Table of Contents
Understanding Combustible Flow
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
- 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
Essential conditions for combustion to occur
- 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 the least greenhouse gas (CO2) per unit energy output
- Examples: LPG gas, Syngas, CNG, NG gas, Biogas etc.
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
- The basic steps for the combustion of cola are presented in the following diagrams
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
Here are some key aspects of liquid combustion processes:
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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
- 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 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.
- 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
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 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
- 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
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 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 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 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.
- A Sectional View of 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
- Heat release due to combustion is given as
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.
- 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 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
- 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
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.
- 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
- Role of gravity in candle flame combustion
- 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 reach at the flame
- 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
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 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
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
- 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
- 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