Parametric effects of bubble-microcantilever impacts in a confined channel

Abstract

In this study, we have investigated the impact of bubbles with microcantilever obstacles in a confined channel. Static cantilevers of thickness 100μm were considered. Cantilevers were mounted perpendicular to the mean flow in a vertically oriented channel with width of 2mm, span of 10mm, and length of 585mm. 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 400–2000μm 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 (ρ), interfacial surface tension (σ), 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=ρUbubble2D∕σ), 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.