Irreversibility Isentropic process

For the case when power production is not implemented, the HP steam is throttled to the pressure of the MP header through a let-down station, which is essentially an irreversible, isentropic process through a valve. The superheated steam is then desuperheated at the process user. 

The fact that shock waves continue to steepen until dissipative mechanisms take over means that entropy is generated by the conversion of mechanical energy to heat, so the process is irreversible. By contrast, in a fluid, rarefactions do not usually involve significant energy dissipation, so they can be regarded as reversible, or isentropic, processes. There are circumstances, however, such as in materials with elastic-plastic response, in which plastic deformation during the release process dissipates energy in an irreversible fashion, and the expansion wave is therefore not isentropic. 

Figure 15.5 shows the ideal open cycle for the gas turbine that is based on the Brayton Cycle. By assuming that the chemical energy released on combustion is equivalent to a transfer of heat at constant pressure to a working fluid of constant specific heat, this simplified approach allows the actual process to be compared with the ideal, and is represented in Figure 15.5 by a broken line. The processes for compression 1-2 and expansion 3-4 are irreversible adiabatic and differ, as shown from the ideal isentropic processes between the same pressures P and P2 -... 

In a system undergoing a reversible adiabatic process, there is no change in its entropy. This is so because by definition, no heat is absorbed in such a process. A reversible adiabatic process, therefore, proceeds at constant entropy and may be described as isentropic. The entropy, however, is not constant in an irreversible adiabatic process. 

The ideal finite-time Rankine cycle and its T-s diagram are shown in Figs. 7.14 and 7.15, respectively. The cycle is an endoreversible cycle that consists of two isentropic processes and two isobaric heat-transfer processes. The cycle exchanges heats with its surroundings in the two isobaric external irreversible heat-transfer processes. The heat source and heat sink are infinitely large. Therefore, the temperature of the heat source and heat sink are unchanged during the heat-transfer processes. 

ADIABATIC PROCESS. Any thermodynamic process, reversible or irreversible, which takes place in a system without the exchange of heat with the surroundings. When the process is also reversible, it is called isentropic, because, then the entropy of the system remains constant at every step of the process, fin older usage, isentropic processes were called simply adiabatic, or quasistatic adiabatic the distinction between adiabatic and isentropic processes was not always sharply drawn.)... 

No information can be deduced about the entropy variation in the intermediate range of reaction rates however, the process is not isentropic because equation n. C. 2. does not go to zero. Considerarions from irreversible thermodynamics show that the entropy must always rise in a closed thermodynamic system when irreversible reaction processes take place (25). 

International Practical Temperature Scale, 5-6 Irreversibility, 40-41, 554-555 and entropy changes, 155-157, 554 Isentropic process, 153-155, 187-189, 223-231, 235-240... 

A third objective is similarly obvious. If compression and expansion processes can attain more isentropic conditions, then the cycle widening due to irreversibility is decreased, cr moves nearer to unity and the thermal efficiency increases (for a given t). Cycle modifications or innovations are mainly aimed at increasing (by increasing or decreasing a)-... 

At the instant a pressure vessel ruptures, pressure at the contact surface is given by Eq. (6.3.22). The further development of pressure at the contact surface can only be evaluated numerically. However, the actual p-V process can be adequately approximated by the dashed curve in Figure 6.12. In this process, the constant-pressure segment represents irreversible expansion against an equilibrium counterpressure P3 until the gas reaches a volume V3. This is followed by an isentropic expansion to the end-state pressure Pq. For this process, the point (p, V3) is not on the isentrope which emanates from point (p, V,), since the first phase of the expansion process is irreversible. Adamczyk calculates point (p, V3) from the conservation of energy law and finds... 

To close this chapter we emphasize that Hie statistical mechanical definition of macroscopic parameters such as temperature and entropy are well designed to describe isentropic equilibrium systems, but are not immediately applicable to the discussion of transport processes where irreversible entropy increase is an essential feature. A macroscopic system through which heat is flowing does not possess a single tempera-... 

The vapor-compression cycle incorporating an expansion valve is shown in Fig. 9.1h, where line A- 1 represents the constant-enthalpy throttling process. Line 2 + 3, representing an actual compression process, slopes in the direction of increasing entropy, reflecting the irreversibility inherent in the process. The dashed line 2 - 3 is the path of isentropic compression (see Fig. 7.6). For this cycle, the coefficient of performance is simply... 

A reversible adiabatic process is isentropic, meaning that a substance will have the same entropy values at the beginning and end of the process. Systems such as pumps, turbines, nozzles, and diffusers are nearly adiabatic operations and are more efficient when irreversibilities, such as friction, are reduced, and hence operated under isentropic conditions. 

The above relate to Figure A.2, which shows an enhanced version of Figure A.l, designed to allow operation of the cell at any selected high temperature and pressure. Isentropic circulators are incorporated to generate the increased conditions. The cell generates heat which is passed without temperature difference to a Carnot cycle to generate power, a reversible process free from the irreversibility of combustion. 

The sliaft work Wj (isentropic) is the maximum tliat can be obtained from an adiabatic hirbine with given inlet conditions and given discharge pressure. Actual turbines produce less work, because the actual expansion process is irreversible. We therefore define a turbine... 

The HS diagram of Fig. 4-2 compares the path of an actual expansion in a turbine with that of an isentropic expansion for the same intake conditions and the same discharge pressure. The isentropic path is the dashed vertical line from point 1 at intake pressure Pi to point 2 at P2. The irreversible path (solid line) starts at point 1 and terminates at point 2 on the isobar for P2. The process is adiabatic, and irreversibilities cause the path to be directed toward increasing entropy. The greater the irreversiblity, the farther point 2 hes to the right on the P2 isobar, and the lower the value of q. 

Shock compression is an irreversible adiabatic compression that heats the material behind the front [1]. The temperature rise can be divided into two parts. The minimum temperature rise would result if shock compression were slow enough that it approximated a reversible adiabatic compression from Vq to Vj. This process, where zlS = 0, is also called an isentropic compression [1]. Due to the irreversible nature of shock compression, an additional rise is produced that results from the entropy increase zl5,vr across the shock front. This additional rise depends on the detailed nature of the shock front. Shock compression is hotter than isentropic compression. The new temperature Tj cannot be determined from the Hugoniot-Rankine equations alone. Some kind of equation of state (EOS) is also needed (for state-of-the art examples see Refs. [12-14]), and the usual choice is a Griineisen equation of state. The temperature Ti is given by [1],... 

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