The effect of exit pressure pulsation on the performance and stability limit of a turbocharger centrifugal compressor

Abstract

Surge is a system instability related to interaction between the compressor, the ducting and the throttle in a given compression system. This limit is critical for automotive turbochargers as it dictates the minimum flow rate of the internal combustion (IC) engine. The inlet valve train of the IC engine cylinder arrangement influences the operation of a turbocharger compressor. It is this valve motion that sets a pulsating exit state for the compressor and, as such, it can impose a further limit on the compressor operation range limiting further the engine minimum flow rate. This thesis presents an experimental evaluation of the surge dynamics on a compressor with induced downstream pulsating flow. Different pulsation levels will be characterized by means of the frequency and amplitude of the pulse. Multiple experiments are carried out in a purpose-built test bench that has components similar to a turbocharger engine system: compressor, duct, plenum and throttle. Experimental data will be presented for a range of frequencies, amplitudes at a given compressor speed, and the effect of these parameters will be analysed independently. The main effect on the surge margin of the compressor has been found to be due to the presence of a volume in the system for all cases, whether steady or pulsating condition, and at all frequencies. It was found that the magnitude of the pulse frequency determines the hysteresis behaviour of the system that leads to a phase difference between the convected terms (volume flow) and the acoustic dominated terms (pressure), and therefore this affects the onset of flow instability, surge, in the compression system under study. Based on these results, the compressor performance, and particularly its stability limit, is strongly influenced by the downstream conditions of the system where the compressor is placed.