十端 Demonstration Take off turbine Engine TET temperatur wooL D Materid capablity cast alloys Convent ly cast alloys 19401950196019601980199020002010 A gure A-8: Rolls-Royce high temperature Fi Figure A-9: Turbine blade coolin technology [Pratt WhitneyI For a given level of turbine technology (in other words given maximum temperature)a design question is what should the compressor tr be? What criterion should be used to decide this? Maximum thermal efficiency? Maximum work? We examine this issue below. Cycle with Th→T。 P Cycle with lower T T=T Figure A-10: Efficiency and work of two Brayton cycle engines The problem is posed in Figure A-10, which shows two Brayton cycles. For maximum efficiency we would like TR as high as possible. This means that the compressor exit temperature approaches the turbine entry temperature. The net work will be less than the heat received; as Tb T the heat received approaches zero and so does the net work The net work in the cycle can also be expressed as fPdv, evaluated in traversing the cycle. This is the area enclosed by the curves, which is seen to approach zero as T-T1A-9 Figure A-8: Rolls-Royce high temperature Figure A-9: Turbine blade cooling technology technology [Pratt & Whitney] For a given level of turbine technology (in other words given maximum temperature) a design question is what should the compressor TR be? What criterion should be used to decide this? Maximum thermal efficiency? Maximum work? We examine this issue below. Figure A-10: Efficiency and work of two Brayton cycle engines The problem is posed in Figure A-10, which shows two Brayton cycles. For maximum efficiency we would like TR as high as possible. This means that the compressor exit temperature approaches the turbine entry temperature. The net work will be less than the heat received; as T T b c → the heat received approaches zero and so does the net work. The net work in the cycle can also be expressed as ∫ Pdv, evaluated in traversing the cycle. This is the area enclosed by the curves, which is seen to approach zero as T T b c → . V T = Tatm = Ta T = Tc Cycle with Tb → Tc Tb2 Tb1 Patm P Cycle with lower Tb