Week 10 - Simulating Combustion of Natural Gas.

Natural gas combustion is an exothermic chemical reaction in which natural gas and oxygen react, producing heat and

several chemical byproducts. This reaction can be controlled and harnessed to generate heat for cooking and heating. It can

also be used to power an electrical generator used to create electricity which can be used for lighting and other

purposes. Natural gas is comprised primarily of methane. Sources for natural gas include fossil fuel deposits which can be

processed to yield natural gas and biofuel generators which can be used to make methane from biological material. The gas

is treated to make it as pure as possible, removing compounds which could impair the combustion process or generate

pollution which would make combustion harmful to the environment.

In the past years strategies for the reduction in soot emissions relied mainly on correlations, experience and trial-and-error

attempts. The increasingly diffusion of CFD is allowing to analyze and optimize combustion devices with sufficient confidence.

However, the uncertainty increases significantly when soot is concerned: the complexity of the gas-phase chemistry and the

numerous mechanisms involved in soot formation process, which are strongly coupled with mixing and radiative heat transfer

and depend on soot volume fraction itself, make the prediction of soot emissions still a challenging task, leading to large

errors in exhaust concentration even with small mispredictions in the formation rates

Different mechanisms are in fact involved in the determination of the final concentration at the exit of the combustion device,

such as inception, coagulation, surface growth and oxidation. Even though reasonable predictions can be achieved on laminar

diffusion flames , the scenario is further complicated when turbulent flames burning common fuels such as diesel or kerosene

are concerned, as the applicability of models developed for simple aliphatic hydrocarbon fuels (such as methane) is

questioned. Soot production in methane-air flames usually takes place from small species (e.g. acetylene) that grow to form

polyaromatic hydrocarbons (PAH) and eventually soot.

Soot is a mass of impure carbon particles resulting from the incomplete combustion of hydrocarbons

Geometry

The 3D geometry is loaded onto the space claim and it is Splitted using 'Split by plane' tool at centre of the Cylinder 

It is again Split into Respective halves

One face is copied representing the 2D image of the cylinder as shown in the pictures

The copied face it pasted on to a new spaceclaim and the whole face is selected and Share topology is enabled to enable the

interaction between the cylinders

Cylinders of different sizes is used here to mesh the respective zones effectively

Mesh

Named selection is added to the edges as shown is the pictures

mesh size of 5e-2m is given to the Outer cylinder

mesh size of 5e-3m is given to the Middle cylinder

mesh size of 1e-3m is given to the Inner cylinder

Capture proximity is turned on with number of cells across gaps as 3

Setup

Steady state , pressure based and Axis symmetris calculation is selected

Energy is enabled since the calculation involves temperature terms

Viscous model is set to K-epsilon 

Species is turned on With species transport equation

Within the species menu Vomlumetric reaction is selected , Inlet diffusion and Diffusion energy sources are also added

For species the mixture material is set to Methane-Air-2step to enable the presence of water

Boundary conditions for Air-Inlet are ,Velocity = 0.5m/s ,Temperature = 300k , Species->  Oxigen with mass fraction

0.23.And for Fuel inlet Velocity is set to 80m/s with Species as 1 mass fraction for CH4 (mathane). The respective mass

fractions are calculated as show below

So Since air contains mostly (79% nitrogen and 21% Oxigen )we have the equation for the reactants

as $C{H}_4+ar(O_2+3.76N_2)$

where ar is the stoichiometric coefficient. And we have the product side as $aC{O}_2+b{H}_{2}O+c{N}_2$

,

we have to balance the product side and reactant side using the coefficients a,b,c

Balancing carbon atoms , a =1.

Balancing Hydrogen atoms , 4=2b $\Rightarrow$b = 2.

For balancing 'ar' Take the coefficients of oxigen and nitrogen from products side and equate them

hence 2ar = 2a+b

ar =a + b/2

= 1+1 =2 

Now we can fine the inlet Mass fractions for Fuel and air

Air contains 2 moles of Oxigen (O2) and 7.52 moles of Nitrogen , The Oxigen Mole fraction would be 2/(2+7.52) = 2/9.52 =

0.21 , when converted to mass fraction it would be 0.23

Since CH4 is only of 1 mole we can Input the fuel inlet with mass fraction of 1 for CH4 in Fuel inlet

Outlet is Pressure based  

Inorder to capture the mass fractions Soot and NOx , The respective options under Species is enabled. The Soot model is set

to one-step with Fuel as CH4 and oxident as oxigen.

