Glossary
to accompany
Thermodynamics: An Engineering Approach, 5th edition
by Yunus A. Çengel and Michael A. Boles8Closed system consists of a fixed amount of mass (control mass), and no mass can cross
its boundary. But energy, in the form of heat or work, can cross the boundary.
Closed system exergy (nonflow exergy) is the reversible work that could be done by a
closed system undergoing a reversible process to equilibrium with its surroundings. For a
mass m the exergy is X = (U - U 0 ) + P 0 (V - V 0 ) - T 0 (S - S 0 ) + mV^2
r
/2 + mgz. On a unitmass basis, the exergy of a closed system is expressed as φ = (u - u 0 ) + P 0 (v - v 0 ) - T 0 (s -
s 0 ) + V^2
r
/2 + gz where u 0 , v 0 , and s 0 are the properties of the system evaluated at the deadstate. Note that the exergy of a system is zero at the dead state since u = u 0 , v = v 0 , and s
= s 0 at that state. The exergy change of a closed system during a process is simply the
difference between the final and initial exergies of the system.
Coefficient of performance COP is the measure of performance of refrigerators and
heat pumps. It is expressed in terms of the desired result for each device (heat absorbed
from the refrigerated space for the refrigerator or heat added to the hot space by the heat
pump) divided by the input, the energy expended to accomplish the energy transfer
(usually work input).
Cogeneration is the production of more than one useful form of energy (such as process
heat and electric power) from the same energy source.
Cold-air-standard assumption combines the air-standard assumptions with the
assumption that the air has constant specific heats whose values are determined at room
temperature (25°C, or 77°F).
Combined cycle (see combined gas–vapor cycle)
Combined efficiency (see overall efficiency)
Combined gas–vapor cycle, or just the combined cycle, is the gas-turbine (Brayton)
cycle topping a steam-turbine (Rankine) cycle, which has a higher thermal efficiency
than either of the cycles executed individually.
Combustion is a chemical reaction during which a fuel is oxidized and a large quantity
of energy is released.
Combustion air is dry air which can be approximated as 21 percent oxygen and 79
percent nitrogen by mole numbers. Therefore, each mole of oxygen entering a
combustion chamber will be accompanied by 0.79/0.21 = 3.76 mol of nitrogen. To supply
one mole of oxygen to a combustion process, 4.76 mol of combustion air are required.