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.