Week 10 - Simulating Combustion of Natural Gas.
In this project, a simulation of the combustion of natural gas will be performed.
Natural gas primarily consists of methane (CH4), and upon combustion produces carbon dioxide (CO2) and water (H2O). Since the gas usually contains some impurties, other oxides like nitrous oxide (NOx) are produced as well. Improper combustion of natural gas can lead to production of carbon monoxide (CO) and soot.
`CH_4 + 2O_2 -> CO_2 + H_2O`
The simulation will be performed in a long cylindrical wind tunnel. Fuel enters at the center of the tunnel, the fuel is surrounded by air. The variation of mass fraction of all the combusting products will be seen at various points from the inlet. The effect of adding water in the fuel on the pollutants will also be seen. The major molecules involved in this reaction are CO2, H2O, CH4, N2, O2, NOx and soot.
This project is divided into two major parts. The first part being the analysis of mass fractions of all molecules involved in the reaction along the combustion cylinder, while the second part is about reducing emissions of NOx and soot by changing the % of water in the fuel from 5% to 30% of molar fraction.
Geometry
The initial geometry consits of a long cylinder of length and height mm disuvev with a tiny cut-out and the fuel inlet at the center.
This is the initial geometry:
Since all cylindrical simulations are axi-symmetric, the simulation can be conducted on a half-wedge to reduce computational resources and time.
The extra parts of the geometry are removed, and the resulting geometry is a rectangle as seen below:
Zooming in,
Note: the rectangle has to be in the XY plane with the axis of the cylinder being along the X axis.
Meshing
The default mesh with an element size of 1mm was used.
Capture proximity was turned on to ensure more cells are used near the fuel/air inlets.
This is the final mesh:
Zooming in,
Mesh Metrics
As seen from the metrics, nearly all elements have a quality >0.9. Hence the mesh quality is satisfactory.
The mesh contains 68326 nodes and 67422 elements.
Simulation Setup
Solver - General
A pressure-based, steady state, axismemetric simulation was used.
Models
Energy was checked as combustion occurs.
Viscous
k-epsilon model with standard wall functions was used.
Species
methane-air-2step was used so that the fuel can be diluted with water for part 2.
Boundaries
inlet-air: velocity-inlet, inlet velocity 0.5m/s, temperature 300K
inlet-fuel: velocity-inlet, velocity 80m/s
axis: axis
outlet: pressure-outlet
wall-side: wall
Other boundaries were left unchanged.
Solution Methods
The following methods were used for this project:
Reports
Only a Residual report was generated.
Contour
Temperature contour was saved for every iteration, an animation was created for the same.
Simulation Results - Part 1
The above simulation setup was ran in a steady state solver for 250 iterations of until the residuals have converged. The results are below.
Residuals
The residuals have dropped well below the 1e-3 mark and are decreasing steadily. The solution has converged.
Animation
An animation of the temperature contour was saved for each timestep: https://youtu.be/sS7J9hXtpmA
There is no change in temperature towards the end of the video, indicating the solution has converged.
Variation of mass fraction along the radius of the cylinder
Line plots were created at set interval (x=50mm to x=400mm) along the radius of the cylinder, the lines are seen below:
Each molecule's mass fraction was extracted from the lines.
CO2
As distance from inlet increases, concentration of CO2 increases as more combustion of CH4 occurs.
H2O
Mass fraction of H2O increases as distance from inlet increases, as more combusting products are generated.
CH4
There is a lot of CH4 near the inlet, and as distance from the inlet increases, mass fraction of CH4 decreases. As distance from center increases, mass fraction of CH4 decreases as CH4 undergoes combustion.
N2
N2 does not react with any molecules, but the concetration of N2 is low near the combustion region as N2 is displaced from the combustion region.
O2
Near the inlet, concentration of N2 is low because O2 is used in the combustion process. It plateau-s at the inlet levels away from the inlet.
