Investigation of Two-Phase Flow in Rectangular Micro-Channels

This study addresses air-water, two-phase flows in micro-channels fabricated on poly-methyl-methacrylate (PMMA) with walls that are partially non-wetting (typical static contact angle 65° in stock form) and not molecularly smooth. Two different types of chips were prepared: Micro-milled micro-channels of aspect ratios 1, 2 and 3 with fixed hydraulic diameter on PMMA and micro-channels of unity aspect ratio replicated using hot embossing of PMMA with a micro-milled brass mold insert. Flow-maps obtained using the same gas-liquid injection geometry and method for the three aspect ratio micro-channels are presented, and regime boundaries are compared with those found by other investigations. The results indicate that the bubbly flow regime boundary is shifted to higher liquid and/or lower gas superficial velocities for the higher aspect ratio channels, while transition to the Annular and Annular-Dry regimes remains the same to within experimental uncertainty. The emphasis of what is presented is on the Segmented flow regime. Regular and irregular Segmented flow regimes of three types are assessed on the basis of the statistical variation in the associated phase length scales from flow observations over a substantial channel length. Comparison between results of the two different injection geometries and micro-channel manufacturing techniques indicate that feedback effects are a significant but not the only cause of segmented flow irregularity. The variability in the size of the liquid plug separating gas bubbles in Segmented flow is found to be substantially higher than that of the bubbles even when the flow is regular (low variability of bubble size). The average bubble length associated with a part of Segmented flows, regular and irregular alike is shown to scale approximately with the capillary number to the 2/3 power (liquid volumetric flow ratio to the −2/3). Irregular Segmented flow is favored by higher liquid superficial velocities, lower liquid volumetric flow ratios and lower channel aspect ratios. Of the three aspect ratios examined, the microchannel with aspect-ratio 3 displayed the broadest window of regular Segmented flow. Two-phase flow pressure drop was measured for test channels of unity aspect ratio. Each flow regime identified on the basis of topological observations is associated with different trends of the pressure drop variation with respect to volumetric flow ratio.