Untitled Quiz

Combustion Fundamentals

• Combustion is a key process in many modern technologies, accounting for approximately 85% of global energy use.
• It converts chemical energy to thermal or propulsive force.

Combustion Applications

• Gas turbines and jet engines
• Rocket propulsion
• Piston engines
• Guns and explosives
• Furnaces and boilers
• Materials processing (e.g., carbon black production, fullerenes, nano-materials)
• Forming of materials
• Fire hazards and safety

Combustion Processes

• Combustion is a complex interaction of physical (fluid dynamics, heat and mass transfer) and chemical (thermodynamics, chemical kinetics) processes.
• Energy, mass, and momentum transport are key physical processes.
• Thermal energy conduction, species diffusion, and gas flow are consequences of chemical energy release.

Relevant Subject Areas

• Thermodynamics: Includes stoichiometry, gas and gas mixture properties, enthalpy of formation, enthalpy of reaction, equilibrium, and adiabatic flame temperature.
• Heat and Mass Transfer: Includes heat transfer by conduction, convection, and radiation, and mass transfer.
• Fluid Dynamics: Includes laminar flows, turbulence, effects of inertia and viscosity, and combustion aerodynamics.
• Chemical Kinetics: Focuses on the application of thermodynamics to reacting systems to determine equilibrium composition and adiabatic flame temperature.
• Additionally, understanding rates of chemical reactions is necessary for accurate modeling of combustion.

Primary Combustion Research Literature Sources

• Combustion and Flame journal
• Combustion Science and Technology journal
• Combustion Theory and Modelling journal
• Progress in Energy and Combustion Science (review journal)
• Proceedings of the Combustion Institute (Biennial Combustion Symposia)
• Combustion, Explosions, and Shock Waves journal (translated from Russian).

Fundamental Definitions

• Chemical Reactions: involve rearrangement or exchange of atoms between colliding molecules.
• Reactants: substances entering a reaction.
• Products: substances produced in a reaction.
• Atom conservation follows reactions. Molecular conservation does not.

Amount of Substance/Mole Numbers (mol)

• One mole of a compound corresponds to 6.023 x 10²³ particles (atoms, molecules, or chemical species).
• Avogadro's constant: 6.023 x 10²³
• Mole fraction (X_i) of species i is the ratio of the number of moles of species i to the total number of moles in the mixture.
• Mass fraction (Y_i) of species i is the ratio of the mass of species i to the total mass of the mixture.

Molar or Molecular Mass (M_i)

• Molar or molecular mass (also known as molecular weight) is the mass of one mole of a substance.
• Units are g/mol. Examples: CH₄ = 16 g/mol, H₂ = 2 g/mol, O₂ = 32 g/mol.

Mean Molar Mass (M)

• Mean molar mass represents an average molar mass of a species mixture.

Ideal Gas Equation of State

• PV = nRT where P = pressure, V = volume, n = number of moles, R = ideal gas constant (8.314 J/mol K), and T = temperature.
• Real gas equations (e.g., van der Waals, Peng-Robinson equations) are necessary for conditions near or above critical temperatures and pressures.

Basic Flame Types

• Premixed flames (Laminar / Turbulent)
• Non-premixed flames (Diffusion) flames (Laminar / Turbulent)
• Partially premixed flames (Laminar / Turbulent)

Stoichiometry

• Stoichiometric flame - Complete fuel consumption
• Fuel-rich system- Excess fuel
• Fuel-lean system- Excess oxidiser
• Common air compositions (O₂ + 3.762 N₂) for combustion reaction calculations

Stoichiometry (Cont'd)

• Air-to-fuel ratios are important in combustion calculations.
• Equivalence ratio (Φ) calculates the combustion proportion of reactants

Laminar (Turbulent) Non-premixed Flames

• Fuel and oxidizer mix in the combustion process.
• A candle flame is often described as a laminar non-premixed or diffusion flame.

Thermodynamics/Thermochemistry

• Basic property relations for ideal gas and ideal gas mixtures
• First law of thermodynamics
• Enthalpy (heat of reaction), adiabatic flame temperature
• Chemical equilibrium

Calorific Equations of State

• Relates enthalpy and internal energy to temperature and pressure.
• Key for calculating changes in enthalpy and internal energy related to combustion
• Ideal gas assumption simplifies relations
• Specific heats (Cv and Cp) values are important for calculation purposes

Ideal Gas Mixtures

• Mole fraction (X_i) and mass fraction (Y_i) in mixtures are fundamental for determining the composition of combustion gases.
• Mixture molar mass is used for extensive property calculations
• Partial pressures

Latent Heat (Enthalpy) of Vaporization

• Heat required to vaporize a unit mass of liquid at a constant pressure
• The Clausius-Clapeyron equation describes the relationship between vapor pressure and temperature for phase changes

First Law of Thermodynamics

• Conservation of energy is fundamental; expressed as the change of energy within a system equal to heat transfer and work done
• Total energy is the sum of internal energy, kinetic energy, and potential energy of the system.

Constant Pressure/Volume Adiabatic Flame Temperatures

• Heat transfer (at constant pressure/volume) within a system is used for adiabatic flame temperature calculations
• The enthalpy of the reactants (at initial state) equals absolute enthalpy of the products (at final state).
• Calculating these temperatures requires knowledge of the reactants and products compositions, and applying the energy conservation equations.

Chemical Equilibrium

• Combustion products are not always a simple mixing
• Relevant in determining the overall stoichiometry
• Identifying the conditions under which components would exist will help to determine the species present

Real Combustion Products

• Products of real combustion reactions include multiple components and species, due to dissociation of major species into minor species.

Second-Law Considerations

• Key for chemical reactions
• Adiabatic reaction vessel - Constant volume (V) where only temperature and pressure change
• Second law condition requires the entropy change to be greater than or equal to 0

Gibbs Function

• Useful for determining the equilibrium at constant T and P.
• Helps calculate equilibrium by establishing a condition to meet.
• Gibbs free energy determines the natural conditions of equilibrium.

Water-Gas Equilibrium

• Important consideration for lean or rich combustion conditions
• Describes the simultaneous presence of CO and H₂, considered as incomplete combustion products
• Relationships are used to calculate the mole fractions of products in the system

Global Mechanisms

• Simplistic models for complex reactions; useful in engineering approximations.
• Useful for combustion calculations; however, have limitations
• Single/Multiple step expressions

Nitrogen Oxide (NO) Kinetics

• NO forms, primarily, from multiple processes
• Equilibrium
• Reaction rates, and the related mechanisms need to be considered to better understand combustion reactions and products

Chemical Kinetics

• Study of chemical reaction rates.
• Reaction orders (first, second, third)
• Rate coefficients
• Mechanisms of reaction - Elementary reaction steps