Читать книгу Fundamentals of Heat Engines - Jamil Ghojel - Страница 59

Entropy is a thermodynamic property that is a measure of process irreversibility or energy degradation and is defined as

(1.88)

where

dS: total entropy change

ds: specific entropy change

dQ: heat transferred reversibly

T: absolute temperature at which heat is transferred

 If heat is added to a system, ds will be positive (entropy increases).

 If heat is removed from a system, ds will be negative (entropy decreases)

 If ds = 0 during a process, the process is isentropic. The frictionless adiabatic process is an isentropic process.

A reversible process occurs when both the system and the surroundings are returned to their original conditions after the process and reverse process have been carried out. Processes in nature are irreversible, however, because reversal always causes some change to occur in the system and/or surroundings. Factors causing irreversibility include:

 Friction

 Unrestricted expansion

 Heat transfer through a finite temperature difference

 Mixing of two different gases

 Chemical reactions