Surge is an unsteady flow phenomenon occurring at low flow rates in the pipeline system including compressors. The surge is a phenomenon that must be avoided because of the danger in the operation: the pipeline equipment can be damaged or the operation cannot be continued. Experimental work is required not only to understand the unsteady behavior but to also validate the CFD used for more localized analysis and development of the understanding of the flow phenomena when operating near surge. Nevertheless, there are still many unclear points not only about the flow phenomenon at the inception of the surge which is important for the prediction of the surge but also about the surge behavior itself. Especially, as for the surge occurring in transonic centrifugal compressors, there are currently few experimental research cases due to the difficulty of the unsteady measurement.

In this research, we measured the time variations in pressure and flow rate in a transonic centrifugal compressor for a vehicle turbocharger which consists of an impeller, vaneless diffuser and scroll. In the experiments, the measurement pipes were set upstream and downstream of the compressor and the velocity and the wall static pressure were measured with an I-type hot wire probe and high response pressure transducers, respectively. In addition, to investigate the process and the occurrence point of the back flow in surge, the wall static pressure was measured by means of high response pressure transducers which were mounted on the shroud upstream of the impeller and the diffuser hub at the two-circumferential positions, respectively. As the result of the experiments, the unsteady flow process during the mild and deep surges was measured and the inception of deep surge was clarified.

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