Flow behaviour in a high speed centrifugal impeller passage under 'design' and 'off design' operating conditions

Complex geometry and very small working space within a modern high speed impeller passage impose severe constraints on any experimental technique to be used for detailed measurement. In this study, using a dual focus laser velocimeter, low energy regions were located in the impeller passage of a small high speed commercial centrifugal compressor passage at three rotational speeds, one close to 100 000 rev/min under 'design' and 'off design' operating conditions. It was deduced that impeller rotation and mass flow rate significantly altered the flow pattern. Higher speed and lower mass flow rate enhanced the process of flow separation resulting in noticeable movement of wake regions to different locations within the impeller passage. Interestingly, however, the overall wake behaviour pattern of high speed impeller flow was quite similar to those observed previously in low speed impeller passage. This finding may have positive implications for future design.