Combustion Basics for Beginner

Table of Contents

Understanding Combustible Flow 

Dr. Sharad Pachpute(PhD, IIT Delhi)

Fuels and Combustion 

Principle of Combustion 

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

 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

Basic Considerations of the Choice of Fuels

     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 least greenhouse gas (CO2) per unit energy output
    3. Example: LPG gas, Syngas, CNG gas etc.

 Main Processes in Combustion of  Fuels

 Solid Fuel Combustion

  • Solid fuel combustion comprises pyrolysis of solid fuels followed by reaction in solid fuel and air
  • Devolatilization of coal is an example of 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
                                             
                     

 Liquid  Fuel Combustion

  • The phase change from liquid to gas phase is occurred 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

 Gaseous  Fuel Combustion

  • Mixing of fuel and gas is fast. Hence, it is the cleanest combustion
  • Gases phase combustion is fast and cleaner compared to liquid and solid-state combustion
  • Gaseous phase combustion is easy to understand and widely used in laboratory
  • Schematic of gas combustion is shown below: fuel  directly mixes with air and forms a flame

 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 and the 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.

 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

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? 
  •          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. 
               
  •          Combustion stoichiometry for general hydrocarbon fuel, CαHβOγ, with air can be expressed 

 Methods of Quantifying Fuel and Air Content of Combustible Mixtures

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

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

For a stoichiometric mixture

  •        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  

  •          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 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. Amount of air less than stoichiometric are called 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

 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
 
 

Fig.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
  • 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 Number 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 Momentum:

 

 Conservation of Energy (Enthalpy) Equation:

Considering one dimension (1-D)

(Enthalpy) Equation:
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

 

 

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

 

 

  • A sectional view  of Premixed Flames
 
 

 Physical Processes in a Premixed Flame

Flame propagation through the unburned mixture depends on two consecutive processes.

1) First, the heat produced in the reaction zone is transferred upstream, heating the incoming unburned mixture up to the ignition temperature.

2) 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
  • Heat release due to combustion is given as
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

 

 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

 

 Effect of Pressure on Laminar Flame Speed

  • Flame speed is negative dependence 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 ).
                        Fig.5.7 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 
     Fig.5.8 Effect of temperature and pressure in 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 the 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 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
  • Example: Use of flame arrestor to stop unwanted flame propagation

 

 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
                                     Fig.5.12 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
  • 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
  • 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 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.

(a) Candle Flame
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
(a) Candle flame combustion
b) Role of gravity for candle flame combustion
c) Flame structure  of candle flame combustion
 
Details  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 reaches at the flame
 
  •   The following figure shows the structure  of 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

 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
 

 Effect of Volume flow rate of fuel on Flame Length 

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

 

  • 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 Stoichiometric air/fuel ratio  and Pressure on on Jet Flame Length

The length of 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 pressure on the flame of non-premixed combustion is show below
  •       With an increase in pressure, the flame height increases

 

 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
 
Fig.6.7  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

 

 Flame Regimes of  Non-premixed Flames

  • Flame regions of non-premixed combustion
 

 Scope of CFD modeling for Combustion 

Flame length of Oil Burner (CFD Results)

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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