Instabilities at the Low-Flow Range of a Turbocharger Compressor

The acoustic and performance characteristics of an automotive centrifugal compressor are studied on a steady-flow turbocharger test bench, with the goal of advancing the current understanding of compression system instabilities at the low-flow range. Two different ducting configurations were utilized downstream of the compressor, one with a well-defined plenum (large volume) and the other with minimized (small) volume of compressed air. The present study measured time-resolved oscillations of in-duct and external pressure, along with rotational speed. An orifice flow meter was incorporated to obtain time-averaged mass flow rate. In addition, fast-response thermocouples captured temperature fluctuations in the compressor inlet and exit ducts along with a location near the inducer tips. At low flow rates, both the large and small volume systems exhibited mild surge as the slope of the compressor characteristics became positive, but the small volume configuration impeded the growth of instabilities and was able to operate without deep surge at mass flow rates well below that of the large volume system. The fundamental mild surge frequency is shown to be dominated by the geometry of the upstream and downstream ducting along with that of the compressor, with in-duct pressure measurements revealing frequencies well below 100 Hz for the configurations studied. For the large volume compression system, the fundamental surge frequency is observed to occur near the Helmholtz resonance of the system during mild surge and decreases with deep surge.