This article discusses how advancement in turbine cooling techniques has helped enhancing the performance and endurance of turbines. Gas turbine thermal efficiencies increase with higher temperatures of the gas flow exiting the combustor and entering the work-producing component—the turbine. 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 the last 50 years, advances have led to an overall increase in turbine and vane cooling effectiveness, from 0.1 to 0.7. It started with convection only and has progressed with film cooling, thermal barrier coatings, and new materials and architectures. Temperature excesses in turbines are now as high as 1400°F (778°C) above alloy melting points. Film cooling is the key to attaining these levels, and to increasing them in the future, for yet higher gas turbine efficiencies.
Large Brayton-Rankine combined cycle electrical power plants are now at record setting thermal efficiencies of 62%, the most efficient heat engines yet perfected by engineers. These have been made possible by modern efficient (as high as 45%) electric power gas turbines whose exit gas path temperatures have been increased enough to allow ample high pressure steam production for the Rankine cycle. Aviation jet engine advances have provided much of the leading edge technology that underlies this power plant revolution.
Gas turbine thermal efficiencies increase with higher temperatures of the gas flow exiting the combustor and entering the work-producing component - the turbine. Turbine inlet temperatures in the gas path of modern highperformance commercial jet engines can reach 3000°F (1649°C), while electric power gas turbines typically operate at 2700°F (1482°C) or lower, and military jets can be in the 3600°F (1982°C) range. (The turbine designer must accommodate for excursions above these nominal temperatures, due to 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 2200°F (1204°C) and 2500°F (1371°C).
This means blades and vanes closest to the combustor can be operating in gas-path temperatures far exceeding their melting point. To endure these temperature excesses of 500 to 1400 F° (278 to 778 C°), they must be cooled to acceptable service temperature (typically eight-to-nine-tenths of their ower melting point) to maintain integrity.
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 (bled from the compressor) 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”.
A New Turbine Cooling Guide
This past June, at ASME Turbo Expo ’17 in Charlotte, the biennial International Gas Turbine Institute Scholar Lecture was given by Ronald Bunker. Ron, a Past IGTI Chair and recently retired General Electric gas turbine heat transfer expert, presented his scholar paper “Evolution of Turbine Cooling” .
Dr. Bunker’s paper can now serve as an up-to-date overview of turbine cooling, complete with a listing of 123 references. 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 design OEM 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. 1 (taken from his Fig. 3 ) this is shown in the form of notional (i.e., expressing a notion) cooling technology curves.
On the Fig. 1 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). (A value of 1.0 would represent “perfect” cooling.)
The Fig. 1 abscissa is the heat load parameter which is the external airfoil heat loading (UAg , where U is an overall hot gas path convective and radiation heat transfer coefficient and Ag is an external surface area), divided into the coolant flow rate (Wc) and the thermal capacity coefficient of the coolant fluid (Cp).
Bunker points out that in the last 50 years, advances have led to an overall increase in turbine and vane cooling effectiveness shown in Fig. 1, from 0.1 to 0.7. It started with convection only (e.g., 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 (e.g., directionally solidified and single crystal turbine blades, which entered service in the 1970-90s).
In Fig. 2 (taken from his Fig. 4 ) are five conventional investment casting cooling geometries in use today. They range from convection only (i.e. internal passage heat transfer only), to film cooling and the combination of both. The reader is referred to Bunker’s paper for a detailed discussion of each.
An Evolution Theme
In writing his ASME IGTI Scholar paper covering this fascinatingly important topic of turbine cooling, Bunker used evolution as a theme. He wrote that the evolution of turbine cooling (since the gas turbine’s invention in 1939) is loosely analogous to that of the Darwinian theory of evolution for animals, starting from highly simplistic forms and progressing to increasingly more complex designs having greater capabilities. Let me continue his evolution theme to end here, with a view of the importance of film cooling for present and future gas turbine technology.
Lieberman [2 reviews current research in human evolutionary biology on the fundamental role our unique sweat cooling system has played in human evolution. Human sweating probably emerged sometime around 2 million years ago, in order to help meat-eating hominids compete with other carnivores. Eating meat led to larger body and brain size than chimpanzees. Sweat cooling allowed our ancestors to forage safely during peak heat in a hot, dry African climate, when heat-dump limited predators were unable to hunt them. Evaporative cooling also allowed persistence hunting, where hominids (well before the bow and arrow) could wear down prey, not by superior speed, but by a sustained pace causing hyperthermic state in the hunted.
An analogy can be made between the role of sweat cooling in human evolution to that of the evolution of gas turbine performance and endurance enhancement due to film cooling. Earlier, we saw what temperature excesses are now in turbines - as high as 1400 F° (778 C°) above alloy melting points. Film cooling is key to attaining these levels, and to increasing them in the future, for yet higher gas turbine efficiencies. (Bunker  discusses “micro cooling”, an advance form of film cooling, akin to sweat cooling.)
Four centuries ago, Ben Jonson wrote: “Who casts to write a living line, must sweat.” Today, a turbine designer who casts to perfect a more efficient gas turbine, must film cool.