Understanding Combustible Flow
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
- Presence of fuel
- Presence of oxidizer (it can be air)
- Right proportions of fuel and oxygen
- The proportion is decided by the flammability limit.
- Ignition energy is essential to provide heat energy for the initiation of burning
Basic Considerations of the Choice of Fuels
- Energy content per volume or per mass
- Combustion and fuel properties
- 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
- 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
- Hydrocarbon solid fuel: CaHbOg with a > b – produce more CO2 when burned
- Examples: wood, coal, biomass
- 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
- liquid hydrocarbon fuels : CaHbOg with a < b
- Examples: Gasoline, oil, diesel etc.
- Coal gas (a mixture of methane and Hydrogen) etc.
- For gaseous fuel: CaHbOg with na <b – have the lowest C/H ratio that results in least greenhouse gas (CO2) per unit energy output
- 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
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.
- 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
- 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:
Note: Refer to the book of “Thermodynamic” for mixture properties in reactive flows
- 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.
Methods of Quantifying Fuel and Air Content of Combustible Mixtures
For a stoichiometric mixture
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
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
- 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 (Qc) and 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
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
- 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 of Energy (Enthalpy) Equation:
Considering one dimension (1-D)
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
- 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
- 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.
- 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 ).
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
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
- 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 (do) of 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
- 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
Increased transport processes of heat and mass by small-scale turbulence
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-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.
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
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
- 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
- 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
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
- Combustion is a complex phenomenon for the measurement of due strong mixing of turbulent and chemical interactions. Species concentration across the flame is a cumbersome process for turbulent flames.
- Computational fluid dynamics (CFD) is useful for numerical investigation of premixed and non-premixed combustion
- Liquid and solid fuel combustion is more complex due to phase change and pyrolysis: Refer to the following topics
- Turbulent combustion modeling using different CFD solvers
- Turbulent multiphase combustion modeling for flames generated from oil and solid fuels
- CFD Modeling of gas-fired burners: burners can be optimized for a design condition using CFD simulations
- CFD modeling of the oil-fired burner
- Modeling of Cement Kiln using a coal gas
- CFD modeling pollutants like NOx, SOx, Soot particles: CFD simulations help to find Concentration of unburnt harmful species
- Sara McAllister, Jyh-Yuan Chen, A. Carlos Fernandez-Pello, Fundamentals of Combustion Process, Springer Publication
- F. El-mahallay, S. El-Din Habik, Funa, Fundamentals and Technology of Combustion, Elsevier Publication
- Stephen Turn, An Introduction to Combustion: Concepts and Applications, Tata Mac Graw hill Publication