Vortex breakdown in dust-collecting return-flow cyclones

This thesis presents the results of an experimental study of the airflows in reverse-flow dust collecting cyclones. Measurements and flow visualisation techniques were used to examine both the 'paired vortex breakdown' theory that was proposed by Abrahamson & Allen (1986), and the 'natural vortex length' that was identified by Alexander (1949) and which has been widely used in predictive models of the collection efficiency. The object of this study was to improve the assumptions concerning the vortex structure that are used in the collection efficiency models. The models available at present are unable to account for the variation in the slope of the grade efficiency curve. The effect of the geometry on the flow was examined for variations on the Stairmand High Efficiency design cyclone, which is a commonly used design with an anomalous grade efficiency curve. The results that are presented in this thesis show that in conical cyclones the vortex is stabilised over cyclone lengths that are far greater than are used in industrial designs. A descriptive theory of the vortex structure was developed that included three types of 'vortex breakdown' phenomena, resulting from different mechanisms. These breakdowns were found to be strongly dependent on the cyclone geometry. Assisted by the stabilising effect of the cone, the vortex reaches the dust exit but forms a recirculating breakdown under the influence of the pressure increase and the diverging streamlines that result from the expansion into the bin. It is proposed that in geometries where the expansion into the bin is delayed the vortex terminates before the dust exit due to the separation of the boundary layer from the wall. Simple measurement techniques were found that could identify the occurrence of the vortex termination