Combustion Basics for Beginner

Table of Contents

Understanding Combustible Flow 

Dr. Sharad Pachpute(PhD, IIT Delhi)

Fuels and Combustion 

Introduction to combustion

  • Combustion is the chemical reaction between a fuel and an oxidizer, resulting in the release of energy in the form of heat, light, and sound
  •  this is one of the complex physical and chemical processes that occur in various forms in our daily lives
  • More than 70% of energy is produced due to combustion in domestic cooking, engines of vehicles and aircraft to boilers and power plants that generate electricity
  • Combustion can also be found in small-scale processes such as the burning of candles, campfires, and gas stoves. 
  • To generate combustion, gas, liquid, and solid fuel are burnt to generate heat energy
  • In a controlled and efficient combustion process, the fuel and oxidizer are mixed in a specific ratio to produce the desired amount of heat energy
  • However, when the combustion process is not properly controlled, it can result in the release of harmful pollutants and greenhouse gases into the atmosphere
  • Therefore, the study of combustion and its processes plays a crucial role in various industries, such as energy production, transportation, and manufacturing.

Principle of Combustion 

  • Combustion is defined as  Rapid oxidation generating heat, or both light and heat, slow oxidation also  accompanied by relatively less heat or no light
  • Different color in combustion is observed  because of light emitted by different excited radicals
Basic of solid fuel combustion mechanism
Basic of solid fuel combustion mechanism
  • Combustible substance: C, H, Mg, S and hydrocarbons etc.  (Fuel is a combustible substance that has Low Electronegativity )
  •  Oxidizer: Oxygen which has the second highest electronegativity after Fluorine
Methane gas combustion reaction in combustion chamber
Methane gas combustion reaction in combustion chamber

 Essential conditions for combustion to occur

    1. Presence of fuel
    2.  Presence of oxidizer (it can be air)
    3.  Right proportions of fuel and oxygen
    4.  The proportion is decided by the flammability limit.
    5.  Ignition energy is essential to provide heat energy for the initiation of burning
Essential condition for combustion
An essential condition for combustion

Basic Considerations of the Choice of Fuels

     The choice of fuel depends on the purpose of the combustion process and several other factors
  1.          Energy content per volume or per mass
  2.          Safety
  3.          Combustion and fuel properties
  4.          Cost of fuel

 Classification of Fuels by Phase at Ambient Conditions

Distribution methods and combustion processes vary based on a fuel’s state of matter, making the phase of fuel at standard conditions a logical basis for classification

   a) Solid Fuel

    1. Solid fuel consists of moisture, volatile matter, fixed carbon, and ash. The ultimate analysis of solid fuels defines the relative amounts of these constituents on a mass basis. The ultimate analysis may be given on the dry basis
    2. Hydrocarbon solid fuelCaHbOg with a > b – produce more CO2 when burned
    3. Examples: wood, coal, biomass
   b) Liquid Fuel
    1. Most liquid fuels are mixtures of many different Hydrocarbons.  Commonly a liquid fuel is treated as a single hydrocarbon with a common formula CxHy� but it can be a mixture of several hydrocarbons
    2.  liquid  hydrocarbon fuelsCaHbOg with a < b 
    3. Examples: Gasoline, oil, diesel etc.
c) Gaseous Fuel
    1. Coal gas (a mixture of methane and Hydrogen) etc.
    2. For gaseous fuel:  CaHbOg with na <b – have the lowest C/H ratio that results in the least greenhouse gas (CO2) per unit energy output
    3. Examples: LPG gas, Syngas, CNG, NG gas, Biogas etc.
Gaseous fuels like LPG and LNG
Gaseous fuels like LPG and LNG

Main Processes in Combustion of  Fuels

 Solid Fuel Combustion Mechanism

  • Solid fuel combustion comprises the pyrolysis of solid fuels followed by a reaction in solid fuel and air
  • Devolatilization of coal is an example of the pyrolysis of coal.
  • Homogenous and heterogeneous reaction:
    • Combustion is observed from the reaction of carbon in the volatile matter and char with air
  • Unburned fuel can result in ash and carbon particles
  • More pollution due to incomplete burning of fuel
  • In rural areas, biomass from agricultural waste is used as fuel. Which is more affordable than LPG