In Nox model only the Thermal Nox is enabled since major part of this compund is formed during high temperatures

For second phase of the calculation we add the water of mass fractions 5% , 10%, 15% , 20% , 25% and 30% of fuel for

each cases by using parametric study which is enabled in the boundary conditionsof fuel inlet-> species

In the report definition output parametres is enabled for Temperature at outlet ,Mass fractions of Soot and Nox at outlet

inorder to observe the changes

A hybrid Initialisation with 400 iteration is setup

Outputs

Part 1 - Baseline Simulation (No water injection with fuel)

Part 2-

Temperature Contours of Each cases

case-1 (5% of water injection)

Case 2 - Water Injection 10%

 

 

Case 3 - Water Injection 15%

Case 4 - Water Injection 20%

Case 5 - Water Injection 25%

Case 6 - Water Injection 30%

 

Plots 

In the post processor I have created four lines as shown in picture below. Four of them are placed in a way to capture the

variation of specific parametres

Series 1 , Series 2, Series 3 and Series 4 represents the Line 1,Line 2,Line 3 and Line 4 respectively

Comparison of Temperature plots of each cases

For Case 1

For case 2

For case 3

For case 4

For case 5

 

For case 6

Observations

As the percentage of water is increased the temperature at the outlet is getting lower

Comparison of soot plots of each cases

Case 1

Case 2

Case 3

Case 4

Case 5

Case 6

Observations

Soot is formed more where the Line 2 lies and as expected the Soot is gettin reduced by injecting water, greater the

percentage of injection lower the Pollutant

Comparison of NO plots of each cases

Case 1

Case 2

Case 3

Case 4

Case 5

Case 6

Nitrogen monoxide is produced more where the line 3 is placed

Also the production of the pollutant is getting lower on each cases (due to more injection of water)

 

Comparison of Oxigen plots of each cases

Case 1

Case 2

Case 3

Case 4

Case 5

Case 6

Observations

Since Line 1 is near to the inlet the oxygen is present more there

On first 2 lines there doesnt seem to have changes but when we observe Line 3 and Line 4 variations we can conclude that

the Oxigen availibility is getting more by the injection of water

 

Comparison of Carbon dioxide plots of each cases

Case 1

Case 2

Case 3

Case 4

Case 5

Case 6

Observations

CO2 seems to have formed more where line 2 lies and there is no much of a change (for line 2 curve) in each cases

Also the CO2 at the outlet is getting lower as injection of water is incresed

Comparison of Carbon Monoxide plots of each cases

Case 1

Case 2

Case 3

Case 4

Case 5

Case 6

 

Observations

Carbon monoxide is also present more at the regions of line 1 and line 2 and quantity of it getting produced is lowered by the

introduction of water

Water plots of each cases

Case 1

Case 2

Case 3

Case 4

Case 5

Case 6

Methane mass fractions of each cases

case 1

case 2

case 3

case 4

case 5

case 6

Nitrogen mass fractions of each cases

case 1

case 2

case 3

case 4

case 5

case 6

 Parametric table

Through the observstion of the values of pollutants and the temperatures obtained for each simulation we can confirm the

fact that injecting water by a 30% is more effective in reducing the pollutants getting formed

Since I haven't selected fuel Nox and prompt Nox which is also expected to occur during mixing of fuel the Nox production

could be more 

 Conclusion

A baseline simulation of 0% water injection in fuel is done and necessary plots and contours were obtained

Simulations with same setup with the percentage of water injection varied in different cases and obtained necessary plots

and contours

The Nox and Soot formed at the outlet were obtained through parametric study

Obtained effect of water addition in fuel    

References

• Modelling soot production and thermal radiation for turbulent diffusion flames - ScienceDirect
• Soot - Wikipedia
• An overview of systems supplying water into the combustion chamber of diesel engines to decrease the amount of nitrogen oxides in exhaust gas | SpringerLink