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ADVANCED THERMODYNAMICS ENGINEERING phần 4

Sự gia tăng dữ liệu ngẫu nhiên như là hai loài khác nhau được trộn lẫn. Điều này có thể được minh họa qua các ví dụ về hai chứa lân cận đoạn nhiệt khối lượng V1 và V2 tại cùng một nhiệt độ, tương ứng, có chứa | Wcv -d Ecv-T0Scv dt ZNQ41-To TRj -meVe miVi -To0 40 The absolute specific stream availability or the absolute specific flow or stream availability V is defined as V T P To er T P - To s T P h T P ke pe - To s T P . 41 where the terms Vi and Ve denote the absolute stream availabilities respectively at the inlet and exit of the control volume. They are not properties of the fluid alone and depend upon the temperature of the environment. The optimum work is obtained for the same inlet and exit states when ớcv 0. In this case Eq. 40 assumes the form Wcvopt - d Ecv-ToScv dt SN1 QRj 1-To TRj -IIỊV e miV i. 42 where the term QRj 1-To TRj represents the availability in terms of the quality of heat energy or the work potential associated with the heat transferred from the thermal energy reservoir at the temperature TR j. When the kinetic and potential energies are negligible V h - To s. 43 For ideal gases s s0- R ln P Pref where the reference state is generally assumed to be at Pref 1 bar. Therefore V V0 R T0 ln P Pref 44 and V0 h0 -T0 s0. The enthalpy h T h0 T for ideal gases since it is independent of pressure. If the exit temperature and pressure from the control volume is identical to the environmental conditions T0 and P0 i.e. the exit is said to be at a restricted dead state in that case Wcvopt wcvopt and the exit absolute stream availability at dead state may be expressed as Ve 0 eT e 0 W - T0 s e 0 0 . Note that eT e 0 h0 since ke and pe are equal to zero at dead state. 2. Lost Work Rate Irreversibility Rate Availability Loss The lost work is expressed through the lost work theorem i.e. LW I Wcvopt - Wcv Toỏcv . 45 46 The terms W_ W_ cv cv opt 0 for expansion processes and Wcv W t o for compression and electrical work input processes. The lost work is always positive for realistic processes. The availability is completely destroyed during all spontaneous processes i.e. those that occur without outside intervention that bring the system and its ambient to a dead .