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Unformatted text preview: Chapter 8 Power and Refrigeration Cycles Ideal and Actual GasTurbine (Brayton) Cycles 857C In gas turbine engines a gas is compressed, and thus the compression work requirements are very large since the steadyflow work is proportional to the specific volume. 858C They are (1) isentropic compression (in a compressor), (2) P = constant heat addition, (3) isentropic expansion (in a turbine), and (4) P = constant heat rejection. 859C For fixed maximum and minimum temperatures, (a) the thermal efficiency increases with pressure ratio, (b) the net work first increases with pressure ratio, reaches a maximum, and then decreases. 860C Back work ratio is the ratio of the compressor (or pump) work input to the turbine work output. It is usually between 0.40 and 0.6 for gas turbine engines. 861C As a result of turbine and compressor inefficiencies, (a) the back work ratio increases, and (b) the thermal efficiency decreases. 844 Chapter 8 Power and Refrigeration Cycles 862E A simple ideal Brayton cycle with air as the working fluid has a pressure ratio of 10. The air temperature at the compressor exit, the back work ratio, and the thermal efficiency are to be determined. Assumptions 1 Steady operating conditions exist. 2 The airstandard assumptions are applicable. 3 Kinetic and potential energy changes are negligible. 4 Air is an ideal gas with variable specific heats. Properties The properties of air are given in Table A21E. Analysis ( a ) Noting that process 12 is isentropic, T h P r 1 1 1 1 2147 = → = = 520 R 124.27 Btu / lbm . ( 29 ( 29 Btu/lbm 240.11 147 . 12 2147 . 1 10 2 2 1 2 1 2 = = → = = = h T P P P P r r R 996.5 ( b ) Process 34 is isentropic, and thus ( 29 Btu/lbm 38.88 2 83 . 265 71 . 504 Btu/lbm 115.84 27 . 124 11 . 240 Btu/lbm 265.83 4 . 17 . 174 10 1 . 174 Btu/lbm 504.71 R 2000 4 3 , 1 2 , 4 3 4 3 3 3 4 3 = = = = = = = → = = = = = → = h h w h h w h P P P P P h T out T in C r r r Then the backwork ratio becomes r w w bw C in T out = = = , , 115.84 Btu / lbm 238.88 Btu / lbm 48.5% ( c ) q h h w w w w q in net out T out C in th net out in = = = = = = = = = 3 2 504 71 24011 238 88 115 84 . . . . , , , , 264.60 Btu / lbm 123.04 Btu / lbm 123.04 Btu / lbm 264.60 Btu / lbm η 46.5% 845 s T 1 2 4 3 q in q out 2000 R 520 R Chapter 8 Power and Refrigeration Cycles 863 [ Also solved by EES on enclosed CD ] A simple Brayton cycle with air as the working fluid has a pressure ratio of 8. The air temperature at the turbine exit, the net work output, and the thermal efficiency are to be determined. Assumptions 1 Steady operating conditions exist. 2 The airstandard assumptions are applicable. 3 Kinetic and potential energy changes are negligible. 4 Air is an ideal gas with variable specific heats....
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 Spring '08
 Chung
 Gas turbine, potential energy changes, Qin, Wnet, Refrigeration Cycles

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