ORIGIN OF DISTURBANCES IN COCURRENT GAS-LIQUID PACKED-BED FLOWS

Visual, video pressure, and conductance techniques were used to study time-varying disturbances in cocurrent flow in packed beds with vertical and horizontal columns. It is found that the trickle-pulse transition, as defined in previous studies, corresponds to conditions where traveling disturbances finally become measurable, not the conditions at which infinitesimal disturbances begin to grow. Observations demonstrate that even if the liquid and gas are uniformly distributed initially, segregated, vertical flowing regions with higher or lower than average liquid holdup form after a short distance. Horizontal packed bed experiments, designed to study how regions of differing liquid holdup interact, indicate that the first type of disturbance is infiltration of gas into the liquid region. A simple model suggests that infiltration occurs if the pressure drop exceeds a value necessary to push gas through liquid-filled pores. Once infiltration is significant enough to form a third ''bubbly'' phase, traveling wave instabilities form and grow into pulses if sufficient column length is available. A three-layer Kelvin-Helmholtz stability model is used to interpret the growth of disturbances in horizontal flows. Video observations of small-scale events in the bed failed to detect significant correlations between different regions. Thus it should be possible to describe flow behavior in these systems with volume-averaged equations, as long as the presence of segregated regions is considered. Column diameter or thickness significantly affects the frequency of disturbances.