Week 10 - Simulating Combustion of Natural Gas.
PROBLEM STATEMENT
1. Perform the simulation analysis using methane:air fuel mixture and plot the variation of mass fraction of different species using the probes at different location of the combustion chamber.
2.Study the effect of adding water to the methane -air mixture and plot the same and discuss the result
Theory
1.Combustion
Combustion, or burning is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) ana an oxidant, usually atmostpheric oxygen, that produces oxidized,often gaseous products, in amixturetermed as smoke-combustion does not alwaysresult in fire,because a flame is only visible when substances undergoing combustion vapourise, but when it does, a flame is a characteristic indicator of the friction.while the activation energy must be overcome to initiate combustion. (e.g using alit much to light a fire),the heat from a flame may provide enough energy to make the reaction self-sustaining Combustion is often a complicated sequence of elementary radical reaction ,solid fuel, such as woodand coal, first undergo exothermic prolysis to produce gaseous fuels whose combustion the supplies the heat required to produce more of them.Combustion is often hot enough that in condescent light in the form of either glowing or a flame is produced. A simple example can be seen in the combustion of hydrogen and oxygen into water vapour,areaction commonly used to fuel rocket engines. This reaction releases 242kj/mol of heat and reduces the enthalpy accordingly (at constant temperature and pressure)
2H2{g} +O2{g}-2H2o{g}
Combustion of an organic fuel in air is always exothermic because the double bond in O2 is much weaker than other doublebonds or pairs of single bonds, and therefore the formulation of the stronger bonds in the combustion products(O2 and H2O results in the release of energy,
Stoichiometric combustion of a hydrocarbon in oxygen.
Generally, the chemical equation of stoichiometric combustionof a hydrocarbon in oxygen is
CxHy + ZO2 - xCO2 + y/z H20
where z = x +y/4
For example , the stoiciometric burning of propane in oxygen is
C2H8 +5O2 - 3Co2 + 4H20
2. Chemical Kinetics
Chemical kinetics, also known as reaction kinetics,is the branch of physical chemistry that is concerned with understanding the rates of chemical reaction. It is to be contrasted with thermodynamics,which deals wioth the direction in which a process occurs but in itself tells nothing about oits rate chemical kinetics includes investigation of how experimental conditions influences the speed of a chemical reaction and yield information about the reaction 's mechanism and transiotion staes zaswell as the construction of mathematical models that also can be describe the characteristics of a chemical reaction
3. Chemical reaction model
Chemical reaction models transform physical knowledge into a mathematical formulation that can be util.ized in computational simulation of practical problem in chemical engineering. Computer simulation provides the flexibility to study chemical processes under a wide range of condition . Modeling a chemical reaction involves solving conservation equation describing convection, diffusion and reaction source for each coimponent species.
series transport equation
Ri is the rate of production of species : by chemical reaction and S is the rate of creation by addition from dispersed phase and the user defined source.I is the diffusion flux of species 22 which arises due to concentration gradients and differs in both laminar and turbulant flows. In turbulant flows, computatinal fluid dynamics also considers the effect of turbulant diffusivity . The net source of chemical species i due to reaction , Ri which appeared as the source term in the species transport equation is computed as the sum of the reaction source over the Wr reaction among the specires.
REACTION RATE MODEL
2. laminar finite rate model:
The laminar finite rate model computes the chemical source terms using the arrehenius expreesion and ignores tiurbulance fluctuatiomn. This model provides the extract solution for laminar flames but gives in accurate solution for turbulence highly affects the chemistry reaction rates due to highly non-linearArrhenious chemical Kinetics. However , this moldel may be accurate for combustion with small turbulance fluctuation for example supersonic flames.
b. EDDY DISSIPATION MODEL.
