Evolution of Turbine Cooling

Turbine cooling is a battle between the desire for greater hot section component life and the techno-economic demands of the marketplace. Surprisingly little separates the haves from the have nots. The evolution of turbine cooling 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. Yet even with the several generations of design advances, limitations are becoming apparent as complexity sometimes leads to less robust outcomes in operation. Furthermore, the changing environment for operation and servicing of cooled components, both the natural and the imposed environments, are resulting in new failure modes, higher sensitivities, and more variability in life. The present paper treats the evolution of turbine cooling in three broad aspects including the background development, the current state-of-the-art, and the prospects for the future. Unlike the Darwinian theory of evolution however, it is not feasible to implement thousands of small incremental design changes, random or not, to determine the fittest for survival and advancement. Instead, innovation and experience are utilized to direct the evolution. Over the last approximately 50 years, advances have led to an overall increase in component cooling effectiveness from 0.1 to 0.7. Innovation and invention aside, the performance of the engine has always dictated which technologies advance and which do not. Cooling technologies have been aided by complimentary and substantial advancements in materials and manufacturing. The state-of-the-art now contains dozens of internal component cooling methods with their many variations, yet still relies mainly on only a handful of basic film cooling forms that have been known for 40 years. Even so, large decreases in coolant usage, up to 50%, have been realized over time in the face of increasing turbine firing temperatures. The primary areas of greatest impact for the future of turbine cooling are discussed, these being new engine operating environments, component and systems integration effects, revolutionary turbine cooling, revolutionary manufacturing, and the quantification of unknowns. One key will be the marriage of design and manufacturing to bring about the concurrent use of engineered micro cooling or transpiration, with the ability of additive manufacturing. If successful, this combination could see a further 50% reduction in coolant usage for turbines. The other key element concerns the quantification of unknowns, which directly impacts validation and verification of current state-of-the-art and future turbine cooling. Addressing the entire scope of the challenges will require future turbine cooling to be of robust simplicity and stability, with freeform design, much as observed in the “designs” of nature.