Comparison of Two Highly Loaded Low Pressure Turbine Cascades Under the Influence of Wake-Induced Transition

Abstract

Recent research has revealed positive effects of unsteady flow on the development of profile boundary layers in turbine cascades at conditions with a laminar suction side separation bubble. Compared to steady flow, a reduction of total pressure loss over a broad range of Reynolds-numbers has been shown. A new design of turbine blades with an increased blade loading (high lift) is possible if the effects of rotor-stator interaction are taken into account. With previous investigations at low speed cascade wind tunnels just one of the parameters Mach- or Reynolds-number could be adjusted during the tests. In order to verify the promising results gained at low speed cascade wind tunnels also at realistic Mach- and Reynolds-number combinations the present investigation has been carried out at the High Speed Cascade Wind Tunnel of the University of the Federal Armed Forces Munich. Being built inside a large pressure tank this high speed cascade wind tunnel offers the possibility to vary the Mach- and the Reynolds-number in the test section independently of each other in order to correctly simulate the flow conditions inside turbomachines. Thereby the experimental gap between investigations at low speed cascade wind tunnels and investigations at turbine-rig setups can be closed. In turbomachines, periodically unsteady flow is caused by the relative motion of rotor and stator rows. A wake generator has been designed and built in order to simulate a moving blade row upstream of a linear turbine cascade in the High Speed Cascade Wind Tunnel of the Universität der Bundeswehr München. The wakes are generated with cylindrical bars moving with a velocity of up to 40 m/s in the test section upstream of the cascade inlet plane. Measurements have been performed on two highly loaded low pressure turbine cascades (turbine cascade A and B) at varying Reynolds-numbers with steady and unsteady inlet flow conditions. For the unsteady inlet flow conditions, the frequency (Strouhal-number) of the wake passing has been altered by varying the speed of the bars. The turbulence intensity and the velocity deficit of the bar wakes have been measured with a 1D hot-wire probe. Wake-induced transition is qualitatively mapped out by employing a simultaneous surface hot-film anemometry system. Measurements of the surface pressure distribution and wake traverses have been performed. Due to an enlarged pitch to chord length ratio, turbine cascade B has a 15% larger lift than turbine cascade A, despite both having the same inlet and outlet conditions. Thereby the turbine cascades A and B have different airfoil shapes in order to take maximum advantage of the positive effects of rotor-stator interaction. Both cascades show a positive influence of unsteady inlet flow conditions to the boundary layer of the suction side, compared to steady inlet flow conditions, with respect of measured losses. With cascade A a maximum reduction of total pressure loss of 34% and with cascade B of 28% has been achieved, both compared to the appropriate steady inlet flow case. At design conditions of the turbine cascades (β1 = 135°, Ma2th = 0.7, Re2th = 100000) with unsteady inlet flow, both cascades have very similar low losses. Consequently, by taking into account the positive effects of wake-induced transition during the design process, new high lift blading with nearly the same low losses at unsteady inlet flow conditions could be achieved. This leads to a reduction of weight and cost of the whole turbine module for a constant stage loading.