The eddy dissipiation model,based on the work of magnissuen2 and hyertager, is a turbulant chemistry reaction model.Most fuel are fast burning and the overallk rate of reduvtion is controlled by turbulance mixing . in the non-premixed flames, turbulance slowly mixes the fuel and oxidizer into the reaction zones where they burn quickly .In premixed flames, the turbulance slowly mixes cold reactants and hot products into the reaction zones.Where the reaction occurs quickly. In such cases the combustion is said to be mixing limited , and the complex and often unknown chemical kinetics can be safely neglected .In this model ,the chemical reaction is governed by large-eddy mixing time scale.Combustion initiates whenever there is turbulance present in the flow.It does not need an ignition sourceto initiate the combustion .This type of model is valid for the non-premixed combustion.but for the premixed flames the reactant is assumed to burn at the moment it enters the computational model which is a short coming of the model as in practice the reactant needs some time to get to the ignition temperature to initiate the combustion.
Factors affecting reaction rate
1. Nature of reactants
2.Physical state
3.surface area of solid state.
4. concentration.
5.temperature
6. catalyst
7.pressure
8.Absorption of light.
APPLICATION
The mathematical models that describe chemical reaction kinetics provide chemists and chemical engineers with tools to better understand and describe chemical processes such as food decomposition,micro-organism growth, statospheric ozone description and the chemistry of biological systems,Thesemodels can also be used in the design or modification of chemical reactors to optimize product yield.more efficiently separate products and eliminate environmentally harmful by-products . When performing catalytic cracking of heavy hydrocarbons into gasoline and light gas . For example,kinetic modelscan be used to find the tempedrature and pressure at which the highest yield of heavy hydrocarbon into gasoline will o9ccur.
Chemical kinetics is frequently validated and explored through modelling in specialized packages asa function of ordinary differential equation -solving (ODE-SOLVING) and curve fitting.
4.mass fraction
A mass fraction can also be expressed,with a denominator of 100, asa percentage by mass. It is one way of expressing the composition of amixture in a dimensionless size;mole fraction (percentage by moles ;mol%) and volume fraction (percentage by volume vol%) are others
when theprevalences of interest are these of individual chemical ewlements, rather than of compounds of other substances, the term massfraction can also refer to the of the mass of an elements to the total massof a sample. In these context an alternstives term isa mass percent composition. The massfraction of an element in acompound can be calculated from the compounds's emperical formula or its chemical formulation.
5.Nitrogen oxide (Nox) emission
nitrogen dioxide isan irritant gas., which at high concentration causes inflammation of the airways. when nitrogen isreleased during fuel combustion it combines with oxygen atomsto create nitric oxide(NO). This further combines with oxygen to create nitrogen dioxide(NO2)
Nitric oxide is not considered to be hazardous to health at typical ambient concentration.but nitrogen dioxide can be nitrogen dioxide and nitric oxide are referred to together as oxides of nitrogen(NoX)
NoX gases react to form smog and acidrain as well as being control the formation of fine particles (PM)
and ground-level ozone,both of which are associated with adverse health effects.
6.soot formation
soot forms when hydrocarbon fuels such as oil,natural gas, and wood are burned .soot particles form when gaseous molecules are heated to high temperature and they don't easily turn back into gaseous molecules then way water droplets do when they are heated .soot as an air borne contaminant in the enviroment hasmany different sources, allof which are results of some form of pyolysis .They includesoot form coal-burning internal combustion engine ,power =plant boilers , hog-fuel,boilers, ship boilers, central steam - heat boilers, waste incineration, local yield burning house firesc,forest fires/fireplacess
and furnaces.These exterior sourcrs also contribute to the indoor environment sources such assmoking of plant matter,cooking , oil lamps,candles,quardz/halogen bulbs with settled dust,fireplaces exhaust emission from vehicles,and defectivefurnaces
types ofcombustion simulation
a) Non-premixed combustion (direct injection,late injection)
b) Premixed combustion(carburetor)
c) partially premixed
NON-PREMIXED COMBUSTION
In non-premixed combustion , fuel and oxidizer enter the reaction zone in distinct streasms.This is incontrast to premixed systemin which reactantsare mixedat the molecular level before burning.
Example of non -premixed combustion include pulverizedfurnaces,diesel internal-combustion engine ,and pool fires.
