A Study of the Flow Characteristics in the Inducer Bleed Slot of a Centrifugal Compressor

A broad operating range between surge and choke is so important for turbocharger compressors and many other applications that a vaneless diffuser, with its reduced efficiency, is usually adopted. With the demand for increased pressure ratio the operating range naturally reduces and techniques to extend the range are necessary. The inducer bleed slot is a technique which has been adopted in turbocharger compressors. This approach was first reported by Fisher (1988) and was described as a Map Width Enhancement slot (MWE). The flow conditions in the MWE slot and impeller inlet duct were investigated with a view to developing an improved understanding of the flow mechanisms involved as the flow rate was reduced from choice to surge. Mean temperature and pressure measurements were recorded in the MWE passage, the main inducer duct to the impeller and the inlet duct upstream of the compressor. In addition the development of flow pulsations were monitored with pressure transducers in the MWE passage, the main inducer duct and the inlet duct, together with the application of flow visualisation techniques. The transient pressure measurements showed that low frequency flow pulsations developed in the MWE passage at high flow rates. As the flow rate was reduced the low frequency pulsations disappeared and flow reversal through the MWE passage developed. It was shown that flow reversal through the MWE passage commenced at flow rates close to the peak efficiency point for the compressor.