Wall Shear Effect on Bubble Formation in Turbulent Flows

Abstract

Experimental bubble formation studies have been undertaken on air injection through a porous plate into fully developed turbulent salt-water flows in vertical rectangular channels of varying aspect ratio. This work was aimed at better understanding the effects of variables including pore size, channel size, channel pressure and gas-to-liquid mass flow ratio on the bubble size and bubble standard deviation. In this research, channel pressure was varied from 138–414 kPa (20–60 psi), the hydraulic channel diameter ranged from 1.82–10 mm which corresponded to aspect ratios from 10–1, mean porous plate pore sizes of 0.2 and 100 micron were used (media grade 0.2, 100) and liquid Reynolds numbers from 3400–30,000 were studied. Image processing techniques were used to measure bubble diameters from digital images taken of the bubbly flows which were back-lit via a high-intensity strobe light. The mean bubble diameters produced ranged from 106–1250 microns. Results show that the combined effects of channel pressure, channel geometry and flow rate on average bubble diameter can largely be captured by using wall shear stress at the air injection site whose calculation was based on previously published literature. Bubble diameter and standard deviation are reduced nonlinearly at higher wall shear stress and it is shown that for the bubbly flow regime the gas-to-liquid mass flow ratio has little effect on average bubble size over the conditions of this study.Copyright © 2013 by ASME