Direct injection is used in heavy-duty vehicles.Similar to diesel engines,high thermal efficiency and power density are maintained in such direct injection natural gas engines.insuch enginde,natural gas injected directally into the combustionchamber.Then the gas mixed with the high pressure air in the combustion chamberand combustion occurs.Due to the non-premixed nature of the combustionoccuring in such engines, a non-premixed combustion model of ANSYS FLUENTcan be used to simulate the combustion process.
PREMIXED COMBUSTION
In premixed combustion,fuel and oxidizer are mixed at the molecular level prior to ignition.Combustion occursas a flame front propogating into the unburnt reactants Examples of premixed combustion include aspirated internal combustionengine,lean-premixed gasturbine combustors,and gas-explosions
Premixed combustion is difficult to model that non-premixed combustion.The reaction for this is that premixed combustion usually occurs asa thin propogating flame that is stratched and contorted by turbulence.For subsonic flows,the overall rate of propogation of the flame is determined by both laminar and turbulant eddies.The laminar flame speed is determined by the rate that species and heat diffuse upstream into the reactants and burn.To capture the laminar flame speedy the internal flame structure would need to be resolved,as well as the detailed chemical kinetics and molecular diffusion processes since practical laminar flame thicknesses of the order of millimeters or smaller; resolution requirements are usually unaffordable.
The effect of the turbulance is to wrinkle and stretch the propogating laminar flame sheet,increasing the sheet area and in turn the effective flame speed.The large turbulant eddies tend to wrinkle and corrugated the flame sheet while the small turbulant eddies if they are smaller than the laminar flamethickness may penetratre the flame sheet and modify the laminar flame structure.
PARTIALLY PREMIXED COMBUSTION
Partially premixed combustion systemsare premixed flames with non-uniform fuel-oxidizer mixturessuch flames,include premixed jetsdischarging into a quitescent atmostphere,lean-premixed combustors with diffusion pilot flame and coding air jets ,and imperfectly mixed inlet.
The partiallypremixed model in FLUENT isa simple combination of the non-premixed model and the premixed model.The premixed reaction progress variable determines the position of the flame front. Behind the flame front.Behind the flame front , the mixture is burnt and equilibrium or laminar flamelet mixture fraction solution is used.Ahead of the flame front the species mass fractions,temperature, and density are calculated from the mixed but unburnt mixture fraction. With in the fame, a linear combination for the unburnt and burnt mixtures is used.
b) BASED ON PHASE
1.Fluid phase :- In this model,the fuel is in the liquid phase and combustion reactions arebhappening at the volume ofthe chamber.
2 Wall:- In this model, The combustion reactors are done at the surface of the combustion chamber.
3.Particles:- In this model,the reactions are being performed at the surface of the particles.
4.Porous regions- In this model,the reactions are being performed in the porous regions
COMBUSTION SOURCE CALCULATION
FINITE RATE CHEMISTRY
This model derives ODE for all species and solves them.it results in accurate solution but the computational time required to solve the ODE is increased as the number of species involved is more.
MIXING BASED MODEL
EDDY DISSIPATION MODEL
In this model,the combustion takes place when there is a presence of turbulence,hence it does mot require any heat heat source toinitiate the combustion reaction.This model is valid for non-premixed fuel mixture.
FINITE RATE EDDY DISSIPATION MODEL
It isthe combination of both finite rate chemistry and the eddy dissipation model,in this modelthe kinetic rate is calculated in addition to the reaction rate calculated in addition to the reaction rate calculated from the eddy dissipation model.
Then the slowest reaction is chosen,
1.If turbulance is low, the species mixing rate is low this limit the reaction rate.
CONTENT
1.GEOMETRY
2 Upload the combustor model into space claim. Since it is a 3D model and we are only interested in the 2Dgeometry of the combustor Therfore,we need to spilit the body by 3/4thand select the XY plane region of geometry since we are using axxix symmetric condition.
3D model of the combustor.
