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FTFS Chap08 P057

# FTFS Chap08 P057 - Chapter 8 Power and Refrigeration Cycles...

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Unformatted text preview: Chapter 8 Power and Refrigeration Cycles Ideal and Actual Gas-Turbine (Brayton) Cycles 8-57C In gas turbine engines a gas is compressed, and thus the compression work requirements are very large since the steady-flow work is proportional to the specific volume. 8-58C 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. 8-59C 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. 8-60C 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. 8-61C As a result of turbine and compressor inefficiencies, (a) the back work ratio increases, and (b) the thermal efficiency decreases. 8-44 Chapter 8 Power and Refrigeration Cycles 8-62E 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 air-standard 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 A-21E. Analysis ( a ) Noting that process 1-2 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 3-4 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 back-work 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% 8-45 s T 1 2 4 3 q in q out 2000 R 520 R Chapter 8 Power and Refrigeration Cycles 8-63 [ 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 air-standard 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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FTFS Chap08 P057 - Chapter 8 Power and Refrigeration Cycles...

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