Flow instabilities in centrifugal compressors are one of the most limiting factors to the performance of the system where the compressor is installed (i.e., internal combustion engines, fuel cell, etc.). A better understanding of the surge phenomenon allows to properly set-up the operative region of the turbomachine in order to avoid a decrease in performance and to prevent the risk of damage. Actually, the instability phenomenon of surge can seriously damage the compressor for vibrations and temperature stress. To avoid damage to the compressor, the maximum boost pressure is generally limited with a by-pass system, reducing the mass flow rate provided by the machine.

The results of a broad experimental investigation performed at the turbocharger test bench of the University of Genoa are reported to deeply analyze the behavior of a small centrifugal compressor. A specific circuit adaptable in volume and length is used to study the effect of different configurations on steady compressor performance with special reference to the unstable operation. Measurements of instantaneous pressure and mass flow rate signals are performed in different sections located upstream and downstream the turbomachine. The difficulties related to measurement of instantaneous fluid-dynamics parameters in the turbocharging circuit and data post-processing are highlighted under unsteady flow conditions occurring in surge operation.

The effect of the compressor operating conditions (i.e., turbocharger rotational speed) and the circuit geometry is studied during the transition from steady to unsteady operation. In particular, the hysteresis loops surrounding the steady state map are reported highlighting filling and emptying and wave action phenomena, also in the region of zero mass flow and in the case of reverse flow condition. All measured quantities presented in this paper are the time synchronous averages of a large number of consecutive cycles in deep surge operation.

The aim of the paper is to extensively analyze the compressor behavior in unstable operating conditions in order to improve simulation models such as a physics-based compressor model previously developed by the Authors.

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