An experimental study of the liquid entry region during flooding is reported for a 50.8 mm i.d. vertical tube. The liquid entry device was a porous plastic tube into which film thickness measurement probes were mounted. Simultaneous measurements of the time-varying film thickness were made at locations below, inside and above the porous inlet. The weight of the evidence from cross-covariance analysis of these data does not appear to support models which assume upward propagation of waves above the feed point as the mechanism by which steady-state flooding takes place. Neither was there evidence that waves grow large enough to bridge the tube. Instead, the liquid film appears to undergo a transition from countercurrent to cocurrent flow inside the liquid feed. The pressure drop across the length of the porous tube was also measured and found to be significantly higher than that previously reported at locations above the liquid inlet. In part II [Int. J. Multiphase Flow20, 235–247 (1994)] a film model based on these data is explored.

A new flooding mechanism is explored for porous tube feed systems based on the concept that flow reversal takes place inside the liquid film at the feed location. The model is developed, assuming that the flow field in the liquid film at any axial position can be represented by the time-average film thickness existing there. Thus, it makes the radical assumption that the interfacial waves exert their influence only by modifying the shear stress in the gas flowing over the film. A 2-D numerical simulation was constructed which solved for both the flow field inside the film and the shape of the free interface. The numerical results show that this film mechanism is a viable approach for modeling flooding, as reasonable values of the interfacial shear stress from the upward gas flow can reproduce the experimentally measured film thickness. This work also suggests that the pressure gradient developed inside the entry is only partially due to interfacial shear. Gas-phase accelerations, due to the wave motion, contribute the remainder of the measured pressure gradient.

The structure of thin, wavy falling films was studied to evaluate whether the random-appearing wave structure is a result of deterministic chaos or a purely stochastic process. The time-varying film thickness was obtained at different spatial locations near the point of wave inception for flow rates in the range of Re = 3-10. Under all conditions the wave structure was aperiodic in nature and displayed none of the known transitions to chaos. However, the power spectra followed an exponential decay law at high frequencies that is characteristic of chaotic systems. The estimated attractor dimension, used to characterize the complexity of a chaotic system, was much higher than those of known model chaotic systems. It is demonstrated that these high values could be explained due to small levels of noise present in experimental situations. Since experimental data are seldom noise free, a basic limitation in applying these methods to experimental measurements is demonstrated.