PARAMETRIC STUDY OF SURFACE ROUGHNESS AND WAKE UNSTEADINESS ON A FLAT PLATE WITH LARGE PRESSURE GRADIENT

X. F. Zhang, H. P. Hodson

Whittle Laboratory University of Cambridge, UK

Abstract The combined effects of surface roughness and upstream unsteady wakes were investigated on a flat plate subjected to the same pressure distribution as that on the suction surface of an ultra high lift low-pressure (LP) turbine blade. The parametric study included the effects of roughness size, type and location under steady and unsteady flow conditions. The roughness elements included straight wires, straight square steps, wavy steps and wavy wires. The experimental re­sults show that the combined effects of surface roughness and upstream unsteady wake can further reduce the profile losses of the ultra highly loaded LP turbine blades. For each flow condition, there is a height of roughness element, which results in the lowest loss. The step-type roughness elements are more effective at inducing boundary layer transition than the wire-type roughness due to the higher disturbance level generated after the sharp edges. The wavy roughness elements are more effective at inducing transition and further reduce the profile losses under steady flow conditions. However, the strong stream-wise vortexes generated by the wavy elements will increase the loss in unsteady conditions. The optimum location of the roughness element is between the blade suction peak and the separation onset location.

Keywords: high lift, LP turbine, boundary layers, separation bubble, unsteady flow, wakes,

surface roughness, loss

1. Introduction

The recent studies of wake-separated boundary layer interaction have made the development of the high lift low-pressure (LP) turbine blades possible. The wakes from the previous blade row impinge on the blade surfaces and induce transition around the separation onset location on the suction surface. The wake-induced turbulent region as well as the calmed region that follows it will suppress the separation bubble. The calmed region, which is laminar – like in nature and attached, will reduce the production of loss (Schulte and Hodson, 1998). The understanding of the unsteady fhw behaviour has been

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K. C. Hall et al. (eds.),

Unsteady Aerodynamics, Aeroacoustics and Aeroelasticity of Turbomachines, 331-344. © 2006 Springer. Printed in the Netherlands.

successfully incorporated into the design of new high lift LP turbine profiles (Howell et al., 2002).

Further increases in the blade loading will result in a higher adverse pressure gradient and a larger separation bubble on the blade suction surface. Howell et al. (2002) showed that the wake unsteadiness still worked well on the ultra high lift blades (Zw = 1.2) at Reynolds numbers of about 200000. However, in the case of lower Reynolds numbers, the incoming wakes may not be strong enough to suppress the large separation and the separation bubble will have more time to re-establish due to the lower wake passing frequency. There will be a larger separation bubble between the wake pass resulting in a larger profile loss. Therefore, another loss reduction mechanism is required.

The current study, funded by the European Commission, is concerned with the combined effects of wake unsteadiness and surface roughness on the bound­ary layer development on an ultra high lift LP turbine blade – T106C. The object is to determine if these disturbances and their effects enable further in­creases in blade loading and if so, to further understand the physical mecha­nisms involved. The T106C LP turbine cascade has a pitch that is 20 percent larger than the well-studied T106A cascade, (Stieger, 2002 and Brandt et al., 2000), resulting in a Zweifel coefficient of 1.19. The first part of the study con­cerned this parametric study of the effect of roughness elements under steady and unsteady ft>w conditions. The experiments were conducted on a fht plate, which is subjected to the same pressure distribution as that on the suction sur­face of T106C profile.