 

coal combustion processes
coal combustion processes
  • The basic steps for the combustion of cola are presented in the following diagrams
Coal combustion with char and volatiles formations
Coal combustion with char and volatile formations

 Liquid  Fuel Combustion Mechanism

  • The phase change from liquid to gas phase occurs followed by combustion
  • For liquid fuel, the combustion has three steps:
    • evaporation of liquid fuel by absorbing heat,
    • mixing of fuel with air in the gaseous phase,
    • formation of flames or reaction zone
  • Unburned fuel can result in soot particles
  • Some kind of pollution due to soot particles
Principle of Liquid combustion for kerosene oil
Principle of Liquid combustion for kerosene oil

Here are some key aspects of liquid combustion processes:

  1. Fuel Atomization:
    • In liquid combustion, the first step is often the atomization of the liquid fuel.
    • This process breaks down the liquid into small droplets to increase the surface area exposed to the oxidizer, facilitating a more efficient and complete combustion process.
    • Fuel atomization can be achieved using various methods, including mechanical injectors, pressure nozzles, or air-assist mechanisms.
  2. Ignition:
    • Liquid fuels need an ignition source to initiate the combustion process.
    • This can be done using spark plugs (common in gasoline engines) or through compression (common in diesel engines). Once ignited, the fuel-air mixture starts to burn.
      Mechanism of liquid combustion
      Mechanism of liquid combustion
  3. Stoichiometric Ratio:
    • Achieving the correct air-fuel mixture ratio is crucial for efficient combustion.
    • This ratio is often expressed as the stoichiometric ratio, which is the ideal proportion of air to fuel required for complete combustion.
    • Deviations from this ratio can result in incomplete combustion, leading to the production of pollutants such as carbon monoxide (CO) and unburned hydrocarbons.
  4. Combustion Chambers:
    • The design of the combustion chamber varies depending on the application.
    • For example, in internal combustion engines, the combustion chamber is typically found within the cylinder.
    • In industrial burners, it’s part of the combustion equipment. The design and geometry of the combustion chamber affect combustion efficiency, heat transfer, and emissions.
  1. Flame and Heat Release:
    • Combustion results in the formation of flames, which are visible as a result of the release of heat and light energy.
    • The temperature of the flame and the rate of heat release are essential parameters to consider in many applications.
  2. Emissions Control:
    • Controlling emissions, such as nitrogen oxides (NOx), particulate matter, and carbon emissions, is a significant concern in liquid combustion processes.
    • Advanced combustion technologies and emission control systems are employed to reduce environmental impacts.
  3. Applications:
    • Liquid combustion processes are used in various applications, including internal combustion engines (e.g., cars, trucks, and airplanes)
    • Industrial furnaces, boilers, and power generation. Different fuels and combustion processes are used based on the specific requirements of each application.
  • Understanding the characteristics of liquid combustion processes is essential for optimizing combustion efficiency, minimizing emissions, and improving the performance and environmental impact of systems using liquid fuels.
  • Research and development in this field continue to focus on increasing efficiency, reducing emissions, and finding alternative and sustainable liquid fuels.

 Gaseous  Fuel Combustion Mechanism

  • Mixing of fuel and gas is fast. Hence, it is the cleanest combustion
  • Gases phase combustion is faster and cleaner compared to liquid and solid-state combustion
  • Gaseous phase combustion is easy to understand and widely used in laboratory

 

Gas burner Bunsen Flame
Gas burner Bunsen Flame
  • Schematic of gas combustion is shown below: fuel  directly mixes with air and forms a flame
Gas burner air and fuel mixing process
Gas burner air and fuel mixing process

