and the cooling passages are in many cases also coated. The cooling schemesare 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 turbines.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 power obtained by the use of steam is the cheapest MW/$ available. The injection of about 5% of steam by weight of air amounts to about 12% more power. The pressure of the injected be at least 40 Bar above the compressordischarge.Thewaysteamsteamisinjemustcted 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 theHigh Turbine Temperature Technology Program， which was investigating Firing Temperatures of 30000F (16490C)， in the early 1980s.
There are three state points within a turbine that are important whenanalyzing the flow. They are located at the nozzle entrance， the rotor entrance， and at the rotor exit. Fluid velocity is an important variable governing the flow and energy transfer within a turbine. The absolute velocity (V) is the fluid velocity relative to some stationary point. Absolute velocity is important when analyzing the flow across a stationary blade such as a nozzle. When consider-ing the flow across a rotating element or rotor blade， the relative velocity W isimportant. Vectorially， the relative velocity is defined
W二 V-u(9-1)
where u is the tangential velocity of the blade.
This relationship is shown in Figure 9-2. The subscript z used in Figure 9-2denotes the axial velocity， while e denotes the tangential component.
Two angles are defined in Figure 9-2. The first angle is the air angle α， which is defined with respect to the tangential direction. The air angle αrepresents the direction of the flow leaving the nozzle. In therotor， the air angle α represents the angle of the absolute velocity leaving the rotor. The blade angle . is the angle the relative velocity makes with the tangential direction. It is the angle of the rotor blade under ideal conditions (no incidence angle).
.egree of Reaction
The degree of reaction in an axial-flow turbine is the ratio of change in the static enthalpy to the change in total enthalpy
R二h1 -h4 (9-2)h01 -h04
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