By Lee S Langston, professor emeritus, UConn
The most efficient powerplants ever produced are now entering service. 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. This makes them the most efficient heat engines yet perfected by engineers, with all three OEMs striving to reach 65% in coming years.
The gas turbines are state-of-the-art. They make extensive use of turbine blade and vane cooling, thereby allowing the high turbine inlet temperatures required to achieve record-breaking efficiencies. Cooling air drawn from the compressor gas path (as much as 20%) is used to protect hot section parts in both combustors and turbines.
Turbine cooling details. Thermal efficiency increases with the temperature of the gas exiting the combustor and entering the turbine—the work-producing component. 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. (The turbine designer must accommodate for excursions above these nominal temperatures, because of combustor hot streaks, etc.)
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. These so-called superalloys, when conventionally vacuum cast, soften and melt at temperatures between about 2200F and 2500F.
This means blades and vanes closest to the combustor can be operating at gas-path temperatures far exceeding their melting points. To endure temperatures of 500F to 1400F above melting, they must be cooled to acceptable service temperature (typically eight- to nine-tenths of their lower melting point) to maintain integrity (Fig 1).
Thus, turbine airfoils subjected to the hottest gas flows take the form of elaborate superalloy investment castings to accommodate the intricate internal passages and surface hole patterns necessary to channel and direct cooling air within and over exterior surfaces of the superalloy airfoil structure. By turbine design conventions, internal airfoil cooling is usually termed convective cooling, while the protective effect of cooling air over external airfoil surfaces is called film cooling.
Turbine cooling guide. The cooling of turbine blades and vanes goes back to the origin of the gas turbine in early 1940s. During WWII, Germany, faced with increasingly difficult times importing strategic materials such as nickel, used turbine blade and vane cooling extensively in its jet engines. Since then, turbine cooling technology and practice has become a very large segment of the gas turbine world.
In June 2017 at ASME’s Turbo Expo in Charlotte, the biennial International Gas Turbine Institute Scholar Lecture was given by Dr Ronald Bunker. A past IGTI chair, the recently retired GE gas turbine heat-transfer expert, presented his scholar paper, “Evolution of Turbine Cooling.”
Bunker’s paper can now serve as an up-to-date overview of turbine cooling, complete with a listing of 123 references (to order “Evolution of Turbine Cooling” online, visit www.asme.org/events/turbo-expo). His 26-page paper treats the evolution of turbine cooling in three broad aspects, including background development, the current state-of-the-art, and prospects for the future. This is indeed a seminal work by an expert, reflecting his direct research and OEM design experience over a period of several decades.
The author posits that the fundamental aim of a turbine heat-transfer designer is to obtain the highest overall cooling effectiveness for a blade or vane, with the lowest possible penalty on thermodynamic performance. In Fig 2, this is shown in the form of notional (that is, expressing a notion) cooling-technology curves.
On the Fig 2 ordinate, the cooling effectiveness of a turbine blade or vane is made up of its bulk metal temperature (Tm), the hot-gas-path temperature (Tg), and the coolant fluid temperature (Tc). Note that a value of 1.0 would represent “perfect” cooling.
The Fig 2 abscissa is the heat-load parameter, WcCp/UAg, where U is an overall hot-gas-path convective and radiation heat-transfer coefficient, Ag the external surface area, Wc the coolant flow rate, and Cp the thermal-capacity coefficient of the coolant.
Bunker points out that in the last 50 years, advances have led to an overall increase in turbine and vane cooling effectiveness from 0.1 to 0.7, as shown in Fig 2. It started with convection only (for example, the convectively cooled turbine airfoils of the German jet engines of WWII), and has progressed with film cooling, thermal barrier coatings (TBCs), and new materials and architectures—for example, the directionally solidified and single crystal turbine blades, which entered service in the 1970s – 1990s.
Fig 3 shows five conventional investment-casting cooling geometries in use today. They range from convection only (that is, internal-passage heat transfer only), to film cooling, and the combination of both. Bunker’s paper offers a detailed discussion of each. Typical state-of-the-art cooling schemes for high-pressure-turbine vanes and blades are illustrated in Figs 4 and 5, respectively.
Outlook. Turbine blade and vane cooling play a key role in making gas turbines for electric-power service more efficient and reliable. These cooling technologies, especially film cooling, will continue to advance gas turbine efficiency and life in the future. CCJ