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Gems of Turbine Efficiency PUBLIC ACCESS

[+] Author Notes
Lee S. Langston

Professor Emeritus, Mechanical Engineering, University of Connecticut

Mechanical Engineering 136(09), 76-77 (Sep 01, 2014) (2 pages) Paper No: ME-14-SEP9; doi: 10.1115/9.2014-Sep-9

This article discusses the use of turbine single-crystal blades in gas turbines. Single-crystal turbine blades were first used in military engines on Pratt’s F100 engine, which powered the F16 and F15 fighter aircrafts. Their first commercial use was on P&WA’s JT9D-7R4 engine, which received FAA certification in 1982, powering Boeing’s 767 and the Airbus A310. In jet engines, single-crystal turbine airfoils have proven to have as much as nine times more relative life in terms of creep strength and thermal fatigue resistance and over three times more relative life for corrosion resistance, when compared to equiaxed crystal counterparts. Modern high turbine inlet temperature jet engines with long life would not be possible without the use of single-crystal turbine airfoils. By eliminating grain boundaries, single-crystal airfoils have longer thermal and fatigue life, are more corrosion resistant, can be cast with thinner walls, and have a higher melting temperature. These improvements all contribute to higher gas turbine thermal efficiencies.

This year marks the diamond jubilee for the gas turbine. Seventy five years ago, in 1939, the first jet engine aircraft flew, and the first gas turbine powering an electric generator was successfully tested.

This is also the approximate golden jubilee for the invention of turbine single crystal blades for use in gas turbines. These gas turbine “Crown Jewels” [1] were being developed and perfected at Pratt & Whitney Aircraft, when I joined the company, fresh out of graduate school, in 1964. Their first real engine test was in 1967-68 on Pratt's J58 engine, which powered the SR-71, Lockheed's supersonic reconnaissance aircraft. At that time, single crystal technology wasn’t yet ready for this early application. Later, single crystal (SX) turbine blades were first used in military engines on Pratt's F100 engine, which powered the F16 and F15 fighter aircraft. Their first commercial use was on P&WA's JT9D-7R4 engine, which received FAA certification in 1982, powering Boeing's 767 and the Airbus A310 [2].

Since these first applications, SX turbine blades have become standard on many high performance jet engines. They are also being used more recently on high performance non-aviation gas turbines, generating electric power with SX turbine blades in sizes as much as ten times larger than their aviation counterparts. (My research shows that Siemens was the first to use non-aviation SX turbine blades in their .3A series machines in the early 1990s.)

As we know, gas turbine thermal efficiency increases with greater 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 high-performance jet engines can exceed 3,000 ̊F, while non aviation gas turbines operate at 2,700 ̊F or lower. In high-temperature regions of the turbine, special high-meltingpoint nickel-base superalloy blades and vanes are used, which retain strength and resist hot corrosion at extreme temperatures. These superalloys, when conventionally vacuum cast, soften and melt at temperatures between 2,200 and 2,500 ̊F. This means blades and vanes closest to the combustor may be operating in gas path temperatures far exceeding their melting point and must be cooled to acceptable service temperatures (typically eight-to-nine-tenths of the melting temperature) 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. To eliminate the deleterious effects of impurities, investment casting is carried out in vacuum chambers. After casting, the working surface of high-temperature cooled turbine airfoils are coated with ceramic thermal barrier coatings to increase life and act as a thermal insulator (allowing inlet temperatures 100 to 300 Fahrenheit degrees higher).

Grain Boundary Phenomena

Conventionally cast turbine airfoils are polycrystalline, consisting of a three-dimensional mosaic of small metallic equiaxed crystals, or “grains”, formed during solidification in the casting mold. Each equiaxed grain has a different orientation of its crystal lattice from its neighbors’. Resulting crystal lattice misalignments form interfaces called grain boundaries.

Untoward events happen at grain boundaries, such as intergranular cavitation, void formation, increased chemical activity, and slippage under stress loading. These conditions can lead to creep, shorten cyclic strain life, and decrease overall ductility. Corrosion and cracks also start at grain boundaries. In short, physical activities initiated at superalloy grain boundaries greatly shorten turbine vane and blade life, and lead to lowered turbine temperatures with a concurrent decrease in engine performance.

One can try to gain sufficient understanding of grain boundary phenomena so as to control them. But in the early 1960s, researchers at Pratt & Whitney Aircraft (now Pratt & Whitney, owned by United Technologies Corp.) set out to deal with the problem by eliminating grain boundaries from turbine airfoils altogether, by inventing techniques to cast single-crystal turbine blades and vanes.

In jet engine use, single-crystal turbine airfoils have proven to have as much as nine times more relative life in terms of creep strength and thermal fatigue resistance and over three times more relative life for corrosion resistance, when compared to equiaxed crystal counter-parts. Modern high turbine inlet temperature jet engines with long life (that is, on the order of 25,000 hours of operation between overhauls) would not be possible without the use of single-crystal turbine airfoils. By eliminating grain boundaries, single-crystal airfoils have longer thermal and fatigue life, are more corrosion resistant, can be cast with thinner walls — meaning less material and less weight — and have a higher melting point temperature. These improvements all contribute to higher gas turbine thermal efficiencies.

Figure 1 shows creep life progress in turbine blade alloys, as given by NASA [3]. In the plot, the abscissa shows the year of alloy development and the ordinate shows temperature capability in degrees Celsius, for a variety of turbine blade superalloys. The temperature capability is the temperature for creep life posed as the time (1000 hours) the alloy reaches a certain elongation/strain (1%) under a given stress (137MPa = 20,000 psi). As shown, single crystal blades are clearly superior.

Figure 1 Progress in temperature (Celsius) creep rupture limits for various superalloys (Photos are of non-aviation turbine blades. Courtesy of NASA.)

Grahic Jump LocationFigure 1 Progress in temperature (Celsius) creep rupture limits for various superalloys (Photos are of non-aviation turbine blades. Courtesy of NASA.)

About fifty years ago, a small group of gas turbine industry researchers set out to eliminate grain boundaries in superalloy turbine blades. Today, the result is a whole class of single crystal turbine blades that have increased thermal efficiencies and have unmatched resistance to high-temperature creep and fatigue.

Langston, Lee S., 2006, “Crown Jewels”, Mechanical Engineering Magazine, February, pp. 31– 33.
Gell, M., Duhl, D.N., and Giamei, A.F., 1980, “The Development of Single Crystal Superalloy Turbine Blades”, Superalloys 1980,
Copyright © 2014 by ASME
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References

Langston, Lee S., 2006, “Crown Jewels”, Mechanical Engineering Magazine, February, pp. 31– 33.
Gell, M., Duhl, D.N., and Giamei, A.F., 1980, “The Development of Single Crystal Superalloy Turbine Blades”, Superalloys 1980,

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