This paper is concerned with the theoretical and experimental aspects of unsteady flow in a radial vaneless diffuser. Specifically, the appearance of self-excited oscillations of large amplitude is investigated in great detail. The waves appear when the angle between the tangential and total velocity becomes very small. The predicted wave speed and perturbed velocity distribution versus radius agree quite well with the experimentally obtained values, however, an undeterminable constant appears in the equation which describes the disturbed velocity distribution. A stability criterion indicates that such pulsating flow can occur if and only if there is a region of radially inward flow. The hypothesis is proposed that the waves are a result of the interaction of the three-dimensional boundary layer near the side walls and the free stream. Unsteady flows of this type can be linked to unstable centrifugal pump, blower, or compressor operation.

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