Conditioned sampled LDA measurements of throughflow velocities inside two-dimensional fluidized bed bubbles

Abstract

An experimental investigation was carried out to measure the fluid flow through a bubble rising in a transparent two-dimensional fluidized bed using a HeNe laser-based fibre optic laser Doppler anemometer (LDA) system configured in the forward scatter mode. A conditioned sampling technique was developed to characterize the throughflow component (the component of fluid flow in a bubble, relative to the bubble, across a plane normal to the vertical axis of the bubble) in single isolated bubbles injected by a computer-actuated solenoid valve. The technique provided velocity information, along a given vertical plane, at each point in the bubble. The quality of the rising bubble was established by a photodiode-based detector, which triggered the LDA via a preconditioned signal if the bubble was clear, that is, free of raining particles. Amplitude discrimination was employed to differentiate between signals from the seed particles and the larger bed material (300 μm and 500 μm mean diameter glass beads). Detailed velocity measurements, for the first time, reveal that the throughflow velocity in two-dimensional beds increases linearly with increasing distance from the distributor, thereby enhancing the convective component in the interphase mass transfer process. Velocity and image analysis data reveal a dependence of the throughflow on the elongation of the bubble.