How do the liquid properties affect the entrapment of bubbles in gas sheared liquid flows?

Abstract

A large number of industries use fossil fuels for the purpose of cooling and lubrication utilizing a gas stream that acts as a shearing force on the liquid surface. Decreasing the carbon footprint of these processes needs an improved understanding of the effect of liquid properties. To this end this study investigates experimentally the effect of varying surface tension and liquid viscosity on the bubble generation and film statistics in a gas-sheared liquid flow. The experiments were conducted in a horizontal rectangular channel using high speed imaging in conjunction with the Brightness-Based Laser-induced Fluorescence technique (BBLIF) to measure film thickness over an area with a spatial resolution of 40 μm and a temporal resolution of 10 kHz. Two butanol–water solutions were used to give reduced surface tensions (namely 0.049 N/m and 0.04 N/m) compared to our previously-studied water only value. Also, two glycerol-water solutions were chosen to yield surface tension values closer to those of an oil of interest (namely 1.4 cP and 1.9 cP). The results show that compared to water, an increase in film thickness and a decrease in the disturbance wave velocity was observed with either the reduction in the surface tension or increase in the liquid viscosity. The frequency and size of the waves were found to be dominated by the liquid surface tension. Moreover, surface tension was found to have more of an effect on aeration as compared to viscosity. The bubble velocity was found to decrease with increase in viscosity, while it increases with decrease in surface tension. It is suggested that these changes would affect heat transfer and hence should be studied in more detail.