Design and Characterization of Micro-Compressor Impellers

Abstract

A study has been conducted, using steady three-dimensional Reynolds-averaged Navier-Stokes simulations (FLUENT) to investigate dominant performance limiting mechanisms for micro-scale, high-speed compressor impellers with diameter in the range of 5mm to 10mm and peripheral speed ∼ 500 ms−1. Heat transfer to impeller flow (hence non-adiabatic in contrast to nearly adiabatic macro-scale centrifugal compressors for aircraft engine application), casing drag, and impeller passage boundary layer loss are identified as micro-scale impeller performance limiting mechanisms. Heat transfer could lead to up to 25 efficiency points penalty, casing drag to about 10–15 points, and passage boundary layer loss to another 10 points for the investigated micro-impellers. Micro-impeller efficiency of up to 90% is achievable if design is directed at mitigating these performance limiting mechanisms. The effect of heat addition on impeller performance is detrimental and depends on a single non-dimensional parameter (ratio of added heat to inlet stagnation enthalpy). The performance penalty is associated with the physical fact that compression at high temperatures requires more work. Casing drag associated with impeller rotating relative to stationary casing results in a torque on the flow near the casing and impeller blade tip that can be characterized in terms of rotational Reynolds number and ratio of tip clearance to impeller radius. Channel boundary layer loss can be characterized in terms of Reynolds number, geometry (impeller exit-to-inlet diameter ratio, blade angles, chord-to-inlet diameter ratio, average-to inlet span ratio, inlet diameter-to-inlet span ratio), and exit-to-inlet temperature ratio related to work input (rotor geometry and speed). A physics-based model is developed for quantifying each of these performance-limiting processes, given the key design parameters. The results from the models are in accord with CFD (FLUENT) data. Implications on impeller design are discussed and design guidelines are formulated. The paper reports a quantitative investigation of micro-turbomachinery performance limiting mechanisms and offers design guidelines based on physical understanding.