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

Info iconThis preview shows pages 1–4. Sign up to view the full content.

View Full Document Right Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

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 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....
View Full Document

This homework help was uploaded on 04/18/2008 for the course EML 3007 taught by Professor Chung during the Spring '08 term at University of Florida.

Page1 / 31

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

This preview shows document pages 1 - 4. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online