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 vertic al and horizontal columns. It is found that the trickle-pulse transiti on, as defined in previous studies, corresponds to conditions where tr aveling disturbances finally become measurable, not the conditions at which infinitesimal disturbances begin to grow. Observations demonstra te that even if the liquid and gas are uniformly distributed initially , segregated, vertical flowing regions with higher or lower than avera ge liquid holdup form after a short distance. Horizontal packed bed ex periments, designed to study how regions of differing liquid holdup in teract, 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 thr ough liquid-filled pores. Once infiltration is significant enough to f orm a third ''bubbly'' phase, traveling wave instabilities form and gr ow into pulses if sufficient column length is available. A three-layer Kelvin-Helmholtz stability model is used to interpret the growth of d isturbances in horizontal flows. Video observations of small-scale eve nts in the bed failed to detect significant correlations between diffe rent 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 sign ificantly affects the frequency of disturbances.