Challenge on Combustion
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.
COMBUSTION:
It is a high temperature exothermic reaction(redox) between a fuel(reductant ) and an oxidant or the oxidizer to release a large amount of chemically bound energy.A flame is a characteristic indicator of the combustion reaction.lt is a sequence of elementary radical reactions.Solid fuels undergoes endothermic pyrolysis to generate gaseous fuel whose combustion supplies the heat required to produce more of such gaseous fuel products.The formation of these products is very complex and many different species such as H, 0, OH, CH3,CH4 are formed in the process. Besides Carbon di oxide and water a large no. of pollutants like Nitrous oxide, Carbon mono oxide and soot particles are formed.A simple example can be seen in the combustion of Hydrogen and Oxygen into water vapour, a reaction generally used to fuel the rocket engines.The reaction releases 250KJ/mol of heat and reduces the enthalphy at constant temperature and pressure.
Types of combustion models:
Based on combustion:
Non-Premixed Combustion: In non -premixed combustion fuel and oxidizer enter the reaction zone in separate streams and are not mixed before combustion.Whereas in premixed combustion reactants are mixed with the oxidizer at the molecular level before burning or combustion takes place.
Examples: Diesel Internal Combustion Engines and pool fires.This is applied only for turbulent flows by using pressure based solver.
Premixed Combustion:This type of combustion happens when fuel and oxidizer are mixed before burning.They are mixed at the molecular level.Here the flame front propagates into the unburnt reactions as the combustion takes place.
Examples: Gas Leak combustion explosions, premixed gas-turbine combustors.
Pressure based solver is used and is applicable for turbulent flows and subsonic flows.This type of flames called detonations or deflagrations or the explosions, where the combustible mixture is ignited by a heat source behind a shock wave.The Premixed combustion model cannot be used along with NOx pollurtant model and soot model turned on, whereas partially premixed model can be used along with the pollutant model turned on.Only inert particles can be used for simulation by premixed combustion model not the discrete phase particles.
Partially premixed combustion:
Here fuel and oxidizer are primiraly mixed before combustion. They are the premixed flames with non-uniform fuel-oxidizer mixtures.lt is a model combustion process intended to be used in Internal combustion engines of motorized vehicles and other automobiles.lts high specific power, high fuel efficiency and low exhaust pollution made it a promising technology.These premixed flames involves premixed jets discharging into the atmosphere, lean premixed combustors with cooling air jets or diffusion pilot flames.ln order to compute the concentrations of individual species,partially premixed combustion model is used.This is applicable for turbulent flows by the used of pressure based solver.
Based on Phases:
Fluid phase:Combustion happens when fluid is in fluid phase.
Volumetric phase/gas phase: It is for gas-phase reactions.The reaction rates are defined on a volumetric basis.
Wall surface: For gas-phase reactions, the reaction rate is defined on a volumetric basis.Here combustion happens on wall or boundary.For surface reactions, the rate of adsorption and desorption is governed by both chemical kinetics and diffusion process.When the volumetric is enabled, wall surface reactions create sources and sinks of species in the gas phase enabling the calc of wall surface reaction kinetics. They are also applied on the reacting surface.
Particle surface: Combustion happens over surfaces involving multi phase particle surface reaction mechanism.This is applied by using the volumetric turned on.
II)Objective:
1. To perform a combustion simulation on the combustor model given and plot the variations of mass fraction of the individual species that are involved and produced in the process.The results are to be supported by contour plots.
2. To perform the same simulation by adding water from 5% by mole to 30% by mole and observe the effects and the changes in the results.
Geometry:
Only 1/4th part of model is used for analysis . Body split tool is used for spliting bodies. 2D front face is use for analysis by copy-paste tool.
Whole model:
Half model:
1/4th Model:
2D geometry for Analysis:
Meshing with Named Selections:
Elememt size of 1 mm is use with capture proximity number of cells as 5.
Named Selections:
Model setup:
- Solver- Pressure based
- Steady state , Absolute and Axisymmetric
- Viscous Turbulence model- K epsilon>Standard>Scallable wall functions
- Species model Turned ON
Species model Turned ON:
Species:
The Species Model dialog box allows you to set parameters related to the calculation of species transport and combustion.
Species Transport - enables the calculation of multi-species transport (either non-reacting or reacting, depending on the selection for Reactions).
Reactions - contains options related to the modeling of reacting flow.
Volumetric - enables the calculation of reacting flow using the finite-rate formulation.
Inlet Diffusion - includes the diffusion flux of species at the inlet.
Diffusion Energy Source - includes the effect of enthalpy transport due to species diffusion in the energy equation.
Mixture material - Methane-air 2 step is used as it allows addition of water in fuel or air
- NOX and Soot model is turned ON
NOx Model
Mainly consists of Nitric oxide (NO), Nitrogen peroxide (NO2) and Nitrous oxide (N20) to a smaller extent Thermal, Prompt, Fuel and N20 intermediate mechanisms are available.
Thermal NOx - It is formed when nitrogen and oxygen in the combustion air combine with one another at the high temperatures in a flame.
Prompt NOx - Forms from the rapid reaction of atmospheric nitrogen with hydrocarbon radicals.
