Mechanical Inhibition of Foam Formation via a Rotating Nozzle
Entry by Emily Redston, AP 225, Fall 2011
Work in progress
Mechanical Inhibition of Foam Formation via a Rotating Nozzle, by W. D. Ristenpart, A. G. Bick, E. A. van Nierop, and H. A. Stone, J Fluid Eng-T Asme 133 (4) (2011).
bubbles, capillarity, confined flow, drops, foams, nozzles, rotational flow, two-phase flow
Many industrial processes involve a step where two or more liquid streams are combined in one vessel. If one of the liquids is poured, sprayed, or dripped into liquid already in the vessel, then often times air is entrained upon impact, and the consequent bubbles form a foamy layer. Generally these foams are an unintended and unwanted byproduct of the process because they may interfere with unit operations, decrease process efficiency, increase process time, and lead to additional process defects. Consequently, a great number of commercial chemical additives have been developed to minimize the impact of foams. Anti-foaming agents are added to prevent foam formation. Unfortunately, these additive chemicals have several drawbacks: they may contaminate the final product, pose environmental disposal problems, and increase the overall process cost and complexity. Non-chemical strategies are thus desirable. Although some work has suggested that mechanical or ultrasonic vibrations help disrupt foams after they form, to date there has been no demonstration of a nonchemical technique that prevents foam formation in the first place.
In this paper, the authors present a simple mechanical apparatus that, for appropriate flow rates, significantly reduces the amount of foam generated when a liquid is sprayed into a container. Specifically, they demonstrated a technique to substantially prevent bubble entrainment due to what they refer to as “multidrop” impacts. Multidrop bubble entrainment occurs when two successive drops impact a liquid-air interface within a critical time interval. We recently performed systematic experiments that demonstrated the critical time interval is commensurate with the time required for an impact crater formed by the first drop to close by capillarity (approximately 5 ms for millimeter scale water droplets). Note that the multidrop regime is distinct from the “regular” or “irregular” entrainment regimes exhibited by single droplets impacting at sufficiently high velocities. The key implication here is that bubble formation, and hence foam formation, can be minimized in the multidrop regime simply by ensuring that no two droplets impact the air-liquid interface at the same location within the critical time interval. Building on this observation, here we report a design for a rotating nozzle that prevents successive collocated impacts, thereby minimizing bubble entrainment. We demonstrate that a lab-scale prototype can reduce the volume of foam formed by as much as 95% for a given flow-rate, provided the angular velocity of the nozzle is sufficiently high.