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 line probes at different locations of the combustion chamber.
2. Study the effect of adding water to the methane-air mixture and plot the same and discuss the results.

• Theory:

1. Combustion:

Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually, atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion does not always result 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 reaction. While the activation energy must be overcome to initiate combustion (e.g., using a lit match 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 reactions. Solid fuels, such as wood and coal, first undergo endothermic pyrolysis to produce gaseous fuels whose combustion then supplies the heat required to produce more of them. Combustion is often hot enough that incandescent 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, a reaction commonly used to fuel rocket engines. This reaction releases 242 kJ/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 double bonds or pairs of single bonds, and therefore the formation of the stronger bonds in the combustion products CO2 and H2O results in the release of energy.

Stoichiometric combustion of a hydrocarbon in oxygen:

Generally, the chemical equation for stoichiometric combustion of a hydrocarbon in oxygen is:

Where,

For example, the stoichiometric burning of propane in oxygen is

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 reactions. It is to be contrasted with thermodynamics, which deals with the direction in which a process occurs but in itself tells nothing about its rate. Chemical kinetics includes investigations of how experimental conditions influence the speed of a chemical reaction and yield information about the reaction's mechanism and transition states, as well as the construction of mathematical models that also can describe the characteristics of a chemical reaction.

3. Chemical reaction model:

Chemical reaction models transform physical knowledge into a mathematical formulation that can be utilized in computational simulation of practical problems in chemical engineering. Computer simulation provides the flexibility to study chemical processes under a wide range of conditions. Modelling a chemical reaction involves solving conservation equations describing convection, diffusion, and reaction source for each component species.

Series transport equation-

R_i is the net rate of production of species i by chemical reaction and S_i is the rate of creation by addition from the dispersed phase and the user-defined source. J_i is the diffusion flux of species i, which arises due to concentration gradients and differs in both laminar and turbulent flows. In turbulent flows, computational fluid dynamics also considers the effects of turbulent diffusivity. The net source of chemical species i due to reaction, R_i which appeared as the source term in the species transport equation is computed as the sum of the reaction sources over the N_R reactions among the species.

Reaction rate models:

1. Laminar finite rate model:

The laminar finite rate model computes the chemical source terms using the Arrhenius expressions and ignores turbulence fluctuations. This model provides the exact solution for laminar flames but gives inaccurate solution for turbulent flames, in which turbulence highly affects the chemistry reaction rates, due to highly non-linear Arrhenius chemical kinetics. However, this model may be accurate for combustion with small turbulence fluctuations, for example, supersonic flames.

1. Eddy dissipation model:

The eddy dissipation model, based on the work of Magnussen and Hjertager, is a turbulent-chemistry reaction model. Most fuels are fast burning and the overall rate of reaction is controlled by turbulence mixing. In the non-premixed flames, turbulence slowly mixes the fuel and oxidizer into the reaction zones where they burn quickly. In premixed flames, the turbulence slowly mixes cold reactants and hot products into the reaction zones where the reaction occurs rapidly. 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 turbulence present in the flow. It does not need an ignition source to 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 computation model, which is a shortcoming of this 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. Catalysts
7. Pressure
8. Absorption of light

Applications:

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, microorganism growth, stratospheric ozone decomposition, and the chemistry of biological systems. These models 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 models can be used to find the temperature and pressure at which the highest yield of heavy hydrocarbons into gasoline will occur.

Chemical Kinetics is frequently validated and explored through modelling in specialized packages as a 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, as a percentage by mass. It is one way of expressing the composition of a mixture in a dimensionless size; mole fraction (percentage by moles, mol%) and volume fraction (percentage by volume, vol%) are others.

When the prevalences of interest are those of individual chemical elements, rather than of compounds or other substances, the term mass fraction can also refer to the ratio of the mass of an element to the total mass of a sample. In these contexts, an alternative term is a mass per cent composition. The mass fraction of an element in a compound can be calculated from the compound's empirical formula or its chemical formula.

5. Nitrogen Oxide (NOx) emissions:

Nitrogen dioxide is an irritant gas, which at high concentrations causes inflammation of the airways. When nitrogen is released during fuel combustion it combines with oxygen atoms to 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 concentrations, 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 acid rain as well as being central to the formation of fine particles (PM) and ground-level ozone, both of which are associated with adverse health effects.

6. Soot formations:

Soot forms when hydrocarbon fuels such as oil, natural gas, and wood are burned. Soot particles form when gaseous molecules are heated to high temperatures, and they don't easily turn back into gaseous molecules the way water droplets do when they are heated. Soot as an airborne contaminant in the environment has many different sources, all of which are results of some form of pyrolysis. They include soot from coal-burning, internal-combustion engines, power-plant boilers, hog-fuel boilers, ship boilers, central steam-heat boilers, waste incineration, local field burning, house fires, forest fires, fireplaces, and furnaces. These exterior sources also contribute to the indoor environment sources such as smoking of plant matter, cooking, oil lamps, candles, quartz/halogen bulbs with settled dust, fireplaces, exhaust emissions from vehicles, and defective furnaces.

