Impact of Rotational Speed on Turbocharger Compressor Surge Through Particle Image Velocimetry1

Stereoscopic particle image velocimetry is used to characterize the variation of the turbocharger compressor inlet velocity field as a function of rotational speed, with an emphasis on surge. While the velocity magnitudes at choke or mild surge increased with rotational speed, the velocity profiles remained qualitatively similar. The variation in deep surge flow field with shaft speed, however, was more substantial. At 80 krpm, the overall flow field was comparable at different time instances (at different points on the surge cycle): the core flow near the duct center was always directed into the impeller, whereas reversed flow occupied an annular region near the periphery in nearly all time instances. However, at 140 krpm, while the negative flow rate (cross-sectional average flow is directed out of the inducer back into the inlet duct) portion of the surge cycle was still similar to the overall surge flow field at 80 krpm, over a substantial part of the positive flow rate (cross-sectional average flow is directed into the impeller) portion of the surge cycle, there was no sign of reversed flow within the visualization domain. As the rotational speed was increased, the surge loop (obtained by combining the particle image velocimetry (PIV) and pressure transducer data) extended over a wider portion of the compressor map with higher maximum (positive) and minimum (negative) flow rates, along with higher amplitude pressure fluctuations. The mean amplitude of mass flow rate and pressure ratio fluctuations at deep surge increased in nearly a quadratic fashion with rotational speed.