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Parametric effects of bubble-microcantilver impacts in a confined channel

by

Matthew Stegmeir, Ellen Longmire, and Susan Mantell

in

Physics of Fluids, 20, 2008.

Category: Journal Article

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Abstract:

In this study, we have investigated the impact of bubbles with microcantilever obstacles in a confined channel. Static cantilevers of thickness 100 _m were considered. Cantilevers were mounted perpendicular to the mean flow in a vertically oriented channel with width of 2 mm, span of 10 mm, and length of 585 mm. Steady, fully developed upward flows with channel Reynolds numbers based on mean fluid velocity and hydraulic diameter of 800–2400 were considered. Bubbles of diameter of 400–2000 _m were introduced upstream of the test section, and impacts were observed by using a microscope equipped with a high frame rate camera. Observations were made in planes normal to the length of cantilevers backlit by using white light. Liquid density ___, interfacial surface tension ___, bubble velocity immediately prior to impact _Ububble_, bubble diameter _D_, obstacle thickness _t_, impact offset distance _d_, channel width _w_, and beam location are all potential influences on the result of bubble-beam impacts. Five dimensionless combinations of these quantities: Weber number _We=_Ububble 2 D__, impact offset _B=2dD where d is the impact offset distance_, bubble diameter relative to beam thickness _Dt_, bubble diameter relative to channel width _Dw_, and beam offset from channel centerline relative to channel size _O=offsetw, where offset is the distance the beam is displaced from the channel centerline_, were considered in the following ranges: 0_We_12, 8_Dt_90, B_1, Dw_1, and 0_O_0.1. Multiple types of interactions ranging from bouncing with little deformation to wrapping with substantial deformation to splitting into two were observed. Splitting required a minimum Weber number to occur, which was observed to be independent of Dt for all cases considered. Impact offset B and Dw combined to affect impact outcome. For Dw_0.6, the Weber number required for splitting was observed to increase with offset B. As bubble diameter D approached the channel width w _0.6_Dw_0.75_ under laminar flow conditions, channel gradient effects could generate a substantial lift force toward the center of the channel for bubbles approaching offset from the channel centerline. This lift force caused bubbles to cross from one side of the beam to the other; this type of interaction increased the likelihood of splitting and resulted in a number of low-We, high B splitting cases. Bubble impacts with channel walls reduced this phenomenon for Dw_0.75.

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