Thermodynamics

Chapter 9 | 489FIGURE 9–4An automotive engine with thecombustion chamber exposed.Courtesy of General Motorssame temperature limits. However, it is still considerably higher than thethermal efficiency of an actual cycle because of the idealizations utilized(Fig. 9–4).The idealizations and simplifications commonly employed in the analysisof power cycles can be summarized as follows:1. The cycle does not involve any friction. Therefore, the working fluiddoes not experience any pressure drop as it flows in pipes or devicessuch as heat exchangers.2. All expansion and compression processes take place in a quasiequilibriummanner.3. The pipes connecting the various components of a system are well insulated,and heat transfer through them is negligible.Neglecting the changes in kinetic and potential energies of the workingfluid is another commonly utilized simplification in the analysis of powercycles. This is a reasonable assumption since in devices that involve shaftwork, such as turbines, compressors, and pumps, the kinetic and potentialenergy terms are usually very small relative to the other terms in the energyequation. Fluid velocities encountered in devices such as condensers, boilers,and mixing chambers are typically low, and the fluid streams experience littlechange in their velocities, again making kinetic energy changes negligible.The only devices where the changes in kinetic energy are significant are thenozzles and diffusers, which are specifically designed to create large changesin velocity.In the preceding chapters, property diagrams such as the P-v and T-s diagramshave served as valuable aids in the analysis of thermodynamicprocesses. On both the P-v and T-s diagrams, the area enclosed by theprocess curves of a cycle represents the net work produced during the cycle(Fig. 9–5), which is also equivalent to the net heat transfer for that cycle.