Development of Blade Profiles for Low-Pressure Turbine Applications

Abstract

This paper describes a program of work, largely experimental, which was undertaken with the objective of developing an improved blade profile for the low-pressure turbine in aero-engine applications. Preliminary experiments were conducted using a novel technique. An existing cascade of datum blades was modified to enable the pressure distribution on the suction surface of one of the blades to be altered. Various means, such as shaped inserts, an adjustable flap at the trailing edge, and changing stagger were employed to change the geometry of the passage. These experiments provided boundary layer and lift data for a wide range of suction surface pressure distributions. The data were then used as a guide for the development of new blade profiles. The new blade profiles were then investigated in a low-speed cascade that included a set of moving bars upstream of the cascade of blades to simulate the effect of the incoming wakes from the previous blade row in a multistage turbine environment. Results are presented for two improved profiles that are compared with a datum representative of current practice. The experimental results include loss measurements by wake traverse, surface pressure distributions, and boundary layer measurements. The cascades were operated over a Reynolds number range from 0.7 × 105 to 4.0 × 105. The first profile is a “laminar flow” design that was intended to improve the efficiency at the same loading as the datum. The other is a more highly loaded blade profile intended to permit a reduction in blade numbers. The more highly loaded profile is the most promising candidate for inclusion in future designs. It enables blade numbers to be reduced by 20 percent, without incurring any efficiency penalty. The results also indicate that unsteady effects must be taken into consideration when selecting a blade profile for the low-pressure turbine.

An assessment of the laminar kinetic energy concept for the prediction of high-lift, low-Reynolds number cascade flows

“…In the multistage engine environment, the unsteady wake-induced transition plays a key role in reducing the separation effects up to a level compatible with acceptably low losses. Many studies demonstrate how high-lift [1,2] and ultra-high-lift [3,4] airfoils can be operated with loss control by taking advantage of wake-induced transition in LPT low-Reynolds, number flows. The study of wake-induced transition in LP turbines has thus led to improvements in performance of presentgeneration turbomachinery [5,6].…”

Fundamentals and Applications of Modern Flow Control

“…Reductions of 12% in blade count were shown by Howell et al (2002) and increases of 20% in blade lift without an increase in blade profi le loss were obtained by Curtis et al (1997) by adjusting the clocking such that upstream wakes were exploited for separation control of highly loaded airfoils. Since modern LPT effi ciencies are already very high (over 90%) further increases are diffi cult.…”

Comparison of a State of the Art and a High Stage Loading Rotor

“…A large number of research has been performed to understand the potentials and effects of highly loaded blades, an overview of this is given by Hodson et al [2]. These effects can, to a certain extend be avoided by taking advantage of unsteady boundary layer interactions, suppressing the occurrence of said separation bubble [2,3,4,5]. Special focus has also been given to the influence of unsteady bladerow interaction on transition, focusing on the calming effect, the wake passing event has on the suction side boundary layer of the blade [2,6,7,8,9] If the loading is increased further, at a certain point the unsteadiness is not able to avoid the separation completely and therefore significantly increases the blades losses, but still the increase in lift and therefore the reduction in blade count may outweigh this increase [10].…”