An Optical Approach for Quantitative Characterization of Slug Bubble Interface Profiles in a Two-Phase Microchannel Flow

A new measurement technique is developed for quantitatively mapping the liquid-gas interface profiles of air bubbles in an adiabatic microchannel slug flow environment. Water seeded with 0.5 mu m-diameter fluorescent polystyrene particles is pumped through a single acrylic microchannel of 500 mu m x 500 mu m square cross section. A periodic slug flow is achieved by the controlled injection of air into the channel. Particles are constrained to the liquid phase, and their distribution in the flow is visualized through an optical microscope in an epifluorescent configuration with pulsed laser illumination to resolve the instantaneous liquid-gas interface profile to within +/- 2.8 mu m in the focal plane. This approach is able to identify the interface profile within individual focal planes at various depths within the channel, unlike conventional backlit optical profile detection approaches that cart only resolve the interface at the midplane. A similar particle-tracking technique was previously demonstrated for interface reconstruction in annular flows; however, the additional noise within images due to the reflection and refraction of background light at the compound-curvature interfaces characteristic of slug bubbles requires texture-based image analysis to obtain interface profiles. The varying interface profile of the slug bubbles in the streamwise direction also greatly complicates the tracking procedures for achieving a three-dimensional reconstruction of slug bubbles based on the measured two-dimensional interface profiles, which requires spatial alignment of the streamwise position of liquid-vapor interfaces realized at varying depths within the channel. This is addressed during reconstruction by using the measured mid-plane slug bubble cap curvature to assign the relative streamwise positions of interface profiles obtained at other measured depths. The characterization of two different selected bubble morphologies presented here demonstrates a critical improvement in metrological capability which can provide greater insight into microchannel flow phenomena in the slug-flow regime. (C) 2015 Elsevier Ltd. All rights reserved.