燃气涡轮工程手册 Gas Turbine Engineering Handbook 1(22)

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The nomenclature of the Inward-Flow Radial Turbine is shown inFigure 1-36. These turbines are used because of lower productioncosts， in part because the nozzle blading does not require any camber or airfoil design.
.xial-Flow Turbines
The axial-flow turbine， like its counterpart the axial-flow compressor， hasflow， which enters and leaves in the axial direction. There are two types of axialturbines: (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 1-37 is a schematic of an axial-flow turbine， also depicting thedistribution of the pressure， temperature and the absolute velocity.
孔ost axial flow turbines consist of more than one stage， the front stages are usually impulse (zero reaction) and the later stages have about 50% reaction. The impulse stages produce about twice the output of a compar-able 50% reaction stage， while the efficiency of an impulse stage is less than that of a 50% reaction stage.
The high temperatures that are now available in the turbine section are due to improvements of the metallurgy of the blades in the turbines.

Figure 1-... Schematic of an axial flow tur.ine.
Development of directionally solidified blades as well as the new singlecrystalblades， with the newcoatings， and the newcooling schemes， are responsible for the increase in firing temperatures. The high-pressure ratio in the compressor also causes the cooling air used in the first stages of the turbine to be very hot. The temperatures leaving the gas turbine compressor can reach as high as 1200 0F (649 0C).Thus， the present cooling schemes need revisiting and the cooling passages are in many cases also coated. Thecooling schemes are limited in the amount of air they canuse， before there is a negating an effort in overall thermal efficiency due to an increase in the amount of air used in cooling. The rule of thumb in this area is that if you need more than 8% of the air for cooling you are loosing the advantage from the increase in the firing temperature.
The new gas turbines being designed， for the new millennium， are inves-tigating the use of steam as a cooling agent for the first and second stages of the turbines. Steam cooling is possible in the new combined cycle powerplants， which is the base of most of the new High Performance Gas Tur-bines. Steam as part of the cooling as well as part of the cycle power will be used in the new gas turbines in the combined cycle mode. The extra powerobtained by the use of steam is the cheapest孔Wj$ available. The injection of about 5% of steam by weight of air amounts to about 12% more power. The pressure of the injected steam must be at least 60 psi (4 Bar) Bar above the compressor discharge. The way steam is injected must be done very carefully so as to avoid compressor surge. These are not new concepts and have been used and demonstrated in the past. Steam cooling for example was the basis of the cooling schemes proposed by the team of United Technology and Stal-Laval in their conceptual study for the U.S. Department study on the High Turbine TemperatureTechnology Program， which was investigating Firing Temperatures of 3000 0F(1649 0C)， in the early 1980s.
Materials
The development of new materials as well as cooling schemes has seen the rapid growth of the turbine firing temperature leading to high turbine efficiencies. The stage 1 blade must withstand the most severe combinationof temperature，stress， and environment; it is generally the limiting compon-ent in the machine. Figure 1-38 shows the trend of firing temperature and blade alloy capability. Since1950， turbine bucket material temperature capability has advanced approximately 850 0F (472 0C)， approximately 20 0Fj10 0C per year. The importance of this increase can be appreciated by noting that an increase of 100 0F (56 0C) in turbine firing temperature can provide a corresponding increase of 8-13% in output and 2-4% improve-ment in simple-cycle efficiency. Advances in alloys and processing， while expensive andtime-consuming， provide significant incentives through increased power density and improved efficiency.
　
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