2600
(1316 C)
°
2400
(1204 C)
°
2200
2000
(982 C)
°
1800
1600
(760 C)
°
1400
1200
(538 C)
°
1000
importance of this increase can be appreciated by noting that an increase of 1000F (560C) in turbine firing temperature can provide a corresponding increase of 8-13% in output and 2-4% improvement in simple-cycle effi-ciency. Advances in alloys andprocessing， while expensive and time-consuming， provide significant incentives through increased power density and improved efficiency. The cooling air is bled from the compressor and isdirected to thestator， therotor， and other parts of the turbine rotor and casing to provide adequate cooling. The effect of the coolant on the aero-dynamics depends on the type of cooling involved， the temperature of thecoolant compared to the mainstream temperature， the location and directionof coolant injection， and the amount of coolant. A number of these factors are being studied experimentally in annular and two-dimensional cascades.
In high-temperature gas turbines cooling systems need to be designed for turbineblades，vanes，endwalls，shroud， and other components to meet metal temperature limits. The concepts underlying the following five basic air-cooling schemes are (Figure 9-13):
1. Convection cooling
2. Impingement cooling
3. Film cooling
4. Transpiration cooling
5. Water/Steam cooling
Until the late1960s， convection cooling was the primary means of cooling gas turbine blades; some film cooling was occasionally employed in critical regions. Film cooling in the 1980s and 1990s was used extensively. In theyear2001， steam cooling is being introduced in the production of frame type engines used in combined cycle applications. The new turbines have very high-pressure ratios and this leads to compressor air leaving at very hightemperatures， which affects their cooling capacity.
.onvection .ooling
This form of cooling is achieved by designing the cooling air to flow insidethe turbine blade orvane， and remove heat through the walls. Usually， theair flow isradial， making multiple passes through a serpentine passage from the hub to the blade tip. Convection cooling is the most widely used cooling concept in present-day gas turbines.
Impingement .ooling
In this high-intensity form of convection cooling， the cooling air is blastedon the inner surface of the airfoil by high-velocity airjets， permitting an increased amount of heat to be transferred to the cooling air from the metal surface. This cooling method can be restricted to desired sections of theairfoil to maintain even temperatures over the entire surface. Forinstance， the leading edge of a blade needs to be cooled more than the midchordsection or trailingedge， so the gas is impinged.
Film .ooling
This type of cooling is achieved by allowing the working air to form an insulating layer between the hot gas stream and the walls of the blade. This film of cooling air protects an airfoil in the same way combustor liners are protected from hot gases at very high temperatures.
Transpiration .ooling
Cooling by this method requires the coolant flow to pass through the porous wall of the blade material. The heat transfer is directly between the coolant and the hot gas. Transpiration cooling is effective at very hightemperatures， since it covers the entire blade with coolant flow.
Water.Steam .ooling
Water is passed through a number of tubes embedded in the blade. The water is emitted from the blade tips as steam to provide excellent cooling. This method keeps blade metal temperatures below 1000 0F (537.8 0C).
Steam is passed through a number of tubes embedded in the nozzle orblades of the turbine. In manycases， the steam is bled from after the HP Steam Turbine of a combined cycle power plant and returned after coolingthe gas turbineblades， where the steam gets heated in the process to the IP steam turbine. This is a very effective cooling scheme and keeps the blade metal temperature below 1250 0F (649 0C).
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本文链接地址：燃气涡轮工程手册 Gas Turbine Engineering Handbook 2(34)