 Some Important Definitions related to Liquid Combustion

Flashpoint of liquid fuels

  •        Flashpoint is defined as the lowest temperature at which a fuel liberates vapor at a sufficient flow rate such that the vapor forms a mixture with air, That mixture will ignite in the presence using the pilot or an ignition source.
  •         When the fuel reaches its flash point fuel is ready to burn when there is an ignition source.
  •         If a spill of liquid fuel occurs, the chances of fire is very high and the air or fuel temperature reaches the flashpoint.
Ignition point of liquid fuel
The ignition point of liquid fuel

 Fire Point

  •          It  refers to the minimum liquid temperature for sustained burning of the liquid fuel with or without the ignition source
  •         At the fire point, the heat release rate of the establishing flame balances the rate of heat losses to the surroundings

 Auto-ignition Point

  • it occurs through the self-heating of the reactants
Liquid fuel flash point, ignition, fire point with flammability
Liquid fuel flash point, ignition, fire point with flammability

 

 

Thermodynamics of Combustion

Properties of Mixtures

  • The thermal properties of a pure substance are described by quantities including internal energy, u, enthalpy, h, specific heat, Cp, etc.
  • Combustion systems consist of many different gases, so the thermodynamic properties of a mixture result from a combination of the properties of all of the individual gas species.
  • The ideal gas law is assumed for gaseous mixtures, allowing the ideal gas relations to be applied to each gas component.
  • Thermodynamic properties can be defined as either mass basis or mole basis

 

For more details click here:

    1. Gas Mixtures and Partial Pressures
    2. Basic of Volume Fraction and Mole Fraction

Note: Refer to the book of “Thermodynamic” for mixture properties in reactive flows

 

 Combustion Stoichiometry

  •        Air contains 21 mol percent O2 and 79 mol percent of N2 by volume
  •        For a given combustion device, say a piston engine, how much fuel and air should be injected in order to completely burn both? 
Methane gas combustion reaction
Methane gas combustion reaction
  •          This question can be answered by balancing the combustion reaction equation for a particular fuel.
  •          A stoichiometric mixture has the exact amount of fuel and oxidizer such that after combustion is completed, all the fuel and oxidizer are consumed to form products. 
Methane gas combustion reaction with stoichiometry
Methane gas combustion reaction with stoichiometry
  •          Combustion stoichiometry for general hydrocarbon fuel, CαHβOγ, with air, can be expressed 
 Combustion stoichiometry for general hydrocarbon fuel
Combustion stoichiometry for general hydrocarbon fuel

 Methods of Quantifying Fuel and Air Content of Combustible Mixtures

  • Fuel-Air Ratio (FAR): The fuel-air ratio, f, is given by
Fuel Air ratio for combustion
Fuel Air ratio for combustion

                          where mf and ma are the respective masses of the fuel and the air.

For a stoichiometric mixture

For a stoichiometric mixture hydrocarbon reaction
For a stoichiometric mixture hydrocarbon reaction
  •        The fuel-air-ratio (FAR)  for above reaction is given as

         where Mf and Mair (~28.84 kg/kmol) are the average masses per mole of fuel and air, respectively. The range of f is bounded by zero and one.

  •        Most hydrocarbon fuels have a stoichiometric fuel-air ratio, fs, in the range of 0.05–0.07

 The air-fuel ratio (AFR); 

  •         AFR  is also used to describe a combustible mixture and is simply the reciprocal of FAR, 
AFR = 1/FAR = 1/f 
  •        For most hydrocarbon fuels, AFR is  14-20: that means  14–20 kg of air is required to achieve  complete combustion of one  kg  fuel
 

Equivalence Ratio (ϕ):

  •       Normalizing the actual fuel-air ratio (FAR) by the stoichiometric fuel-air ratio provides  the equivalence ratio,

                                  ϕ  = FARa/FARs  

 Equivalence Ratio for combustion
Equivalence Ratio for combustion
  •          The subscript, s denotes the values at the stoichiometric condition.

ϕ <1 is a lean mixture,

ϕ = 1 is a stoichiometric mixture,

ϕ >1 is a rich mixture.