Types of combustion simulation:

1. a) Non-premixed combustion (Direct injection, Late injection)
2. b) Premixed Combustion (Carburetor)
3. c) Partially premixed.

Non-premixed combustion:

In non-premixed combustion, fuel and oxidizer enter the reaction zone in distinct streams. This is in contrast to premixed systems, in which reactants are mixed at the molecular level before burning. Examples of non-premixed combustion include pulverized coal furnaces, diesel internal-combustion engines, 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. In such engines, natural gas is injected directly into the combustion chamber. Then the gas mixes with the high-pressure air in the combustion chamber and combustion occurs. Due to the non-premixed nature of the combustion occurring in such engines, a non-premixed combustion model of ANSYS FLUENT can 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 occurs as a flame front propagating into the unburnt reactants. Examples of premixed combustion include aspirated internal combustion engines, lean-premixed gas turbine combustors, and gas-leak explosions.

Premixed combustion is difficult to model that non-premixed combustion. The reason for this is that premixed combustion usually occurs as a thin, propagating flame that is stretched and contorted by turbulence. For subsonic flows, the overall rate of propagation of the flame is determined by both laminar and turbulent 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 speed, 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 are of the order of millimeters or smaller, resolution requirements are usually unaffordable.

The effect of turbulence is to wrinkle and stretch the propagating laminar flame sheet, increasing the sheet area and in turn the effective flame speed. The large turbulent eddies tend to wrinkle and corrugated the flame sheet while the small turbulent eddies if they are smaller than the laminar flame thickness, may penetrate the flame sheet and modify the laminar flame structure.

Partially premixed Combustion:

Partially premixed combustion systems are premixed flames with non-uniform fuel-oxidizer mixtures. Such flames include premixed jets, discharging into a quiescent atmosphere, lean-premixed combustors with diffusion pilot flame and cooling air jets, and imperfectly mixed inlets.

The partially premixed model in FLUENT is a 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, the mixture is burnt and the 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. Within the fame, a linear combination for the unburnt and burnt mixtures is used.

1. b) Based on phase:
2. Fluid phase:- In this model, the fuel is in the liquid phase and combustion reactions are happening at the volume of the chamber.
3. Wall:- In this model, the combustion reactions are done at the surface of the combustion chamber.
4. Particles:- In this model, the reactions are being performed at the surface of the particles.
5. 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 solutions 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 not require any heat source to initiate the combustion reaction. This model is valid for non-premixed fuel mixture.

Finite rate Eddy dissipation model:

It is the combination of both finite rate chemistry and the eddy dissipation model. In this model, the kinetic rate is calculated in addition to the reaction rate calculated from the eddy dissipation model.

Then the slowest reaction is chosen.

1. If turbulence is low, the species mixing rate is low, this limits the reaction rate.
2. If turbulence is high, the kinetic rate is low, this limits the reaction rate.

• Content:

1. Geometry

a.Upload the combustor model onto SpaceClaim, since it is a 3D model and we are only interested in the 2D geometry of the combustor. Therefore, we need to split the body by 3/4th and select the plane XY plane region of geometry since we are using the axisymmetric condition.

3D model of the combustor:

splitting the body:

After splitting the body, select the 2D Plane normal to the Z-axis and copy that geometry to a new design sheet and delete the old one since we do not need it.

2D geometry:

Mesh setup:

Generate base mesh with capture proximity settings and set the cell gap to 3 cells or as per your requirement.

Mesh settings:

Provide name selections for the necessary inlets, outlet and walls.

Named selections:

Axis:

Fuel inlet:

Air inlet:

Walls:

Outlet:

Fluent setup:

Standard K-epsilon turbulence model with standard wall function is chosen as we are interested in the analysis of the combustion reaction than the wall phenomenons.

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 of the model is chosen. The combustion

Chemical interaction chosen is the Eddy dissipation model. This model is chosen because it requires less computational time when compared to other models and also requires fewer hardware requirements. In this model, the start time of combustion is calculated based on the turbulence kinetic energy 'k' and epsilon. Here the heat source of the combustion is the turbulence of the fuel mixture.

Axis symmetric model is chosen as the bottom of the model axis boundary condition is applied.

General settings:

Viscous model and species:

Species modelling:

The species modelling is to set parameters related to species transport and combustion.

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 transport of chemical species by solving conservation equations describing convection ,diffusion, and reaction sources for each component species.

Mixture materials - To set up the species transport and the reacting flow , a set of species and a list of rules governing their interactions are followed by mixture material selection.

• A list of constituent species reffered to as fluent materials
• A list of mixing laws to 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 the overall rate of reaction is controlled by turbukent mixing . In non premixed flames , turbulence slowly convects or mixes fuel and oxidiser into reaction zones where they burn quickly. In premixed flames, the turbulence slowly convects cold -reactors and hot products into reaction zones where reaction occurs rapidly. such cases , combustion is said to be mixing limited to species.

