Near-wall flow response to large air bubbles rising in inclined water channels

Abstract

The two-strip electrodiffusion probe is employed to investigate the wall shear rate induced by large air bubbles rising in stagnant or coflowing water within inclined rectangular channels. Synchronized video recordings of bubble movements captured by a high-speed camera provide additional information on the bubble shapes and terminal velocities. The measurements are carried-out in a channel with easily adaptable geometry (three heights and various widths) over a wide range of operation parameters (air volumes and liquid velocities) and inclination angles (from horizontal to vertical arrangement). The main objective of this experimental study is to elucidate the influence of individual operating parameters on the bubble-induced wall shear rate. The typical profile of wall shear rate measured at the center of a flat channel can be characterized by a positive peak at the bubble front location, a negative plateau corresponding to the reverse flow in a liquid film around the bubble, and highly fluctuating values in a wake behind the bubble. Just as the large bubbles in flat channels exhibit a close similarity in their frontal shapes, so do the wall shear rate profiles induced by differently sized bubbles. The magnitude and profile of measured wall shear rate is found to be controlled primarily by the distance between two opposite walls squeezing the rising bubble, thus in flat channels by the channel height. By contrast, widening the channel has no significant effect, even though it contributes to a significant increase in the bubble rise velocity. When the channel is tilted, the rising bubbles are pushed towards the roof wall and thus two distinct wall shear rate profiles are measured at the opposite walls of the channel. The liquid coflow then contributes positively not only to the bubble velocity but also to the magnitude of wall shear rate in the near-wall flow region.