The Role of Reynolds Number Effect and Tip Leakage in Compressor Geometry Scaling at Low Turbulent Reynolds Numbers

In compressor design, a convenient way to save time is to scale an existing geometry to required specifications, rather than developing a new design. The approach works well when scaling compressors of similar size at high Reynolds numbers but becomes more complex when applied to small-scale machines. Besides the well-understood increase in surface friction due to increased relative surface roughness, two other main problems specific to small-scale turbomachinery can be specified: (1) the Reynolds number effect, describing the non-linear dependency of surface friction on Reynolds number and (2) increased relative tip clearance resulting from manufacturing limitations. This paper investigates the role of both effects in a geometric scaling process, as used by a designer. The work is based on numerical models derived from an experimentally validated geometry. First, the effects of geometric scaling on compressor performance are assessed analytically. Second, prediction capabilities of reduced-order models from the public domain are assessed. In addition to design point assessment, often found in other publications, the models are tested at off-design. Third, the impact of tip leakage on compressor performance and its Reynolds number dependency is assessed. Here, geometries of different scale and with different tip clearances are investigated numerically. Fourth, a detailed investigation regarding tip leakage driving mechanisms is carried out and design recommendations to improve small-scale compressor performance are provided.