Experimental Studies of High-Speed Dense Dusty Gases

Abstract

An experimental study of the flow of high-speed dense dusty gases has been conducted in a novel shock tube facility. The flow is generated through rapid depressurization and subsequent fluidization of a stationary packed bed of particles loaded under pressure in the vertical driver section of the shock tube. The flow was studied with high-speed photography and fast-response pressure transducers. The studies have been exploratory in nature. The entire process of lofting and disassembly of packed particle beds has been documented. A wide spectrum of dusty flows with particle loadings ranging from that of a fully packed plug to that of a dilute disperse particle flow was observed in this facility. Only extreme flow fields like packed plug flows and very dilute disperse particle flows were found to be uniform. All other flow fields, with intermediate particle loadings, were characterized by the simultaneous presence of dense filamentary structures and dilute dispersions of particles. Typically, while operating with 0.5 mm glass beads, flows reached speeds of 60 meters per second in a period of 25 milliseconds. Two lofting configurations of the packed beds were set up. In the first configuration, the rapid depressurization of the interstitial bed fluid and the consequent initiation of bed expansion was examined. Bed expansion starts along horizontal fractures that partition the bed into slabs. While the bed is accelerating, particles rain down from the bottom surfaces of the slabs partitioning the fractures into bubbles with a characteristic honeycomb pattern. The bubbles eventually compete and the dominant ones prevail. The observed instability of the bottom surfaces of the slabs is analogous to the Rayleigh-Taylor instability observed in continuous media. The flow development in this configuration was not influenced by any wall effects. The second lofting configuration is a high-speed fluidization configuration. Here, the role of the fluid entering from below the bed, in continuing the bed expansion initiated by the rapid depressurization of the interstitial bed fluid, was examined. The bed expansion occurs along expanding and elongating bubbles and the bubble walls are stretched into dense filamentary structures. Beds initially stacked with a gradient in particle size or density or both showed drastic differences in response to fluidization. The morphology of the expanded flow field in all cases was essentially the same: nonuniform, interspersed with dense filamentary structures and dilute dispersions of particles. In the second lofting configuration, only the late stages of flow development were influenced by wall effects. Wall effects manifest as faster moving fluid along the walls and denser accumulation of flow structures towards the center of the channel. The bottom of the dusty flow is characterized by the presence of a tail; a concentric dense particle column formed by the accumulation of particles, initially present in the bottom regions of the flow. The tail terminates in a bulbous and streamlined bottom from which particles are slowly eroded by the coflowing fluid. A multi-transducer probe was installed in the dusty gas flow for making dynamic pressure measurements and for correlating observations with those made through extensive flow visualization.