Stoichiometric and  Excess air

  •      The minimum amount of air that supplies the required amount of oxygen for the complete combustion of a fuel is called the stoichiometric or theoretical air
  •      The amount of air in excess of the stoichiometric air is called excess air. It is usually expressed in terms of the stoichiometric air as percent excess air. The amount of air less than stoichiometric is called a deficiency of air
  •      Percent Excess Air: The amount of air in excess of the stoichiometric amount is called excess air. The percent excess air, %EA, is defined as
Formula for excess air over the stoichiometric amount
Formula for excess air over the stoichiometric amount

 Heating Values

  • The thermal properties of a pure substance are defined using internal energy, u, enthalpy, h, specific heat, Cp, etc.
  • Heating values of a fuel ( kJ/kg or MJ/kg) are generally used to define the maximum amount of heat that is generated by combustion with air at standard conditions ( STP). The temperature and pressure at standard conditions are  398 k (25°C) and 101.3 kPa.
  • The amount of heat released during the combustion of the fuel depends on the phase of water in the products. If water is found in the gas phase in the products, the value of total heat release is called as the lower heating value (LHV).
  • For water vapor in the condensed form, additional heat energy (which is equal to the latent heat of vaporization) is extracted. Then the total energy release is called higher heating value (HHV).
  • The value of the LHV is calculated from the  HHV and latent heat of  phase change for water to vapor
In above equation NH2O,P is the number of moles of water in the products. Latent heat for water at STP is hfg= 2.44 MJ/kg = 43.92 MJ/kmol. In combustion literature, the LHV is normally called the enthalpy or heat of combustion (Qcand is a positive quantity.

 Enthalpy of Combustion

  • In combustion processes, reactants are consumed to form products and energy is released.
  • This energy comes from a rearrangement of chemical bonds in the reactants to form the products.
  • The standard enthalpy of formation, ∆hoi , quantifies the chemical bond energy of a chemical species at standard conditions.
  • The enthalpy of formation of a substance is the energy needed for the formation of that substance from its constituent elements at STP conditions (25°C and 1 atm).
  • The ‘absolute’ or ‘total’ enthalpy = enthalpy of formation + sensible enthalpy
  • One way to determine the enthalpy of formation of a species is to use a constant pressure flow reactor
  • When phase change is encountered, the total enthalpy needs to include the latent heat

Adiabatic Flame Temperature

  • One of the most important features of a combustion process is the highest temperature of the combustion products that can be achieved.
  • The temperature of the products would be highest when there are no heat losses to the surrounding environment and all of the energy released from combustion is used to heat the products
 
 
Graphical interpretation of adiabatic flame temperature
Graphical interpretation of adiabatic flame temperature

.

Chemical Kinetics 

      Chemical kinetics provides important information related to the rate at which species are consumed and produced and the rate at which the heat of reaction is released

The Nature of Combustion Chemistry

  • The chemical reaction can be described by an overall stoichiometric relation as
  • The collection of elementary reactions that define the overall or global reaction is called a reaction or combustion mechanism.
  • In intermediate stages, many chain initiation or chain branching reactions are involved
  • The detailed combustion chemistry of hydrocarbon involves so many intermediate species and steps. Hence, only major species are considered for analysis or modeling
                        

 Elementary Reaction Rate

  • Forward Reaction Rate and Rate Constants 
    • The chemical expression of an elementary reaction can be described by the following general expression
                       
    • The corresponding rate of reaction progress is  expressed by the following  form (often referred to as the law of mass action)
    • The rate at which reaction proceeds: The reaction rate is proportional to the concentration of reactants

Where k is the Arrhenius rate constant, A0 is the  pre-exponential factor, Ea is the activation energy,  R^u is the universal gas constant (1.987 cal/mol-K, 1 cal =4.184 J)

    • The activation temperature (K)

The rate of the reaction

Where, [A] = XAP/(Ru T) and [B] = XBP/(Ru T)as XA and XB are mole fractions of A and B

  •        The consumption rate of reactant A is then expressed by

 Simplified Model of Combustion Chemistry

  • For a general hydrocarbon fuel with an overall combustion stoichiometry
Simplified Model of Combustion Chemistry
Simplified Model of Combustion Chemistry
  • The consumption rates of fuel and oxygen are determined as
  • The production rates of CO2 and H2O are determined as

