Mumbai university > Mechanical Engineering > Sem 3 > Thermodynamics

Marks: 8M

Year: Dec 2015,

i)Combustion: A chemical fuel is a substance which releases heat energy on combustion.The principal combustible elements of each fuel are carbon and hydrogen. Though sulphuris a combustible element too but its presence in the fuel is considered to be undesirable

ii)Enthalpy of formation: A combustion reaction is a particular kind of chemical reaction in which products are formed from reactants with the release or absorption of energy as heat is transferred to as from the surroundings. In some substances like hydrocarbon fuels which are many in number and complex in structure the heat of reaction or combustionmay be calculated on the basis of known values of the enthalpy of formation. Hfof the constituent of the reactants and products at the temperature T0 (reference temperature). The enthalpy of formation(Hf) is the increase in enthalpy when a compound is formed from its constituent elements in their natural form and in a standard state.

iii)enthalpy of combustion: The difference between the enthalpy of the products at a specified state and the enthalpy of the reactants at the same state for a complete reaction is called the enthalpy of reaction hR. For combustion processes, the enthalpy of reaction is usually referred to as the enthalpy of combustion hC, which represents the amount of heat released during a steady-flow combustion process when 1 kmol (or 1 kg) of fuel is burned completely at a specified temperature and pressure.

iv)Adiabatic flame temperature: In the absence of any work interactions and any changes in kinetic or potential energies, the chemical energy released during a combustion process either is lost as heat to the surroundings or is used internally to raise the temperature of the combustion products. The smaller the heat loss, the larger the temperature rise. In the limiting case of no heat loss to the surroundings (Q =0), the temperature of the products reaches a maximum, which is called the adiabatic flame or adiabatic combustion temperature of the reaction.