Page 131 - Mechanical Engineers' Handbook (Volume 4)

120 Exergy Analysis, Entropy Generation Minimization, and Constructal Theory

SUBSCRIPTS
() B base
() C collector
() C Carnot
() H high
() L low
() m melting
() max maximum
() min minimum
() opt optimal
() p pump
() rev reversible
() t turbine
() 0 environment
() free stream

2 EXERGY ANALYSIS
Figure 1 shows the general features of an open thermodynamic system that can interact
˙
thermally (Q ) and mechanically (P dV/dt) with the atmospheric temperature and pressure
0
0
reservoir (T , P ). The system may have any number of inlet and outlet ports, even though
0
0
only two such ports are illustrated. At a certain point in time, the system may be in com-
munication with any number of additional temperature reservoirs (T , ..., T ), experiencing
1
n
˙
˙
˙
the instantaneous heat interactions, Q ,..., Q n . The work transfer rate W represents all the
1
possible modes of work transfer: the work done on the atmosphere (P dV/dt) and the
0
remaining (useful, deliverable) portions such as PdV/dt, shaft work, shear work, electrical
work, and magnetic work. The useful part is known as available work (or simply exergy)
or, on a unit time basis,

Figure 1 Open system in thermal and mechanical communication with the ambient. (From A. Bejan,
Advanced Engineering Thermodynamics. 1997 John Wiley & Sons, Inc. Reprinted by permission.)