Investigating the Effects of Surface Topography and Roughness on Turbine Aerodynamic Performance

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Reducing the SFC of aircraft engines has become an increasingly significant task in order to achieve Advisory Council for Aeronautics Research in Europe's (ACARE) 2050 flightpath plan to reduce aircraft emissions. A part of this challenge is the requirement to improve the overall aerodynamic efficiency of gas turbines by increasing machine stage efficiencies. Within aeroengines, entropy is a useful measure of inefficiency and approximately one third of the entropy generated within an aeroengine turbine is associated with the "aerodynamic friction" between the air flow and the blade surfaces. A major contributor to the rate of entropy production within a boundary layer is the "skin friction" between the air and blade which depends strongly on the roughness (topography) of the blade surface. When the roughness elements protrude through the laminar sublayer, the perturbations resulting from the roughness determines the rise in skin friction. In the rare instance that roughness is accounted for, the capabilities of determining the impact on the skin friction are constrained by insufficient correlations utilising coarse measurements of the centreline averaged roughness height, Ra, as the singular responsible geometric parameter. A surface with the same Ra can have various streamwise roughness distributions where the surface is akin to rolling hills or steep mountains. This determines whether the laminar sublayer perturbs over the topography or is protruded through. An added dimension to the PhD problem is the presence of a pressure gradient in turbine blades and particularly the region of diffusion on the suction surface. The aim of this PhD is to develop understanding of the relationship between surface topography (roughness) and aerodynamic loss. One of the deliverable of this research will be the development of correlations for the effects of surface topography that can be applied to all blade surfaces (i.e engine-run, novel ceramic matrix composites and as-manufactured). An improved understanding of how surface roughness affects skin friction is not only important to current designs but essential for the successful implementation of new materials and manufacturing processes. The PhD will take an experimental and numerical approach. A flat-plate liner working section in the Rhoden wind tunnel (situated in the Low Speed Whittle Lab) will be used to simulate scaled-up engine representative boundary layer, allowing the impact of the surface topography on skin friction to be measured. A numerical tool will be used to determine which surfaces to be 3-D printed and experimentally tested. This will be the main method used to determine correlations which can be applicable to all real turbine blade surfaces. To broaden the scope of the research, high fidelity computational fluid dynamics (in particular, large eddy simulation) will be performed to see if the fluid-structures responsible for the roughness induced aerodynamic loss can be captured.