SOOT Formation.  
• Water addition leads to an increase in OH radicals that might have a significant impact in SOOT Oxidation and reduce the SOOT formed in Gas Phase. 
• Added water dilutes calories generated by combustion. That decreases combustion temperature. If temperature reduces sufficiently, NOx cannot form in great concentration. 
• We can see a small decrease in temperature. Reducing combustion temperature means avoiding stoichiometric ratio. If this ratio is low, it generates lower concentration of thermal NOx. 
• So, with these or various types of technologies, we can reduce NOx emissions and SOOT formation, which helps us to meet regulatory requirements by government. And we can reduce its harmful effects on the environment and humans. 

CONCLUISION 

The simulation provides a complete temperature distribution along the geometry. Contours of various species can be studied. 

By adding water, the pollutant soot and NOx formation at the outlet is reduced. The plots of the same is also looked into.