Glossary
to accompany
Thermodynamics: An Engineering Approach, 5th edition
by Yunus A. Çengel and Michael A. Boles24Heat transfer (heat) is defined as the form of energy that is transferred between two
systems (or a system and its surroundings) by virtue of a temperature difference. It is the
area under the process curve on a T-S diagram during an internally reversible process;
however, this area has no meaning for irreversible processes.
Heating value of a fuel is defined as the amount of heat released when a fuel is burned
completely in a steady-flow process and the products are returned to the state of the
reactants. In other words, the heating value of a fuel is equal to the absolute value of the
enthalpy of combustion of the fuel.
Helmholtz function a is defined as a = u – Ts.
Henry’s constant is the product of the total pressure of the gas mixture and the mole
fraction of a specie in the liquid phase.
Henry’s law states that the mole fraction of a weakly soluble gas in the liquid is equal to
the partial pressure of the gas outside the liquid divided by Henry’s constant.
Higher Heating value HHV of fuel is the amount of heat released when a specified
amount of fuel (usually a unit of mass) at room temperature is completely burned and the
combustion products are cooled to the room temperature when the water formed during
the combustion process is completely condensed and leaves as a liquid.
Humidity ratio (see absolute humidity)
Humidifying is the process of adding moisture to atmospheric air.
Hydrocarbon fuels are the most familiar fuels and consist primarily of hydrogen and
carbon. They are denoted by the general formula CnHm. Hydrocarbon fuels exist in all
phases, some examples being coal, gasoline, and natural gas.
Hypersonic flow occurs when a flow has a Mach number M >> 1.
Ideal cycle is an actual cycle stripped of all the internal irreversibilities and complexities.
The ideal cycle resembles the actual cycle closely but is made up totally of internally
reversible processes.
Ideal gas is a gas that obeys the ideal-gas equation of state.
Ideal-gas equation of state (or ideal-gas relation) predicts the P-v-T behavior of a gas
quite accurately within some properly selected region where Pv = RT.
Ideal gas specific heat relation is Cp = Cv + R.