For mixed-flow compressors the downstream vaneless diffuser can be either a straight or curved annular passage. Investigations have been carried out on model diffusers six times larger than those employed in a turbocharger study, Whitfield et al (1978). This made it possible to carry out detailed internal flow measurements using a five hole yaw probe and a hot wire anemometer. The development of the flow profiles throughout the diffuser were studied with particular reference to the onset of unstable conditions. Flush mounted wall pressure transducers were also used to detect the onset of unstable flows and to measure the extent and development of the unsteady conditions.

Two curved and one straight annular diffusers have been tested in order to study their relative merits. In addition, the results obtained with a straight radial diffuser are included for comparison. It has generally been observed that the straight annular diffuser was the least stable, with the radial diffuser exhibiting the best stability characteristics. Of the two curved annular diffusers tested flow stability was improved by reducing the passage width. In both cases, however, the curved diffuser proved to be clearly superior to the simpler straight design.

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