Heat Analysis and Thermodynamic Effects Part 8

Tham khảo tài liệu 'heat analysis and thermodynamic effects part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | On the Optimal Allocation of the Heat Eexchangers of Irreversible Power Cycles 199 T. _ 1- t4s T3X X T 12s 41 where X is given by the equation 40 . If a non-isentropic Brayton cycle without external irreversibilities see 1-2-3-4 cycle in Fig. 3 is considered with isentropic efficiencies of the turbine and compressor m and m2 respectively and from here the following temperature relations are obtained Aragón-González et al. 2000 42 T - T T - T n 13 4 m T2S 1 T3-T4 2 t -T 1 - X Ì T2 T111 t- I T4 T3 1 - n1 1 - X I n2X Now if we consider the irreversible Brayton cycle of the Fig. 3 the temperature reservoirs are given by the constant temperatures Th and TL. In this cycle two single-pass counterflow heat eXchangers are coupled to the cold-hot side reservoirs Fig. 2 and Fig. 3 . The heat transfer between the reservoirs and the working substance can be calculated by the log mean temperature difference LMTD equation 2 . The heat transfer balances for the hot-side are equations 1 and 6 Qh Uh AhLMTDh mcp T3 - T2 Ql Ul Al LMTDh mcp T4 - 1 43 where are given by the equations 4 . The number of transfer units NTU for both sides are equation 7 N Uh Ah T3 - T2 N Ul Al T4 - 1 H mc LMTD l mc LMTD pH p L 44 Then its effectiveness equation 9 Eh 1 - eN L T - T II lí el 1 - eN1 T4 - 1 T4 - 1 45 As the heat eXchangers are counterflow the heat conductance of the hot-side cold side is UhAh UlAl and the thermal capacity rate mass and specific heat product of the working substance is CW. The heat transfer balances results to be QH CWEH TH T2 CW T3 T2 QL CWEL T4 TL CW T4 T1 46 The temperature reservoirs TH and TL are fiXed. The eXpressions for the temperatures T2 and T4 including the isentropic efficiencies ni and m2 the effectiveness eh and el and LI TL TH are obtained combining equations 41 42 and 45 V-1 - f. 1V1 - X rE X E u 1 - E V - 1 - x ni T _ LElBx eh 1 - El m x t t _ LEhX Uh 1 - Eh x X T 12 el Eh 1 - El 1h 14 el Eh 1 - eL 1h 7 And the dimensionless eXpressions q Q .