Abstract (Summary)
Low-pressure turbines (LPT) experience large changes in chord Reynolds number as the turbine engine operates from take-off to cruise conditions. Due to prevailing conditions at high altitude cruise, the Reynolds number reduces drastically. At low Reynolds numbers, the flow is largely laminar and tends to separate easily on the suction surface of the blade, and this laminar separation in particular leads to significant degradation of engine performance due to large re-circulation zones. Therefore, a better understanding of low-Reynolds number flow transition and separation is very critical for an effective design of LPT blade, and in exploring various possibilities for implementing flow control techniques, passive or active, to prevent or delay the flow separation in the low-pressure turbine. The objective of the present study is to understand the three-dimensional flow separation that occurs inside an LPT cascade at very low Reynolds numbers, and a high-order accurate numerical solution procedure is used to attain the same. A multi-block, periodic, structured grid generated by the grid generation software, GRIDPRO, is used to represent the flow domain. A MPI-based higher-order, parallel, chimera version of the FDL3DI flow solver, developed by the Air Force Research Laboratory at Wright Patterson Air Force Base, is extended for the present turbomachinery application. A sixth-order accurate compact-difference scheme is used for the spatial discretization, along with second-order accurate temporal discretization. Up to tenth-order filtering has been applied to minimize the numerical oscillations, and maintain numerical stability. Simulations have been performed for Reynolds numbers (based on inlet velocity and axial chord) 10,000 and 25,000. The effect of these low-Reynolds numbers on the flow physics for a low-pressure turbine cascade has been studied in detail. At Re = 10,000, the flow undergoes more separation than at Re = 25,000 as expected and the separation remains significant over the entire blade for both the Reynolds number. The location of the onset of separation matches with an available LES simulation and with the available experimental data. In addition to the above simulations, another study was carried out to understand the effect of two different sets of inflow/outflow boundary conditions on the flow solution. The two sets of boundary conditions include static inflow with extrapolated outflow (BC1), and dynamic inflow (BC2) that accounts for upstream influence in the subsonic flow. The computed Cp distribution for the LPT flow shows good agreement with the available experimental data. Application of BC2 boundary condition predicted a bounded region of separation, while BC1 boundary condition predicted significant separation over the entire blade of an LPT.
Bibliographical Information:


School:University of Cincinnati

School Location:USA - Ohio

Source Type:Master's Thesis

Keywords:low pressure turbine flow separation filtering compact difference schemes


Date of Publication:01/01/2005

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