Glossary
to accompany
Thermodynamics: An Engineering Approach, 5th edition
by Yunus A. Çengel and Michael A. Boles43Second law distinction between heat transfer and work states that an energy
interaction that is accompanied by entropy transfer is heat transfer, and an energy
interaction that is not accompanied by entropy transfer is work.
Second-law efficiency ηII is the ratio of the actual thermal efficiency to the maximum
possible (reversible) thermal efficiency under the same conditions. The second-law
efficiency of various steady-flow devices can be determined from its general definition,
ηII = (exergy recovered)/(exergy supplied). The second law efficiency measures how
well the performance of actual processes approximates the performance of the
corresponding reversible processes. This enables us to compare the performance of
different devices that are designed to do the same task on the basis of their efficiencies.
The better the design, the lower the irreversibilities and the higher the second-law
efficiency.
Second law of thermodynamics (increase of entropy principle) is expressed as the
entropy of an isolated system during a process always increases or, in the limiting case of
a reversible process, remains constant. In other words, the entropy of an isolated system
never decreases. It also asserts that energy has quality as well as quantity, and actual
processes occur in the direction of decreasing quality of energy.
Seebeck effect results when two wires made from different metals are joined at both ends
(junctions), form a closed circuit, and one of the ends is heated. As a result of the applied
heat a current flows continuously in the circuit. The Seebeck effect is named in honor of
Thomas Seebeck, who made its discovery in 1821.
Sensible energy is the portion of the internal energy of a system associated with the
kinetic energies of the molecules.
Shaft work is energy transmitted by a rotating shaft and is the related to the torque T
applied to the shaft and the number of revolutions of the shaft per unit time.
Shock angle (wave angle) is the angle at which straight oblique shocks are deflected
relative to the oncoming flow as the flow comes upon a body.
Shock wave is an abrupt change over a very thin section of flow in which the flow
transitions from supersonic to subsonic flow. This abrupt change in the flow causes a
sudden drop in velocity to subsonic levels and a sudden increase in pressure. Flow
through the shock is highly irreversible; and, thus, it cannot be approximated as
isentropic.
Simple compressible system is a system in which there is the absence of electrical,
magnetic, gravitational, motion, and surface tension effects. These effects are due to
external force fields and are negligible for most engineering problems.