The chemical reaction rate is dependent on kεkε combustion proceeds whenever kεkε is greater than 0 and an ignition source is not required in that case for initiating combustion.

Species transport is turned on. The reaction is volumetric. Explicit source chemistry solver is opted for. Inlet diffusion and diffusion energy source is turned on. The turbulence rate chemistry interaction is eddy dissipation. As material , methane air 2 step is chosen as it allows addition of water .

Soot model is turned on:

One step model is 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).

Bounadry Conditions:

There are two inlets , Fuel inlet and Air inlet ;

For Air inlet :

The momentum condition is, velocity of 0.5 m/s, perpendicular to the boundary , The thermal conditions are 300K and the species boundary conditions involve every species to be 0, except, oxygen(O2) which is set to 0.23 (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 0 , except methane (CH4) that is the fuel species primary.

In second part of the project, water is added upto 30% .

Three xases are studied ; with 5% water, for this condition CH4 is 0.95 and H2O is 0.05 ,

With 10% water ; CH4=0.90, H2o =0.10

With 30% water ; CH4 = 0.70, H2O = 0.30

• Result and observations:

Lines probes created at different locations in the combustor:

Part 1:

Residuals:

Temperature contour and mass fraction chart:

From the temperature contour plot, the temperature increases as the combustion proceeds and is maximum at the midsection of the combustion chamber.

CO2 contour and mass fraction chart:

Mass fraction of CO2CO2 contour plot describes that the combustion zone that has taken place and that its concentration increases as the combustion proceeds.

H2O contour and mass fraction chart:

Mass fraction of the H2O contour plot describes that the water concentration is decreased as the combustion takes place.

CH4 contour and mass fraction:

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 0 for the rest of the domain which indicates complete combustion has taken place.

N2 contour and mass fraction:

From the above contour plot, it is observed that the N2 concentration 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

O2 contour and mass fraction

Mass fraction of O2 describes that there is an untouched part of the air in the domain which indicates that those places where combustion has not taken place and places, where the mass fraction of O2 is zero, indicating there is complete combustion.

NOx emissions conotur and mass fraction chart:

From the above contour plot, the NOx pollutant formation 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 midsection of the combustion chamber which leads to the formation of NOx pollutants.

Soot formations:

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 midsection of the chamber because as the combustion proceeds the amount of percentage of O2O2 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 engines with water.

Minimizing temperature peaks during the combustion process in diesel engines, hence NOx concentration in the exhaust gas can be done by means of supplying water into the cylinder.

The emulsion supplied into the cylinder causes the increase 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 soot and CO formation. Emulsifying the mixture reduces local zones with temperature peaks which leads to decreasing nitrogen activity and NOx concentration.

Water supplied into the combustion chamber, apart from lowering the combustion temperature also decreases the concentration of atomic oxygen by reaction

H2O→OH+H

H2O+O→OH+OH

H2+O→H+OH

H+O2→OH+O

which influences the reaction speed:

O+N2↔NO+N

O+N2+OH↔N2O+OH

OH formed this way is more difficult to interact with atomic N which lowers the amount of NO.

1. For 10% water addition:

Residuals:

1. Temperature contour and mass fraction chart:

CO2 contour and mass fraction chart:

H2O contour and mass fraction chart:

CH4 contour and mass fraction:

N2 contour and mass fraction:

O2 contour and mass fraction:

NOx emissions conotur and mass fraction chart:

Soot formations:

For 20% water addition:

Temperature contour and mass fraction chart:

CO2 contour and mass fraction chart:

H2O contour and mass fraction chart:

CH4 contour and mass fraction:

N2 contour and mass fraction:

O2 contour and mass fraction:

NOx emissions conotur and mass fraction chart:

Soot formations:

1. For 30% water addition:

Residuals:

Temperature contour and mass fraction chart:

CO2 contour and mass fraction chart:

H2O contour and mass fraction chart:

CH4 contour and mass fraction:

N2 contour and mass fraction:

O2 contour and mass fraction:

NOx emissions conotur and mass fraction chart:

Soot formations:

• Conclusion and discussions:

Comparing the NOx and soot levels for different water addition levels:

Percentage of water addition (%)	The ratio of methane and water mixed (CH4:H2O)	Mass fraction of NOx emissions	Mass fraction of soot formations
10	0.9:0.1	6.245e-03	5.37e-12
20	0.8:0.2	5.260e-03	3.360e-12
30	0.7:0.3	2.287e-04	1.679e-12

From the above experiment and the data from the table, it can be concluded,

Inference:

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 is reduced which in turn delays the combustion to takes place which leads to an efficient mixture of fuel and air. With the proper mixture of fuel and air complete combustion takes place because of which nitrogen oxides formation is reduced as there is no oxygen left to react with nitrogen to form nitrogen oxides.

Soot is formed as a result of the burning of fuel in presence of less oxygen at low temperatures. As complete combustion takes place due to efficient mixing of the 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 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.