An experimental investigation of the flow field between two radial plates

Abstract

The flow field in a radial diffuser downstream of a radial impeller is highly complex, as the flow is turbulent, unsteady, viscous, and three-dimensional. Depending on the initial conditions at the diffuser inlet, the flow may separate from one or both walls of the diffuser. The objective of this experimental investigation of flow characteristics in a radial vaneless diffuser of a centrifugal blower is to enhance the performance of centrifugal compressors over a wide operating range. Numerical and experimental investigations of rotating stall effects on compressors and blowers have been conducted over the past four decades to achieve this objective. The flow characteristics in a radial diffuser were measured while changing the impeller speed and keeping other parameters such as diffuser radius and width ratio constant. The velocity and pressure distributions in the diffuser were measured along a radial path using a miniature X-wire probe and dynamic as well as static pressure transducers. The axisymmetrical radial and tangential mean velocity distributions and their statistics, as well as pressure distributions, are reported. Some of the results presented in this paper agree with those from earlier work on this subject. For example, the flow angle α (measured at half the diffuser width and from the diffuser inlet to its outlet) did not exceed 65°. This means that α < αc, although a recirculation region was present on the diffuser shroud wall. The critical flow angle αc required to initiate self-excited flow oscillations has been generally reported to be about 78°. Current data also indicate that the flow exiting the impeller is skewed, as revealed by the triple velocity product correlations. The current data are valuable for vaneless diffuser numerical model developers, since there are very limited similar data available in the open literature. The current data support the opinion that the flow separation is not enough to trigger or initiate stall at the diffuser.