Gas–liquid flow of sub-millimeter bubbles at low void fractions: Experimental study of bubble size distribution and void fraction

Abstract

This work studies gas–liquid flow of small bubbles (<1 mm) at low void fractions (<10−1) that is encountered in human bloodstream during Decompression Sickness and is also relevant to two-phase applications such as flow boiling in macro-channels. Two fundamental parameters are experimentally investigated: Bubble Size Distribution (BSD) and void fraction. Experiments are conducted in co-current upward bubbly flow. Water and blood simulant are used as test liquids, while bubble size is controlled using prescribed surfactant (SDS) concentrations. BSDs are determined employing digital image analysis of bubbly flow images captured at three radial positions across the flow cross-section. Volumetric and cross-sectional area averaged void fraction is measured at three axial locations along the flow by Differential Pressure (ΔP) and Electrical Resistance Tomography (ERT), respectively. BSDs are well-fitted by the log-normal distribution. ERT and ΔP measurements are in fair agreement, with void fraction being practically equal along the flow. The influence of gas/liquid phase velocities and surfactant concentration on the measured void fraction and BSDs’ average value and width is discussed in detail. Interestingly, high SDS concentration in blood simulant results in the formation of bubble clusters, whose role on the examined parameters is investigated.