Review of Transport Equations and Properties

 Overview of Heat and Mass Transfer

  • Fourier’s law of heat conduction:
where k is the thermal conductivity of the material, A is the area, and ▼T is the temperature gradient.
  • Convective heat transfer: adjacent to the wall with fluid flow
  • Radiation is energy transfer: between surface and surrounding
  • Mass transfer: diffusion and  advection 
    • Fick’s law of Mass Diffusion
    • Advection is the transport of species through fluid motion as described by

 Some Important Dimensionless Numbers for Heat and Mass Transfer

 Some Important Dimensionless Numbers for Heat and Mass Transfer
Some Important Dimensionless Numbers for Heat and Mass Transfer

Governing Equations for Combustible Flow

Conservation of Mass and Species

 

Because combustion does not create or destroy mass, the conservation of mas (or continuity) equation

Conservation of Mass and Species for combustion
Conservation of Mass and Species for combustion

 Conservation Momentum:

 

 Conservation of Energy (Enthalpy) Equation:

Considering one dimension (1-D)

(Enthalpy) Equation:
Conservation of Energy Equations for combustion
Conservation of Energy Equations for combustion
Total Enthalpy is defined as

Assuming that ∂P/ ∂t = 0 and k = constant, the simplified 1-D energy equation in terms of temperature (with constant cp and k)

Dimensionless (Normalized) of Energy (Enthalpy) Equation

Defining the characteristic quantities as tc for time, lc for the spatial variable, Tc for temperature, yc for mass fraction, uc for velocity and Pc for pressure

The dimensionless energy equation for the combustion 

Non-dimensional groups that determine the physics of combustion processes

 

Non-dimensional groups that determine the physics of combustion processes
Non-dimensional groups that determine the physics of combustion processes

 

Premixed Flames

  • Definition of Flame: A flame is a self-sustaining propagation of a localized combustion zone at a subsonic velocity
  • Premixed flames refer to the combustion mode that takes place when fuel and oxidizer have been mixed prior to their burning
  • Classical examples of premixed flames: 1) Gas stove, 2) Bunsen burner

 

 

Premixed Flames
Premixed Flames
  • A Bunsen flame is a type of gas flame used in laboratory settings for various heating and combustion purposes.
  • It is named after Robert Bunsen, a German chemist who popularized the use of this type of burner in the mid-19th century.
  • The Bunsen burner is a simple device that produces a controlled, non-luminous (non-glowing) flame by mixing flammable gas with air in a specific way.
  • Here are the key characteristics of a Bunsen flame
  • The flame produced in this burner is typically blue in color.
  • It appears blue because it is a nearly complete combustion, which means that there is sufficient oxygen for the gas (usually natural gas or propane) to burn completely, producing primarily carbon dioxide and water vapor.
Premixed flame Bunsen burner
Premixed flame Bunsen burner
  • A Sectional View of Premixed Flames
 
Flame envelope of gas premixed flames
Flame envelop of gas-premixed flames
 

 Physical Processes in a Premixed Flame

  • Flame propagation through the unburned mixture depends on two consecutive processes.
  • First, the heat produced in the reaction zone is transferred upstream, heating the incoming unburned mixture up to the ignition temperature.
  • Second, the preheated reactants chemically react in the reaction zone.
  • Both processes are equally important and therefore one expects that the flame speed will depend on both transport and chemical reaction properties
  • Temperature of reactant and products
    • Tp = flame (product) temperature
    • Tig= ignition temperature ~ auto ignition temperature
    • Tr = reactant temperature  
  •  Flame speed and heat release rate (HRR)  of premixed combustion
 Flame speed and heat release rate (HRR)
Flame speed and heat release rate (HRR)

 

  • Heat release due to combustion is given as
 Flame speed and heat release rate (HRR)
Flame speed and heat release rate (HRR) where Qc is the heat of combustion (LHV)
 

