Influence of Reynolds number and blade geometry on low pressure turbine performance

A first-of-a-kind computational experiment was implemented to assess the influence of Reynolds number and blade design (for identical gas flow angles) on low pressure turbine performance. The objective is to link the time-average performance of a low pressure turbine blade with the flow unsteadiness accompanying a separation of the suction side boundary layer. Specifically, this is accomplished using a time-accurate, two-dimensional, incompressible direct Navier-Stokes simulation of flow in three low pressure turbine blade designs for Reynolds numbers ranging from 20,000 to 120,000. Comparisons of the three geometries reveal that the separated region for the aft loaded blade design is characterized by a time-average separation bubble that is shorter in length than the forward loaded blade. The difference in design translates into a "lagging" of the shear layer roll-up process on the suction side for the forward loaded blade as compared to the aft loaded blade; for a given Reynolds number, the aft loaded design is closer to a point of periodic boundary layer re-attachment relative to the forward loaded design. Forward loading produces greater profile loss due to the higher velocities in the forward half of the passage. However, the unsteady features for the aft loaded blade are very different from those seen for the forward loading case; this is reflected in the time-averaged flow, with the forward loading having a smaller trailing edge flow blockage. Thus, one can deduce that the mixed-out total pressure loss will be larger for the aft loading situation. Differences in the the time-average and unsteady flow features between the two designs are even more apparent at a Reynolds number of 120,000. The performance of the low pressure turbine blade increases significantly when periodic re-attachment of the suction side boundary layer occurs. The total pressure loss generated in the passage drops by nearly a factor of 2 between the fully separated boundary layer and the periodically re-attaching boundary layer. This investigation has demonstrated the feasibility of using a high-order scheme to potentially link the unsteady flow features with the time-average low pressure turbine blade performance. The ability of this high-order Navier-Stokes flow solver to compute differences in flow behavior and performance between seemingly similar blade designs for Reynolds numbers up to 120,000 highlights the usefulness of the technique as a design tool. Thesis Supervisor: Dr. Choon S. Tan Title: Principal Research Scientist, Gas Turbine Laboratory Acknowledgments First, I would like to thank Pratt and Whitney for their involvement in this research. I appreciate the input and direction provided by Dr. Om Sharma, Brent Staubach, and Gary Stetson. I would also like to thank NASA Lewis Research Center for the use of their computational facilities. Specifically, the time and effort put forth by Dr. David Ashpis in answering my questions and helping me to produce the videos of the calculations was of great help to this investigation. Thanks to Choon for taking me on as a student, and doing everything possible to help me from the first day we met. Thanks to my friends who made working (and almost living) in the GTL an enjoyable experience. Finally, I can not express enough appreciation towards my parents, whose love and support gave me the opportunity to earn this degree. Thanks.