Week 10 - Simulating Combustion of Natural Gas.

Question:

Part I

Perform a combustion simulation on the combustor model and plot the variation of the mass fraction of the different species’ in the simulation using line probes at different locations of the combustor as shown in Fig. You need to plot for CO2, H2O, CH4, N2, O2, NOx emissions & Soot formation. 

Part II

As you must have observed from the above simulation, the Nox and soot is getting formed at the outlet of the combustor. Such formation has harmful effects on the environment and humans. The stringent government norms also demand the least formation of Nox and soot and to satisfy those requirements, you need to check the effect of adding the water in the fuel.

In this part, you need to add the water content in the fuel from 5% to 30% by mole and observe the effect of it on the results. It is necessary to provide line plots and contours to prove your claim.

You can use the parametric study approach if required. 

Note:-

1. Use methane-air-2step as a mixture material than methane-air. [It allows to add water in the fuel or air]

2. Use a one-step soot model to calculate the soot formation. 

Aim: To perform a combustion simulation on given combustor model and to observe the variation of mass fraction of different species in the simulation and study the effect of adding water in to the fuel.

Objective :

Part 1

1) Perform a combustion simulation and plot the variation of the mass fraction of the different species’ in the simulation using line probes at different locations of the combustor.

2) Setup NOx and soot model to observe NOx emissions and soot formation

Part 2

1) Add water content in the fuel from 5% to 30% by mole.

2) Provide necessary plots and contours from the CFD results

Introduction :

Combustion of natural gas is an exothermic chemical reaction in which natural gas and oxygen react to produce heat and several chemical byproducts. This reaction can be controlled and used to produce heat for cooking and heating. It can also be used to power an electrical generator used to produce electricity that can be used for lighting and other purposes. Natural gas consists primarily of methane. Natural gas sources include fossil fuel deposits that can be processed into natural gas and biofuel generators that can be used to produce methane from biological material. The gas is treated to be as clean as possible, removing compounds that could impair the combustion process or create pollution that would make combustion harmful to the environment.

In years past, soot reduction strategies relied mainly on correlations, experience, and trial-and-error. The ever-increasing expansion of CFD makes it possible to analyze and optimize combustion plants with sufficient reliability. However, the uncertainty increases significantly when it comes to soot: the complexity of the gas-phase chemistry and the numerous mechanisms involved in the soot formation process, which are strongly coupled to mixing and radiative heat transfer and depend on the soot volume fraction itself, make the prediction of soot emissions still challenging a task that leads to large errors in exhaust concentration even with small mispredictions in formation rate

In fact, various mechanisms such as formation, coagulation, surface growth, and oxidation are involved in determining the final concentration at the exit of the incinerator. While reasonable predictions can be achieved on laminar diffusion flames, the scenario is further complicated when it comes to turbulent flames burning common fuels such as diesel or kerosene, as the applicability of models developed for simple aliphatic hydrocarbon fuels (such as methane) is questioned. Soot production in methane-air flames usually occurs from small species (e.g., acetylene) that grow to form polyaromatic hydrocarbons (PAHs) and possibly 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 5mm is given to the Outer cylinder
• mesh size of 2mm is given to the Middle cylinder

Mesh details:

Mesh quality:

The name selection are given which shown in figure below:

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 CH4+ar(O2+3.76N2) 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 - No water injection with fuel

Part  - With water injection with fuel

• Case 1 : 5% water injection with fuel

• Case 2 : 15% water injection with fuel

• Case 3 : 30% water injection with fuel

CONCLUSION:

1. With the increase in the Mass fraction of water in the fuel-air mixture NOx pollutant is reduced.

2.  With the increase in the Mass fraction of water in the fuel-air mixture Soot formation is reduced.

3. With the increase in the Mass fraction of water in the fuel-air efficient mixture of fuel-air is achieved.

4. With the increase in the Mass fraction of water in the fuel-air mixture temperature of the exhaust gases is reduced.

5. With the increase in the Mass fraction of water in the fuel-air mixture velocity of the exhaust gas is reduced.