Spiliting the body
b.After splitting the body , select the 2D plane normal to the z-axis and copy that geometry to a now design sheet and delegte the old one since we donot need it
2D Geometry
2 Mesh -setup
2. Generate base mesh with capture proximity setting and set the cell gap to3 cells or as per your requirement
Mesh setting
b.provide name selection for the necessary inlet,outlet and walls
Named selection
a.Axis
b.fuel inlet
c.Air inlet
d. walls
e.outlet
MESH QUALITY
3.FLUENT SET UP
1. Standard k-epsilon turbulance model with standard wall function is chosen aswe are interested in the analysis ofthe combustion reaction than the wall phenomenon.
2.Species model is chosen as species transport as there is a separate inlet for air and fuel.The fuel chosen is methane-air. The volumetric combustion phase ofthe model is chosen the combustion.
3.chemical interaction chosen is the eddy dissipation model.This model is chosen because it requiores less computational time when compared to other models and also requires fewer hardwere requirements.In this model,the start time of combustion iscalculated based on the turbulance kinetic energy 'k' and epsilon.Here the heat source of the combustion isthe turbulance ofthe fuel mixture.
4.Axis symmetric model is choosen as the bottom of the model axis boundary condition is applied.
general setting
viscous model and species
SPECIES MODELLING
The species modelling is to set parameters related to species transport and comnbustion.
Species transport enables the calculation of multispecies transport either reacting or non reacting depending on type of reaction.
What ANSYS fluent does is it models mixing and transpport of chemical species by solving conservation equations describing convection diffusion,and reaction sources for each components species
mixture materials:- To set up the species transport and the reacting flow, aset of species and a list of rules governing their interaction are followed by mixture material selection.
. A listof constituents species refered to as fluent material
. A list of mixing lawsto dictate how mixture properties are to be derived from property of individual species.
. A set of reactions including a reaction type (finite rate,eddy dissipation)
Eddy dissipation chemistry interaction
Most of the fuels are fast burning and overall rate of reaction is controlled by turbulant mixing,In non premixed flames , turbulance slowly convects or mixes fuel and oxidiser into reaction zones where they burn quickly , In premixed flames,the turbulance slowly convectscold reactors and hot products into reaction zones
where reaction occurs rapidly,such cases, combustion issaid to be mixing limited tto species.
The chemical reaction is dependent on k/e combustion, proceeds whenever k/e is greater than 0 and an ignition source is not required in that case for initiating combustion.
Specires transport is turn on, The reaction is volumetric.Explicit source chemistry solver is opted for inlet diffusion and diffusion energy source is turned on.The turbulance rate chemistry interaction is eddy dissipation.As matrerial methane air 2 steps is chosen as it allows addition of water.
soot model is turned on
one step modelis chosen, with species definition as methane (CH4) as fuel and oxygen (O2) as oxidant.
NOx Modelling
All the four pathways, Thermal NOx,Prompt NOx, Fuel NOx and N2O intermediate is turned on. The Primary fuel species is methane (CH4)
Boundary Condition
There are two inlet: Fuel inlet and Air inlet.
For air inlet
The momentum condition as velocity of 0.5m/s,perpendicular to the boundary,The thermal conditions are 300K and the species boundary conditions involve every species to be O.except,oxygen(O2) which is set to 0.023 (mass fraction)
Fuel inlet
The velocity is 80m/s,perpendicular to the boundary,the thermal condition is 300k
the species boundary condition is every species is set to O, except methane (CH4) that is the fuel species primary.
In second part of the project,water is added to 30% .
Three xashes are studied: with 5% water, for the condition CH4 is 0.95 and H20 is 0.05
with 10% water: CH4 -0.90.H2o = 0.10
with 30% water :CH4=0.70,H20 = 0.30
Result and Observation
Lines probes created at different location imn the combustor.
2.Temoerature contour and mass fraction chart
From the temperature contour plot,the temperature increases as the combustion proceeds and is maximum at the mid section of the combustion chamber.
b CO2 contour and mass fraction chart
Mass fraction ofthe CO2 contour plot describes that the water concentration is decreased as the combustion takes place.
c H2o contour and mass fraction chart
d. CH4 contour and mass friction.
