Week 10 - Simulating Combustion of Natural Gas.
Aim:To Perform a combustion simulation on the combustor model using Ansys 2021 R1
Part 1: 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 2: In this part we 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.
Introduction:
Combustion:Combustion is burning of fuel inside a chamber.A chemical reaction in which fuel reacts with oxygen to give large amounts of heat.Unline IC engines the process is different in the combustor,in the inlet high pressured oxygen rich air is sent into the chamber and there is an there is an inlet for the fuel which is sprayed at high velocity through nozzle to atomize and the highly turbulent air mixes with the fuel.It is made sure that the combustion takes place only after proper mixing of the fuel and air. Combustor is a chamber in which combustion takes place.Combustors are mostly important for air craft and rocket propulsion.These combustors produce sufficient heat and thrust for an aircraft or rocket to be moved.
There are two types of combustion Internal cpombustion and external combustion.Examples of internal combustion are IC Engines's,Internal combustion chambers.Examples of external combustion are boilers in which combustion is done outside the chamber.
In this report simple combustion chamber is used which is shown in the figure below.
To simulate the combustion process ansys provides various setups based on the mixing of fuel and Phases
Based on Mixing:
Pre-Mixed Combustion:Fuel is mixed with air thoroughly before entering the combustion chamber.This is done with the help of carburettor in which fuel is sprayed as atomized particles into the stream of air.
Non-Pre Mixed combustion:In this type the fuel is not mixed with the air before entering the chamber and fuel is allowe to mix only after entering the combustion chamber.Some time is given for the mixing before combustion the spark plug is kept after some distance from the inlet.
Partially pre-mixed:Some of the fuel is pre mixed
Based on Phases:
Fluid phase:In this volumetric combustion
Wall:surface combustion
particle:surface combustion
Porous region
Geometry:
Geometry is imported into the space claim
As the geometry is axi symmetric it can be slitted into a plane an simulation is performed to get better results and reduced computational power and time.
The geometry is splitted about xy plane and then xz plane
The xy plane is copies and pasted in the new design and the earlier geometry is deleted
Then combine tool is used to combine all the boundaries
Meshing:
Meshing with quadrilateral elements as they produce better results.
element size =2mm
Capture proximity: Yes
Number of elements :17175
Naming is given to the air,fuel inlet outlet,axis and wall
Pre-Processing:
In the setup pressure based solver is used and velocity formulation is absolute.Time is steady state.The 2D space is set to axisymmetric
K-epsilon model is used with standard.
Energy is turned on as temperature is involed
Species transport is selected and the reactions are set to volumetric.The methane air mixture is selected.
Inlet diffusion
Diffusion energy source
The important one is the selecting eddy dissipation model this allows for combustion as soon as the fuel enters thye combustion chamber
Nox model is selected
Soot model is also activated
The required contours are selected
Boundary conditions:
Air inlet:
velocity 0.5m/s
mole fraction of O2: 0.23
Fuel inlet:
velocity: 80 m/s
Mole fraction of Ch4 :1
Post processing:
Position of line probes:
The line probes are created at a distace 0.1,0.2,0.3,0.4,0.5m
The contours fo the temperature and species are created.
Charts showing the mass fraction of different species at different line probes are created
Part 1: Without addition of water
Methane air mixtire is used
Boundary conditions:
Air inlet:
velocity 0.5m/s
mole fraction of O2: 0.23
Fuel inlet:
velocity: 80 m/s
Mole fraction of Ch4 :1
Residuals:
Temperature contour:
Plots and contours of mass fraction of different species at line probes
Mass fraction of Ch4:
Mass fraction of Co2:
Mass fraction of H2O:
Mass fraction of O2:
Mass fraction of N2:
Mass fraction of pollutant Soot:
Mass fraction of NOx emmision:
Conclusion part 1:
- The amount of oxygen present before combustion is more this is because it is utilised in the combustion and mass fraction of oxygen is reduces.
- N2 mass fraction is more in the chamber throughout because the air consists of around 70% of N2.The mass fraction is mostly same throughout because of its inertness but there is a slight decrease at the end of combustion chamber due to the Nox formation.
- The highest temperature is noted to be approximately 60% of the length of the chamber.
- It is observed that the soot and Nox formation are pretty high.Nox pollutant are formed when the temperature is very high.The nitrogen is inert at certain temperature but reacts at higher temperatures.To reduce the temperature of the chamber water is introdues along with the fuel.
- The soot and nox emissions are high.
Animation of the combustion:
Part 2: Adding water along with the fuel
Methane air mixtire is used
Boundary conditions:
Air inlet:
velocity 0.5m/s
mole fraction of O2: 0.23
Fuel inlet:
velocity: 80 m/s
Mole fraction of Ch4 : 0.95%(parametric study is provided)
Mole fraction of H2O: 0.5%(parametric study is provided)
Output parameters are defined mass fraction of soot and nox emissions at the outlet.
Species model:
5% water
Temperature contour:
Mass fraction of pollutant Soot:
Mass fraction of NOx emmision:
10% water
Temperature contour:
Mass fraction of pollutant Soot:
Mass fraction of NOx emmision:
15% water:
Temperature contour:
Mass fraction of pollutant Soot:
Mass fraction of NOx emmision:
20% water
Temperature contour:
Mass fraction of pollutant Soot:
Mass fraction of NOx emmision:
25% water
Temperature contour:
Mass fraction of pollutant Soot:
Mass fraction of NOx emmision:
30% water
Temperature contour:
Mass fraction of pollutant Soot:
Mass fraction of NOx emmision:
Parmetric table:
Conclusion part 2:
- It is seen from the plots that the NOx and soot emissions are reducing with the increasing the percentage of water.
- we can see that the there is a drastic decrease in the soot and the Nox emissions with just introduction of water of about 5%.
- With the increase in the water percentage from 5% t0 30% Mass fraction of the NOx emmisions reduced from 2.071e-5 to 5.25e-6.
- With the increase in the water percentage from 5% t0 30% Mass fraction of the Soot emmisions reduced from 0.0004625 to 6.82e-5.
- It is observed that with the temperature in the chamber is also reduced and we know that the Nox and soot emissions increase with the increase in the temperature of the chamber.