 Expressions for Laminar Flame Speed and Thickness

  •     Expression for laminar flame speed 

 Structure of Premixed Flames

  • Due to the very  small thickness of premixed flames (~ a few millimeters at 1 atm), it is quite difficult to measure the species concentrations accurately
  • Computations of premixed flames with detailed chemistry and transport have been useful in illustrating the structure of a typical premixed flame
  • Computed radical profiles and their net production rate are presented for a laminar one-dimensional stoichiometric methane-air premixed flame initially at ambient conditions
  •  Flame structure of premixed combustion

 Flame structure of premixed combustion
Flame structure of premixed combustion

 

 Dependence of Flame Speed on Equivalence Ratio and Temperature

  • Since the flame speed depends on the chemical reaction rate, one expects a strong dependence of SL on temperature and consequently on equivalence ratio
  • Flame temperatures of several fuels versus equivalence ratio showing that the peak flame temperatures occur at a slightly rich mixture. 
  • Flame speed of propane-air versus equivalence at 1 atm with various initial temperatures
Dependence of Flame Speed on Equivalence Ratio and Temperature
Dependence of Flame Speed on Equivalence Ratio and Temperature

 

 Effect of Pressure on Laminar Flame Speed

  • Flame speed is negatively dependent on pressure considering the global rate of progress

  • Laminar flame speed using the global reaction rate

normally n= +1=1~2

  • Note that since (n = a+b) is normally larger than zero, flame speed is found to decrease with pressure for most hydrocarbon fuels
  • A correlation of flame speed of stoichiometric methane-air mixture as function of pressure showing a decreasing trend 

 

Where,ϕ is the equivalent ratio and ѱ is the empirical constant

5.  Flammability Limits

  •       If the  combustible mixture is too rich or too lean, the flame temperature decreases and  flame speed drops significantly 
  •        The flame cannot move when the equivalence ratio crosses the its higher limit or lower limit. These two limits are called as the rich and the lean flammability limits (RFL and LFL ).
 Flammability limit of methane
Flammability limit of methane
                   
 

 

 

    Effects of Temperature and Pressure on Flammability Limits

  •        A flame approaching a conducting material loses heat to the material, reducing the temperature of the reaction and consequently its reaction rate
  •         If the heat losses are significant, the reaction may not be able to continue and the flame is quenched
  •       With an increase in the temperature the flammability limit increases 
Effect of temperature on the flammability limit 
Effect of temperature on the flammability limit
Effect of pressure on the flammability limit 
Effect of pressure on the flammability limit
 

 Flame Quenching

  •         Flame quenching has many serious implications in combustion processes, from fire safety to pollutant emissions.
  •         One of the  important parameters in flame quenching is the minimum distance at which a flame can approach a material surface before quenching
  •         This distance is called the “quenching distance” . It  determines such parameters as the spacing in flame arrestors or the amount of unburned fuel left in the combustion chamber walls of an engine cylinder
  •        Flame quenching distance formula is used to decide the gap of walls
 
 Flame Quenching Formula
Flame Quenching Formula
 
Flame Arrestor
  • A flame arrestor is designed to stop unwanted flame propagation through a gas delivery system.
  • Flammable gases pass through a metal grid, or mesh, which is generally designed with spacing smaller than the quenching distance for the conditions under consideration
  • A flame arrester is a device designed to prevent the propagation of flame from the exposed side of a pipe or vessel to the protected side while allowing the flow of gases, liquids, or vapors to continue.
  • Flame arresters are commonly used in various industrial applications where flammable gases or vapors are present, such as in pipelines, storage tanks, or processing units.
  • The primary purpose of a flame arrester is to provide safety by preventing the spread of flames and protecting equipment and personnel from the potential hazards associated with combustible materials.
  • Flame stability is crucial in ensuring that the flame arrestor effectively performs its intended function. Here are some factors related to flame stability in the context of flame arrestors:
  • Example: Use of flame arrestor to stop unwanted flame propagation
Use of flame arrestor with metal grid
Use of flame arrestor with metal grid
Use of flame arrestor in burner
Use of flame arrestor in burner
  •  Flame

 Effect of equivalent ratio and pressure on flame thickness

  • Flame thickness and quenching distance (doof methane air versus equivalence ratio
  • Dependence of quenching distance on pressure 
                          
