LDV measurements and analysis of gas and particulate phase velocity profiles in a vertical jet plume in a 2D bubbling fluidized bed.

Abstract

Fluidized beds are commonly used as chemical reactors and solid fuel combustors, where high-speed gas jets are employed to introduce reactants to the system. Quantitative gas and particulate phase velocity measurements are needed to characterize the transport phenomena of these jet plumes into the particulate emulsion. Two component Laser Doppler velocimetry (LDV) was used to investigate the gas and particulate phase velocities and resulting transport of a vertically injected gas jet plume in a two-dimensional bubbling bed. LDV measurements of this optically dense multiphase flow are challenging due to laser intensity fluctuations, which mix with the recorded burst frequencies. This problem was resolved by optimizing the Bragg Cell configuration and burst signal processing. The jet gas was seeded with ice crystals, and bursts from the bed particles and gas tracers were simultaneously acquired. These bursts were differentiated based on their intensity and coincidence to determine the gas and particulate phase velocities. The behavior of the single-phase gas jet in the empty bed was examined. The self-similar velocity profile growth was consistent the development of a free two-dimensional turbulent jet. The bed was filled with high-density polyethylene microspheres. The gas and particulate phase velocity profiles of the jet were measured. Similarity profiles are presented and the resulting void fraction, mass flow and momentum transport calculations are analyzed. The effect of fluidization velocity on the jet dynamics was examined and was shown to influence the rate of mass entrainment into the jet plume and the momentum exchange between the phases. The ratio of the maximum gas and particulate phase velocities appeared to follow a similar trend for all test conditions. Hence, the ratio of the drag force coupling to the particulate or gas phase momentum is constant at a given axial location from the jet inlet. Additionally, the mass-averaged velocity ratio for the two phases, which is equivalent to the ratio of the centerline velocities, develops in a similar fashion. This phenomenon is observed for high-speed gas jets in bubbling beds with spherical particle entrainment when <italic>Re_p</italic> > 1,000 so that <italic>C_D</italic> &sim; 0.4.