This means blades and vanes closest to the combustor can be operating at gas-path temperatures far exceeding their melting points. These so-called superalloys, when conventionally vacuum cast, soften and melt at temperatures between about 2200F and 2500F. In the highest-temperature regions of the turbine, special high-melting-point nickel-base-alloy cast blades and vanes are used because of their ability to retain strength and resist hot corrosion at extreme temperatures. (The turbine designer must accommodate for excursions above these nominal temperatures, because of combustor hot streaks, etc.) Turbine inlet temperatures for modern high-performance commercial jet engines can reach 3000F, while gas turbines in electric-power service typically operate at 2700F or lower and military jets in the neighborhood of 3600F. Thermal efficiency increases with the temperature of the gas exiting the combustor and entering the turbine-the work-producing component. Cooling air drawn from the compressor gas path (as much as 20%) is used to protect hot section parts in both combustors and turbines. They make extensive use of turbine blade and vane cooling, thereby allowing the high turbine inlet temperatures required to achieve record-breaking efficiencies. The gas turbines are state-of-the-art, says Lee S Langston, professor emeritus, UConn, an ASME Life Fellow who joined Pratt & Whitney Aircraft as a research engineer after receiving his PhD from Stanford University in 1964. This makes them the most efficient heat engines yet perfected by engineers, with all three OEMs striving to reach 65% in coming years. These latest and largest combined cycles-powered by GE’s 7HA, Siemens’ SGT6-8000H, and Mitsubishi Hitachi’s M501J engines-are all clocking in at 62% – 63% thermal efficiency. The most efficient powerplants ever produced are now entering service.
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