The effect of roughness and inflow unsteadiness on turbine flows

Abstract

The aim of this work is to characterise the effects of unsteady incoming wakes with different wake profiles and roughness on the turbine blade and loss mechanisms of a linear low-pressure turbine (LPT) cascade. The main focus lies on the boundary layer development as well as the profile and overall losses, which are affected by periodically incoming wakes and surface roughness due to boundary layer interactions. Large eddy simulations with the WALE model of the T106A linear LPT cascade at Reynolds number Re = 100,000 were carried out for the investigations within this work. At this Reynolds number, the highly loaded blade profile is prone to a large separation bubble on the suction surface in an unperturbed environment. Unsteady wakes and surface roughness can exert a beneficial effect by minimising, and in some cases preventing, unwanted laminar separation. Within this work three different incoming wake profiles, generated by means of upstream bars and the Magnus effect, are investigated. It has been found that it is paramount to simulate the correct incoming wake strength compared to actual blade row wakes. The combined effects of wake-blade boundary layer interaction and the wake distortion losses within the turbine passage mutually affect the overall turbine losses. Additionally to the interaction between incoming wakes and the blade’s boundary layer, surface roughness and its implications on the profile and overall losses is considered. In order to do so, a simple roughness model was used and a set of different roughness parameters was simulated. The model, which keeps the computational costs low compared to fully resolved roughness surfaces, was able to reproduce the behaviour observed in the literature in terms of the onset of transition and the effect on the laminar and turbulent boundary layer. For comparison, and for a more detailed analysis of flow mechanisms, a real as-cast roughness patch, scaled down to match representative roughness parameters, was simulated by means of an immersed boundary method. The roughness height definition of the roughness model corresponds well to the roughness height of the as-cast roughness based on the evaluation of the onset of transition, boundary layer development and losses.

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ORCID for Neil Sandham: orcid.org/0000-0002-5107-0944

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