Spark tracing technique is used to provide detailed physical understsnding of flow phenomena in impellers which significantly influence fluid dynamic performance of turbomachinery. A stationary image of cenfrifugal impeller passages is observed with the aid of an Image Derotator in which a rectangular edge prism rotates synchronously at half the rotational velocity of the impeller. Patterns of main flow and flow near vane wall are visualized. Features of them are discussed with regard to a hydraulic pump impeller (outlet vane angle is 13 deg.) and a blower impeller (75 deg.). Closer observation reveals that a significant adverse flow appears at a partial flow rate.

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