Compact axial compression systems are of interest to the domestic appliance industry. The associated low Reynolds number leads to high losses compared to large-scale compressors due to a transitional flow field with large regions of separation. Operation off design and close to stall leads to unsteady flow that generates increased noise radiation. This paper investigates how Reynolds number variations affect the three-dimensional flow-field in a compact compressor when operating prior to stall and how this relates to both the unsteady behavior near stall and the compressor in stall characteristics. 

An experimental study has been conducted using a scaled-up single stage axial compressor across a Reynolds number range of  to . Steady and unsteady casing static pressure measurements, along with rotor downstream unsteady flow velocity measurements have been used to observe the rotor flow field over a range of operating conditions including the design point, the near stall region, and the condition immediately after stall inception. These measurements have been repeated at several Reynolds numbers.

The steady results showed that, below a critical value of 60,000, as the Reynolds number reduces, the pressure rise coefficient of the compressor decreases. This fall off in performance corresponds to an increase in the compressor rotor secondary flows. As the Reynolds number reduced from 60,000 to 20,000 the hub and casing end wall separations were found to double in size, eventually occupying 40% of the passage area downstream of the rotor.

The casing unsteady pressure measurements were used to produce spectra that feature a broadband hump at around 50% of the blade passing frequency. The magnitude of this hump increases with reducing flow coefficient and at Reynolds numbers below the critical value, multiple equally spaced peaks also appear around the 50% blade passing frequency prior to stall. The spacing of these peaks has been found to be exactly equal to the measured stall cell speed once rotating stall is established. When operating in stall, the stall cell is found to increase in size and slow down as Reynolds number decreases. For a Reynolds number decrease from 60,000 to 20,000 the cell size increased by 15% at the same time as the speed decreased by 4%. 

The measured spectra and observed flow structures show that disturbances exist prior to stall at frequencies consistent with the frequencies within stall. The size of the stall cell that forms is related to the extent of the three-dimensional flow-field present prior to stall. Below a critical value, all of these flow features are highly sensitive to Reynolds number.


The Effects of Reynolds Number on the Stall and Pre-Stall Behaviour of Compact Axial Compressors

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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