Fuel NOx - It is formed by the reaction of nitrogen bound in the fuel with oxygen in the combustion air. It is rarely a problem with gaseous fuels.
In our case we will activate Thermal and prompt NOx as there is no nitrogen in input fuel as is is Methane CH4
One step Soot model is used.
Boundary conditions:
- For case 1 , As their is no addition of Water h2O , Air inlet will have velocity of 0.5 m/s with thermal temperature 300 k with species of 0.23 mass fraction of O2. For Fuel inlet , velocity is 80 m/s with thermal temperature 300 k and species of 1 mass fraction of ch4.
- For case 2 , Their is addition of water in terms of 5 , 15 , 25 , 30% in fuel. so all the above conditions remains same with only change in fuel inlet species with 0.05 , 0.15 , 0.25 , 0.3 mass fraction of h2O with ch4 of mass fraction 0.95 , 0.85 , 0.75 , 0.7 respectively. Air inlet species remains same with 0.23 mass fraction of O2.
Solving Methods: Coupled with Hybrid initialisation . Running for 1000 iterations
Solution:
Case 1 : ch4 = 1 , O2 = 0.23
Residual plot:
Static temperature with Volumer rendering:
Mass fraction of ch4 along with chart at varying distances:
Mass fraction of co2 along with chart at varying distances:
Mass fraction of o2 along with chart at varying distances:
Mass fraction of h2O along with chart at varying distances:
Mass fraction of N2 along with chart at varying distances:
Mass fraction of NOx along with chart at varying distances:
Mass fraction of Soot along with chart at varying distances:
Case 2(a): ch4=0.95 , h2O = 0.05
Residual plot:
Static temperature with Volume rendering:
Mass fraction of ch4 along with chart at varying distances:
Mass fraction of co2 along with chart at varying distances:
Mass fraction of o2 along with chart at varying distances:
Mass fraction of h2O along with chart at varying distances:
Mass fraction of N2 along with chart at varying distances:
Mass fraction of NOx along with chart at varying distances:
Mass fraction of Soot along with chart at varying distances:
Case 2(b): ch4=0.85 , h2O = 0.15
Residual plot:
Static temperature with Volume rendering:
Mass fraction of ch4 along with chart at varying distances:
Mass fraction of co2 along with chart at varying distances:
Mass fraction of o2 along with chart at varying distances:
Mass fraction of h2O along with chart at varying distances:
Mass fraction of N2 along with chart at varying distances:
Mass fraction of NOx along with chart at varying distances:
Mass fraction of Soot along with chart at varying distances:
Case 2(c): ch4=0.75 , h2O = 0.25
Residual plot:
Static temperature with Volume rendering:
Mass fraction of ch4 along with chart at varying distances:
Mass fraction of co2 along with chart at varying distances:
Mass fraction of o2 along with chart at varying distances:
Mass fraction of h2O along with chart at varying distances:
Mass fraction of N2 along with chart at varying distances:
Mass fraction of NOx along with chart at varying distances:
Mass fraction of Soot along with chart at varying distances:
Case 2(d): ch4=0.7 , h2O = 0.3
Residual plot:
Static temperature with Volume rendering:
Mass fraction of ch4 along with chart at varying distances:
Mass fraction of co2 along with chart at varying distances:
Mass fraction of o2 along with chart at varying distances:
Mass fraction of h2O along with chart at varying distances:
Mass fraction of N2 along with chart at varying distances:
Mass fraction of NOx along with chart at varying distances:
Mass fraction of Soot along with chart at varying distances:
Comparing results of NOx and Soot mass fraction at Outlet for all cases:
| Addition of Water H2O | Mass fraction of Nox at Outlet | Mass fraction of Soot at Outlet |
| NO | 1.252e-4 | 9.944e-27 |
| 5% | 9.622e-5 | 2.067e-24 |
| 15% | 5.442e-5 | 1.411e-23 |
| 25% | 2.720e-5 | 5.090e-20 |
| 30% | 1.817e-5 | 1.661e-18 |
Graphical Representation of Mass fraction of NOx and Soot with addition of Water H2O:
Conclusion:
- From table and graphical representations , we can see that the value of mass fraction of NOx decreases with increase in addition of water (H2O) in simulation which is great as according to government norms with reduction of NOx formation.
- We can also observe that mass fraction of soot increases with increase in addition of water.
- Thus we can conclude that Combustion simulation is performed successfully
- The whole comustion simulation is done by the Eddy Dissipation model with the volumetric turned on.The model is used along with turbulence model. The contour of Temperature, CO2 and H2o are more or less same which signifies that the species mass fraction depends on the effective combustion taking place The formation of NOx is reduced as we increase the water content from 5% to 20% but in the case of 30% the Formation of NOx is more and also the soot which happens to be in larger mass fraction than the previous cases. NOx concentrations generated in the combustion are generally low. So, it has little influence on the flow field and predictions, temperature ando ther combustion products.
Reference Research Paper:-
1. https://www.sciencedirect.com/science/article/pii/S1876610217337724
2. https://link.springer.com/article/10.1007/s00773-015-0303-8