The effect of endwall contouring on the unsteady flow through a turbine rotor

[摘要] ENGLISH ABSTRACT: With increasing environmental concerns and the drive for a greener economy comes anincreased desire to improve turbine engine fuel efficiency and reduce emissions. Unfortunatelyweight reduction techniques used increase the blade loading, which in turn increasesthe losses. Non-axisymmetric endwall contouring is one of several techniques being investigatedto reduce loss in a turbine. An investigation at Durham University produced anon-axisymmetric endwall design for a linear cascade. An adaption of the most promisingendwall was investigated in an annular rotating test rig at the CSIR using steady stateinstrumentation. The current investigation extends those investigations into the unsteadytime domain.Previous investigations found that a generic rotor endwall contour improved efficiencyby controlling the endwall secondary flow vortex system in both a linear cascade and anannular 1½ stage rotating test turbine. The current research was aimed at determining ifthere were any unsteady effects introduced by the contoured endwall. The approach wasunique in that it investigated the unsteady effects of an endwall contour originally designedfor a linear cascade both experimentally and numerically at three incidence angles (positive,zero and negative to represent increased load, design load and decreased load respectively),the results of which are openly available.Unsteady experimental hotfilm results showed that the endwall contour made the velocityprofile more radially uniform by reducing the strength of the endwall secondary flowvortex system. The fluctuations in the velocity were also reduced producing a more temporallyuniform velocity profile. The FFT magnitude of the velocity at the blade passingfrequency below midspan was also reduced. It was found that the reduction in the endwallsecondary flow vortex system due to the contour increased with increasing loading.Numerical results showed that the oscillations in the flow were small and did not penetratethe boundary layer. The contoured rotor was forward and aft loaded when comparedto the annular rotor, resulting in a weaker cross passage pressure gradient which allowedthe endwall secondary flow vortex system to be less tightly wrapped. Numerical results did not show a significant difference in the oscillations observed in the annular and contouredrotor.A new objective function for use in the endwall optimisation process was proposed thatacts as a proxy for efficiency, but is less prone to uncertainty in the results. When used onthe current results it shows the same trend as efficiency. It remains to be used to designan endwall for full validation.

[发布机构] Stellenbosch University