.ultiple Small-.ole .esign
With this particular design， primary cooling is achieved by film cooling with cold air injected through small holes over the airfoil surface (Figure 9-21). The temperature distribution is shown in Figure 9-22.
These holes are considerably larger than holes formed with porous mesh for transpiration cooling.Also， because of their largersize， they are lesssusceptible to clogging by oxidation. In thisdesign， the shell is supported by cross ribs and is capable of supporting itself without a strut under engine operating conditions.
This design has the highest creep life next to a transpiration-cooled design， and it has the best strain distribution between leading and trailing edges. It is the closest to optimum.
Water-.ooled Turbine .lades
This design has a number of tubes embedded inside the turbine blade toprovide channels for the water (Figure 9-23). In mostcases， these tubes are constructed from copper for good heat-transfer conditions. Thewater，which is converted to steam by the time it reaches the blade tips， is then
Figure 9-21. .ultiple small hole transpiration-cooled blade
Figure 9-22. Temperature distribution for a multiple small-hole design 0F (cooled)
injected into the flow stream. These blades are presently in the experimental stage. They hold great promise for the turbine of the future in which turbine inlet temperatures of 3000 0F (1648.8 0C) are possible. This type of cooling should keep blade metal temperatures below 1000 0F (537.8 0C) so that there will be no hot-corrosion problems.
Steam-.ooled Turbine .lades
This design has a number of tubes embedded inside the turbine blade to provide channels for steam. In most cases these tubes are constructed from copper for good heat-transfer conditions. Steam injection is becoming the prime source of cooling for gas turbines in a combined cycle application. Thesteam， which is extracted from the exit of the HPTurbine， is sent through thenozzleblades， where the steam is heated， and the blade metal temperature decreased. The steam is then injected into the flow stream entering the IP steam turbine. This increases the overall efficiency of the combined cycle.
In the case of the rotatingblades， thesteam， after it is used in the coolingof theblades， is returned through a series of specially designed slip rings to the steam flow entering the IP steam turbine. Steam cooling in combined cycle power plants holds great promise for the turbines of the future in which turbine inlet temperatures of 3000 0F (1649 0C) are possible. This type of cooling should keep blade metal temperatures below 1200 0F (649 0C) so that hot-corrosion problems will be minimized. It also will help increase the efficiency of the total combined cycle power plant by between 1% and 3%.
An evaluation of the six different blade designs is shown in Table 9-1.
Table 9-1 Summar of .reep Life .xperiments
Time to 1. .reep Strain (.rs.
.lade .ooling.esign .ased on Initial .ased on Average .onditions .onditions
Strut design 243047，900 Film convection 18646，700 Transpiration 2530 Infinite Multiple small-hole 480033，500 Water cooled 150 Infinite Steam cooled 15035，000
.ooled-Turbine Aerod.namics
The injection of coolant air in the turbine rotor or stator causes a slightdecrease in turbine efficiency; however， the higher turbine inlet temperatureusually makes up for the loss of the turbine component efficiency， giving an overall increase in cycle efficiency. Tests by NASA on three different types of cooled stator blades were conducted on a specially built 30-inch turbine cold-air test facility. The outer shell profile of all three blade types was thesame， as seen in Figure 9-24.
Total pressure surveys were made downstream of the stators in both the radial and circumferential directions to determine the effect of coolant on stator losses. The wake traces for the stator with discrete holes and the stator with trailing edge slots show that there is a considerable difference in total pressure loss patterns as a function of the type of cooling and the amount ofcooling air supplied. As the coolant flow for the porous blades increases， the disturbance to the flow pattern and the wake thickness increases. Conse-quently， the losses increase. In ablade with trailing edge slots， the lossinitially starts to increase with coolant flow as the wake thickens.However，as the coolant flow is increased， it tends to energize the wake and reduce losses. For ahigher coolantflow， the coolant pressuresmust behigher， resulting in an energization of the flow.
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