Unsteady effects of periodic wake passing frequency on aerodynamic performance of ultra-high-lift low pressure turbine cascades

Abstract

Flow losses and flow field data downstream of an ultra-high-lift aft-loaded low pressure turbine cascade were comprehensively measured for different incoming wake passing frequencies, and the mechanisms through which incoming wakes influence secondary flow were examined via numerical calculations. Reynolds numbers ranging from 25 000–100 000 (based on the axial chord and inlet velocity) were considered in both the presence and absence of the wakes. At low Reynolds numbers of 25 000 and 50 000, increasing wake passing frequency gradually suppressed the suction surface separation bubble and increased the cross-passage pressure gradient, and the unsteady wakes clearly improved the throughflow characteristics of the cascade passage. Furthermore, the larger separation bubble on the suction surface hindered the migration of the secondary vortices from the endwall to span. Consequently, incoming wakes were not beneficial for suppressing secondary flow at low Reynolds numbers. At the high Reynolds numbers of 80 000 and 100 000, increasing wake passing frequency afforded stronger inhibition of the secondary flow owing to the reduction in blade loading originating from the “negative jet” influence of the wakes. Transport of incoming wakes in the cascade passage caused the downstream migration of the position of the saddle point, which is also advantageous for decreasing secondary flow losses.