NOx
Mass fraction of NOx is higher away from the inlet, as NOx is produced later in the combustion process.
Soot
Mass fraction of soot is higher away from the inlet, as soot is produced later in the combustion process.
Part 2
The inlet velocity's boundary conditions were parameterised, specifically mole fraction of CH4 and H2O.
Simulation Results - Part 2
The simulation was conducted for 250 iterations or until the residuals had converged.
Fuel with 5% of water by mole fraction
Line plot of mass fraction of pollutant NOx along the radius
Contour plot of mass fraction of pollutant NOx
Maximum mass fraction of pollutant NOx: 0.00031194338
Line plot of mass fraction of soot along the radius
Contour plot of mass fraction of soot
Maximum mass fraction of soot: 0.053675174
Fuel with 10% of water by mole fraction
Line plot of mass fraction of pollutant NOx along the radius
Contour plot of mass fraction of pollutant NOx
Maximum mass fraction of pollutant NOx: 0.00026463
Line plot of mass fraction of soot along the radius
Contour plot of mass fraction of soot
Maximum mass fraction of soot: 0.0463746
Fuel with 15% of water by mole fraction
Line plot of mass fraction of pollutant NOx along the radius
Contour plot of mass fraction of pollutant NOx
Maximum mass fraction of pollutant NOx: 0.000201252
Line plot of mass fraction of soot along the radius
Contour plot of mass fraction of soot
Maximum mass fraction of soot: 0.0367495
Fuel with 20% of water by mole fraction
Line plot of mass fraction of pollutant NOx along the radius
Contour plot of mass fraction of pollutant NOx
Maximum mass fraction of pollutant NOx: 0.000148268
Line plot of mass fraction of soot along the radius
Contour plot of mass fraction of soot
Maximum mass fraction of soot: 0.0306751
Fuel with 25% of water by mole fraction
Line plot of mass fraction of pollutant NOx along the radius
Contour plot of mass fraction of pollutant NOx
Maximum mass fraction of pollutant NOx: 0.000105258
Line plot of mass fraction of soot along the radius
Contour plot of mass fraction of soot
Maximum mass fraction of soot: 0.0234769
Fuel with 30% of water by mole fraction
Line plot of mass fraction of pollutant NOx along the radius
Contour plot of mass fraction of pollutant NOx
Maximum mass fraction of pollutant NOx: 7.16292e-5
Line plot of mass fraction of soot along the radius
Contour plot of mass fraction of soot
Maximum mass fraction of soot: 0.018127
Summary of Part 2 Results
| Sno. | Mole Fraction of CH4 by % | Mole Fraction of H2O by % | Maximum mass fraction of NOx | Maximum mass fraction of soot |
| 1 | 0.95 | 0.05 | 0.0003119434 | 0.0536751740 |
| 2 | 0.9 | 0.1 | 0.0002646300 | 0.0463746000 |
| 3 | 0.85 | 0.15 | 0.0002012520 | 0.0367495000 |
| 4 | 0.8 | 0.2 | 0.0001482680 | 0.0306751000 |
| 5 | 0.75 | 0.25 | 0.0001052580 | 0.0234769000 |
| 6 | 0.7 | 0.3 | 0.0000716292 | 0.0181270000 |
Variation of Mole Fraction of H2O and maximum mass fraction of NOx
Variation of Mole Fraction of H2O and maximum mass fraction of soot
Conclusions and Observations
1. The simulation runs well and expected and desired results are obtained.
2. The variation of mass fraction along the cylinder of all reaction molecules was explained.
3. In part 2, water is added to the fuel. The mass fractions of the pollutants decrease, this would lead to lower pollution levels.
4. As more water is added, the mass fraction of the polluntants further decreases. Though this decrease may seem beneficial from the results in this simulation, in reality the calorific value of the combustion decreases when water is added. Hence more research is required and the fine balance between the level of pollutants and calorific value must be obtained.