Compressor surge for fully and semi fluctuating flows in automotive turbochargers

Performance of radial compressors are gathered in maps correlating mass flow parameter, compressor speed, total-to-total pressure ratio and efficiency. In automotive turbochargers, a radial compressor is coupled to an expansion turbine via a common shaft. In these turbomachines, compressors performance are measured in steady flow gas-stand following specific code of standards where normalisation for intake pressure and temperature are applied. Compressor operations are delimited by surge and choke conditions due to instabilities onset. In this study, the focus is on the analysis of surge, corresponding to the low mass flow limit of the compressor. Surge is characterised by flow recirculation at the inlet, across the compressor wheel, leading to instability of the turbomachine due to extreme pressure and mass flow oscillations.
In order to adopt turbochargers with internal combustion engines, pressure and mass flow oscillations at turbine inlet and compressor outlet should be considered due to the fluctuation caused by the reciprocating engine. Therefore, a specific engine gas-stand has been developed for powering the turbine and generating real mass flow oscillations due to the motion of the engine exhaust valves. This is referred in the paper as semi fluctuating flow condition of the turbocharger, with turbine solely subjected to pulsating flows. In addition, the compressor is disconnected from the engine intake and the load is controlled by a back-pressure valve, to reflect semi fluctuating flow conditions. In this research study, the Fast Fourier Transform (FFT) and fluctuation of compressor outlet pressure and mass flow have shown the reduced effects of turbine pulsations on compressor stability. In order to capture the recirculation at the compressor inlet, FFT and temperature rise to the ambient intake temperature has been monitored through a thermocouple placed 25mm away from the compressor wheel.
Moreover, the map generated for compressor operating under steady flow with solely flow pulsation at the turbine inlet has been used as a benchmark to exploit surge behaviour of the compressor for fully fluctuating flows in turbochargers. In fact, the compressor outlet has been connected to the engine intake in parallel with the external boost rig. In this layout, the external boost has been supplied in order to control the load on the turbocharger by providing flow to the engine and throttling the compressor, simultaneously. Compressor surge has resulted different between the two configurations, extending to lower corrected mass flow values in fully fluctuating flows in the turbocharger. Investigation has been performed, evaluating compressor pressure and mass flow variations and compressor inlet temperature rise to the ambient conditions in the proximity of the compressor ducting.