 
  • Minimum ignition energy (MIE) variation with mixture composition with different turbulence velocities
                                          
         

 

 Turbulent  Premixed Flame

  • Experimental observations reveal that premixed flames in turbulent flows propagate faster than their counterparts in laminar flows
Laminar and turbulent (wrinkled) premixed flames
Laminar and turbulent (wrinkled) premixed flames
                               
 
  • The enhancement in flame propagation speed can be significant; turbulent flames can propagate two orders of magnitude faster than laminar flames
  • Modeling of  turbulent quantitates in Reynolds averaged equations for momentum, energy and mass
Modeling of  turbulent quantitates in Reynolds averaged equation
Modeling of  turbulent quantitates in Reynolds averaged equation
  • The additional terms in the Reynolds averaged Navier equations are solved by a CFD modeling
  • A variety of turbulence  models are available in CFD simulations

 Turbulent Flame Speed

  • The effect of turbulence on flame propagation may be classified based upon the type of interaction between turbulence and the flame
  • Several regimes can be classified on the basis of length, velocity, and chemical time scales. For instance, two interaction regimes have been proposed for the enhancement of flame speeds in turbulent flows
    1. Increased transport processes of heat and mass by small-scale turbulence 

    2. The increased surface area due to wrinkling of the flame by large turbulent eddies

  • As laminar flame speed depends on transport properties
  • Accordingly for turbulent flames, the flame speed
Formula for the turbulent he flame speed
The formula for the turbulent he flame speed
  • The ratio of turbulent flame speed to laminar flame speed is proportional to the ratio of flame areas
  • One simple model to account for the wrinkled flame surface is

 

Flame Zones for Turbulent Premixed Combustion 

  • Non-dimensional numbers for characterizing the interaction between turbulence and combustion
Flame zone for Premixed Flame
Flame zone for Premixed Flame
                                       Flame region of premixed combustion
  • Flame types of premixed combustion.

Non-premixed  (Diffusion) Flames

  •       Non-dimensional numbers for characterizing the interaction between turbulence and combustion
  •        In many combustion , the fuel and oxidizer are separated before entering the reaction zone where they mix and burn
  •        The combustion reactions  are named as “non-premixed flames”
  •       This combustion is also called “diffusion flames” because the major transport of fuel and oxidizer into the reaction zone of flame is  primarily by diffusion
  •       A candle flame and lighter are common familiar examples of a non-premixed (diffusion) flame.
  •        Examples of non-premixed combustion.
Example of non-premixed flame: Candle Flame
Example of non-premixed flame: Candle Flame
Examples of non-premixed combustion.
Examples of non-premixed combustion.
 

 Description of a Candle Flame

At the flame surface, combustion leads to high temperatures that sustain the flame. The elements of the process are:

    • Heat the flame melts wax at the base of the candle flame.
    • Liquid wax flows upward by capillary action, through the wick towards the flame.
    •  Heat from the flame vaporizes the liquid wax.
    • Wax vapors migrate toward the flame surface, breaking down into smaller hydrocarbons.
    • Ambient oxygen migrates toward the flame surface by diffusion and convection

Mechanism of Candle Flame Combustion

Mechanism non-premixed flame : Candle Flame
Mechanism non-premixed flame: Candle Flame
C
  •  Role of gravity in candle flame combustion
Mechanism non-premixed flame
Mechanism non-premixed flame as Candle Flame
  • Flame structure  of candle flame combustion
  • Details of candle flame combustion.
Flame structure  of candle flame combustion
Flame structure  of candle flame combustion
 
 

Structure of Non-premixed Laminar Free Jet Flames

  • Experimental and Computed temperature distribution of a non-premixed jet flame
  • Both evaporated fuel and air reach at the flame
 
Structure of Non-premixed Laminar Free Jet Flames
Structure of Non-premixed Laminar Free Jet Flames
  •   The following figure shows the structure  of the non-premixed flame
    • The concentration of fuel is higher at the nozzle exit 
    • The concentration of Oxygen and nitrogen is higher away from the flame
    • Flame region formed at the edge of nozzle
lame region formed at the edge of nozzle
lame region formed at the edge of the nozzle

