ENDWALL CONTOURING USING CONTINIOUS DIFFUSION, A BREAKTHROUGH METHOD AND ITS APPLICATION TO A THREE-STAGE HIGH PRESSURE TURBINE
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abstract
Blades of high pressure turbines have a relatively small aspect ratio that produces major secondary flow regions close to the hub and tip. The secondary flows caused by a system of hub and tip vortices induce drag forces resulting in an increase of secondary flow losses, and thus, a reduction of stage efficiency. Given the high level of technological maturity and the current state of turbine aerodynamic efficiency, major efficiency improvement, if any, can be achieved only by significant R&D effort. In contrast, a moderate increase in aerodynamic efficiency is attainable by reducing the effect of parasitic vortices such as those mentioned above. Introducing an appropriate nonaxisymmetric endwall contouring reduces the secondary flow effect caused by the pressure difference between pressure and suction surfaces. Likewise, attaching leading edge fillets reduces the strength of horseshoe vortices. While an appropriate endwall contouring design requires special care, the design of the leading edge fillet is straightforward. In this paper, we present a physics based method which enables researchers and engineers to design endwall contours for any arbitrary blade type regardless of the blade loading, degree of reaction, stage load and flow coefficients. A thorough step-by-step design instruction is followed by its application to the second rotor row of the three-stage research turbine of the Turbomachinery Performance and Flow Research Laboratory (TPFL) of Texas A&M University. Comprehensive numerical calculations of the flow field, including the secondary flow, show the positive impact of an appropriately designed endwall contouring on the efficiency. The results also show how an inappropriately designed contour can be detrimental to turbine efficiency.
