Experimental investigation on surge phenomena in small centrifugal compressors

Centrifugal compressors are key components in propulsion and energy production systems. They are commonly adopted in fuel cells, internal combustion engines, small cogeneration plants and many other systems. The main limitation to obtaining high performance and high boost pressures in centrifugal compressors is due to the instability phenomenon that occurs at low mass flow rates. A better knowledge of this phenomenon can consistently improve the performance of the systems in which centrifugal compressors are installed. The analysis of compressor instabilities, i.e., surge, is an important topic: during surge occurrence the operating flow can assume a highly unsteady behaviour with large fluctuations in pressure and mass flow rate. The compressor can be seriously damaged by phenomena of surge instability due to vibrations and thermal stress. In standard application it is common practice to reduce the operating area of the turbomachinery working away from the surge line to avoid any risk of damage. Usually, to avoid the surge occurrence, the maximum boost pressure is controlled with a by-pass valve commonly called dump valve, it is clear that this solution reduces the mass flow rate provided by the machine and therefore its performance. A better understanding of surge, which can be achieved through dedicated experimental investigations, is essential to develop simulation models capable of accurately predict compressor behaviour and surge occurrence. An in-depth experimental investigation was carried out on a small centrifugal compressor for automotive application at the test rig for components of propulsion system of the University of Genoa. A specific circuit adaptable in length and volume was set up to analyse the effect of different layout configurations on compressor performance with special reference to the low mass flow rate region. The instantaneous value of pressure and mass flow rate are measured in different sections upstream and downstream the compressor and along the circuit. Particular attention was paid in the transition from the steady to the unsteady operation of the compressor, considering different conditions and circuit configurations. Furthermore, the present work describes a methodology to analyse surge cycles, showing their variations with compressor speed and system configuration, applying time synchronous average of a large number of consecutive cycles to evaluate shapes and sizes of surge cycles. Then, this paper highlights the energy content of pressure and mass flow rate fluctuations considering the hysteresis loop during deep surge operations. Finally, a preliminary analysis on the surge precursor is performed by analysing the low and high frequency domain of the pressure fluctuations during the transition from the stable to the unstable zone.