 Laminar Jet Flame Height (Lf)

  • The length, or height, of a non-premixed flame, is an important property indicating the size of a flame
  • Considering a simple free jet flame-based non-dimensional analysis of energy and Species (fuel) mass fraction

  • Fuel Consumption rate
  • Non-dimensional Equations for energy and species
  • Non-dimensional Time Scales

Where D =fixed mass diffusivity of fuel

  •  Jet flame of non-premixed combustion
 Jet flame of non-premixed combustion
Jet flame of non-premixed combustion
 

 Effect of Volume flow rate of fuel on Flame Length 

  •       The flame length (Lf or Hf) for a constant diffusivity, D  

 

The flame length (Lf or Hf) for a constant diffusivity
The flame length (Lf or Hf) for a constant diffusivity
  • For a given fuel and oxidizer (i.e., fixed mass diffusivity D), flame height increases linearly with the volumetric flow rate Such a linear dependence is indeed observed in experiment
  • Empirical Correlations for Laminar Flame Height is a function of inlet temperature of fuel and air, air/fuel ratio 

 

  • Effect of flow rate of  flame length of non-premixed combustion
Effect of flow rate of  flame length of non-premixed combustion
Effect of flow rate of  flame length of non-premixed combustion

Effect of Stoichiometric air/fuel ratio  and Pressure on  Jet Flame Length

The length of the flame increases with increasing the stoichiometric air-fuel ratio (S) and pressure of fuel
• With an increase in the Carbon index, the flame height increases
 
Effect of Stoichiometric air to fuel ratio  and Pressure on  Jet Flame Length
Effect of Stoichiometric air to fuel ratio  and Pressure on  Jet Flame Length
  •       Effect of pressure on the flame of non-premixed combustion is shown below
  •       With an increase in pressure, the flame height increases
Effect of pressure on the flame of non-premixed combustion
Effect of pressure on the flame of non-premixed combustion

 

 Turbulent Jet Flame 

  • Length of turbulent jet flame
  • Estimation of turbulent jet flames with hydrocarbon fuels burning with air:
  • where ρfuel and  ρflame are the densities of fuel and flame, and fs is the stoichiometric fuel-air mass ratio
 
Turbulent Jet Flame 
Turbulent Jet Flame
  • Effect of jet velocity on the flame of non-premixed combustion
Effect of jet velocity on the flame of non-premixed combustion
Effect of jet velocity on the flame of non-premixed combustion

Lift-Off Height (h) and Blowout Limit

  • Experimentally, it is observed that when the velocity of a jet   increases up to a certain point, the flame lifts off of the nozzle
  • Further increase in the jet velocity  can lead to total flame blow out
  •   This effect is related to the the jet velocity and the lower portion of the flame that anchors the flame to the jet nozzle cannot propagate against the flow
  •  See the following figure,  three is a gap between the reaction (flame) and the nozzle tip for non-premixed combustion
  • The lift-off height  is determined using the the relative magnitude of the jet velocity(Vjet) and the premixed flame speed
  • The lift height is the gap between the nozzle exit and flame
Lift-Off Height (h) and Blowout Limit
Lift-Off Height (h) and Blowout Limit

 

 Flame Regimes of  Non-premixed Flames

  • Flame regions of non-premixed combustion
 Flame Regimes of  Non-premixed Flames
Flame Regimes of  Non-premixed Flames

 Scope of CFD modeling for Combustion 

Flame length of Oil Burner (CFD Results)
Scope of CFD modeling for Combustion

References

Books
  1.      Sara McAllister, Jyh-Yuan Chen, A. Carlos Fernandez-Pello, Fundamentals of Combustion Process, Springer Publication
  2.       F. El-mahallay, S. El-Din Habik,  Funa, Fundamentals and Technology of Combustion, Elsevier Publication
  3.       Stephen Turn, An Introduction to Combustion: Concepts and Applications, Tata Mac Graw hill Publication

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