Page 136 - Mechanical Engineers' Handbook (Volume 4)
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3 Entropy Generation Minimization  125

                           The engineer must relate the degree of thermodynamic nonideality of the design to the
                           physical characteristics of the system, namely, to finite dimensions, shapes, materials, finite
                           speeds, and finite-time intervals of operation. For this, the engineer must rely on heat transfer
                           and fluid mechanics principles, in addition to thermodynamics. Only by varying one or more
                           of the physical characteristics of the system can the engineer bring the design closer to the
                           operation characterized by minimum entropy generation subject to finite-size and finite-time
                           constraints.
                              The modeling and optimization progress made in EGM is illustrated by some of the
                           simplest and most fundamental results of the method, which are reviewed in the following
                           sections. The structure of the EGM field is summarized in Fig. 4 by showing on the vertical
                           the expanding list of applications. On the horizontal, we see the two modeling approaches
                           that are being used. One approach is to focus from the start on the total system, to divide
                           the system into compartments that account for one or more of the irreversibility mechanisms,
                           and to declare the rest of the system irreversibility-free. In this approach, success depends
                           on the modeler’s intuition, because the assumed compartments do not always correspond to
                           the pieces of hardware of the real system.
                              In the alternative approach (from the right in Fig. 4), modeling begins with dividing the
                           system into its real components, and recognizing that each component may contain large
                           numbers of one or more elemental features. The approach is to minimize S gen  in a funda-
                           mental way at each level, starting from the simple and proceeding toward the complex.
                           Important to note is that when a component or elemental feature is imagined separately from
                           the larger system, the quantities assumed specified at the points of separation act as con-





































                            Figure 4 Approaches and applications of the method of entropy generation minimization (EGM).
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