In compressor design, a convenient way to save time is to scale an existing geometry to required specifications, rather than development of a new design. The approach works well when scaling compressors to 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:
The Reynolds number effect, describing the non linear dependency of surface friction on Reynolds number.
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.
Firstly, effects of geometric scaling on compressor performance are assessed analytically.
Secondly, prediction capabilities of reduced order models from the public domain are assessed and an alternative model developed by the authors is presented. In addition to design point assessment, often found in other publications, the models are tested at off-design.
Thirdly, 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.
Fourthly, a detailed investigation regarding tip leakage driving mechanisms is carried out and design recommendations to improve small scale compressor performance are provided.