An experimental determination of the trailing-edge base pressure on blades in transonic turbine cascades

Abstract

This thesis documents an experimental investigation of the base (trailing edge) pressure and its approximate distribution on a transonic turbine blade. Since the base pressure plays an important role in determining the profile loss on blades with thick trailing edges, both the base pressure and the blade losses are presented for a range of transonic exit Mach numbers. The overall objective of this work is to provide experimental data for improving current computer-based models used in designing turbine blades.

The two-dimensional cascade was tested in the VPI&SU Transonic Cascade Wind Tunnel, a blow-down type of tunnel facility. The blade design for the cascade was based on the pitchline profile of the high-pressure turbine in a commercial jet engine with a design exit Mach number of approximately 1.2. In order to carefully instrument the thin trailing edge, the blades used in the experiment were made five times the size of the actual engine blade. With this large-scale blade, five static pressure taps were placed around the trailing edge. In addition to these taps, the rearward portion of the suction surface was also instrumented with five static pressure taps. The aerodynamic losses were quantified by a loss coefficient: the mass-averaged total pressure drop divided by the total pressure upstream of the blade row. These measured pressures were taken with a fixed total pressure probe upstream of the cascade and a pitchwise traversing probe in the downstream position. The cascade was tested for an exit Mach number ranging from 0.70 to 1.40.

The results of the experiments indicate a decreasing normalized base pressure (p_B/p_t1) with increasing downstream Mach number (M₂) until the minimum value of p_B/p_t1 = 0.30 at M₂ = 1.30. The approximate base pressure distributions for all transonic downstream Mach numbers indicate nearly uniform pressure around the central 90° of the trailing edge. Results for the profile loss are displayed for exit Mach numbers between 0.70 and 1.35; the trend of increasing loss with decreasing base pressure is shown. The shadowgraph pictures taken reveal the trailing edge region of the flow for several downstream transonic Mach numbers.