燃气涡轮工程手册 Gas Turbine Engineering Handbook 2(29)

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9

Axial-Flow Turbines

Axial-flow turbines are the most widely employed turbines using a compres-sible fluid. Axial-flow turbines power most gas turbine units-except the smaller horsepower turbines-and they are more efficient than radial-inflow turbines in most operational ranges. The axial-flow turbine is also used in steamturbine design; however， there are some significant differences between the axial-flow turbine design for a gas turbine and the design for a steam turbine.
Steam turbine development preceded the gas turbine by many years.Thus， the axial-flow turbine used in gas turbines is an outgrowth of steam turbine technology. In recent years the trend in high turbine inlet temperatures in gas turbines has required various cooling schemes. These schemes are described in detail in this chapter with attention to both cooling effectiveness and aerodynamic effects. Steam turbine development has resulted in the design of two turbine types: the impulse turbine and the reaction turbine. The reaction turbine in most steam turbine designs has a 50% reaction level that has been found to be very efficient. This reaction level varies considerably in gas turbines and from hub to tip in a single-blade design.
Axial-flow turbines are now designed with a high work factor (ratio ofstage workto squareofblade speed)to obtainlowerfuelconsumptionandreducethe noise from the turbine. Lower fuel consumption and lower noise requires the designofhigherby-passratioengines.Ahighby-passratioenginerequiresmanyturbine stages to drive the high-flow， low-speed fan. Work is being conducted todevelop high-work， low-speed turbine stages that have high efficiencies.
Turbine Geometr.
The axial-flow turbine， like its counterpart the axial-flow compressor， hasflow， whichenters andleaves inthe axial direction. There aretwo types ofaxial
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turbines: (1) impulse type， and (2) reaction type. The impulse turbine has its entire enthalpy drop in the nozzle; therefore it has a very high velocity entering the rotor. The reaction turbine divides the enthalpy drop in the nozzle and therotor. Figure 9-1 is a schematic of an axial-flow turbine， also depicting thedistribution of the pressure， temperature， and the absolute velocity.
　
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