The mass fraction of CH4 describes the fuel not burnt at the injection region where the fluid is still flowing and penetrates towards the combustion chamber with a velocity of 80m/s and O for the rest of the domain which indicates complete combustion has taken place.
e+ N2 contour and mass fraction
From the above contour plot,it is observed that the N2 concentation is higher all over the combustion chamber because nitrogen forms a large portion of the atmosphere, and also it less reactive with the other species.
f.O2 contour mass fraction
Mass fraction of 02 describes that there is an untouched part of the air,n the domain which indicates that those places where the mass fraction of O2 is zero, indicating there is complete combustion.
g.NOx emission contour and mass fraction chart
From the above contour plot, the NOx pollutant formulation is high at a distance of 500mm from the inlet of the combustion chamber as the formation of NOx is due to the reaction between nitrogen and oxygen at a high temperature.Due to the combustion process,the temperature is high at the mid section of the combustion chamber which leads to the formation of NOx pollutants.
h.Soot formation
From the above contour plot, the mass concentration of the soot is increasing along the length of the combustion chamber and is maximum at the mid section of the chamber because as the combustion proceeds the amount of percentage of O2 available for combustion decreases and at the mid-section of the combustion chamber,the percentage of oxygen is least and hence there is the high mass fraction of soot at the midsection.
PART 2
The purpose of supplying engine with water.
minimising temperature peaks during the combustionprocess in diesel engines,hence non concentrationin the exhaust gas can be done by mass of supplying water into the cylindder.
The emulsion supplied into the cylinder causes the increases of self-ignition delay which affects the flammable mixture becoming more homogeneous in the whole volume of the combustion chamber.The increased-self -ignition delay extends the time necessary to spread the injected fuel which results in fewer zones with not enough oxygen,That decreases the intensity of shoot and co formation.Emulsifying the mixture reduces local zones with temperature peaks which leads to decreasing nitrogen activity and NOx concentration.
RESIDUAL
1.For 10% water addition
Temperature contour and massfraction chart
b.co2 contour and mass fraction chart
H2O contour and mass fraction chart
d.CH4 contour and mass fraction chart
e.N2 contour and mass fraction chart
f.O2 contour and mass fraction
g NOx emission contour and mass fraction chart
Soot formation contour and mass fraction
2.For 20% water addition
residual
a.Temperature contour and mass fraction chart
b.CO2 contour and mass fraction chart.
c.H2O contour and mass fraction chart
d.CH4 contour and mass fraction chart.
e.N2 contour and mass fraction chart.
f.O2 contour and mass fraction
g.NOx emission cintour and mass fraction chart
h.Soot formulation contour and mass fraction
3. For 30% water addition
Residual
a. Temperature contour and mass production chart
b.CO2 contour mass fraction chart
c.H2O contour and mass fraction chart
d.CH4 contour and mass production chart
e. N2 contour and mass fraction chart
f.O2 contour and mass fraction
g.NOx emission contour and mass fraction chart.
h.Soot formulation contour and mass fraction
CONCLUSION AND DISCUSSION
Comparing the NOx and soot level for different water addition levels.
Percentage of water addition(%) the ratio of methane Mass fraction of Nox Mass fraction of soot Formation
10 and water mixed emission
20 H4-H2O 2.762e-04 3.67e-02
30 0.9:01 1.642e-04 2.89e-02
0.8:02 8.507e-05 1.215-02
07:03
From the above experiment andthe data from the table,it can be calculated
From the above plots,it is observed that the NOx and soot formation is reduced with an increase in the mole fraction of water in the methane-air mixture with the addition of water in the fuel mixture,the temperature isreduced which in turn delays the combustion to takes place which leads to an efficient mixture of fuel and air complete combustion takes place because of which nitogen oxides formation is reduced as there is no oxygen left to react with nitrogen to form nitrogen oxides.
Soot is formed asa fresult of the burning of fuel in persence of less oxygen at low temperature as compldete combustion gtakes place dueto efficient mixing ofthe fuel and air there is very less amount of fuel and oxygen present after combustion as a result the soot formation is reduced .
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 mas-fraction of waterin 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 ofof water in the fuel-air mixture velocity of the exhaust gas is reduced.