Development and implementation of a technique for fast five-hole probe measurements downstream of a linear cascade

Abstract

Flow measurement using a linear compressor or turbine cascade is a well-established technique to characterize the flow in turbomachines with a certain degree of abstraction. A common way to obtain a general characterization of the flow is to measure the flow downstream of the cascade with a five-hole probe, obtaining e.g. total pressure losses and flow turning. Pneumatic five-hole probes are used to capture steady or time-averaged flow quantities, if not otherwise specified. Using standard probes with considerable distance between probe head and pressure transducer, a dynamic calibration is possible, which allows to obtain a transfer function between measured pressure difference and actual pressure at the probe head. The transfer function is dependent on different effects such as overall pressure level and tube length between probe head and pressure transducer. Hence, for every different measurement set-up a dynamic calibration is necessary. In this paper methods proposed by other authors are combined and extended to allow for fast or transient five-hole probe measurements, obtaining the transfer function directly from the measurement itself. The effectiveness of this method is presented for flow measurements downstream of a compressor cascade with attached and stalled flow (by varying the Reynolds number) as well as with steady and periodically unsteady inflow. The new method allows to reduce the measurement time by up to 90 percent without compromising measurement accuracy. In fact, due to higher spacial resolution, the flow downstream of the cascade can better be resolved with the new method.