Nonintrusive Optical Validation of Two-Phase Flow Regimes in a Small-Diameter Tube

Abstract

While the majority of studies examining the relationship between flow regime and thermofluid performance rely on subjective flow regime determination, objective techniques are needed to more firmly establish the nature and repeatability of these phenomena. This study describes a nonintrusive optical flow regime characterization methodology used to study horizontal, adiabatic, two-phase flow of water–water vapor and water–nitrogen gas in small (8.84 mm) diameter tubes. The method relies on shining a fiber-optic light source through the top of a borosilicate glass tube at the outlet of a smooth copper tube, using a CMOS camera to capture light rings resulting from total internal reflection at the liquid–vapor interface, and extracting a film thickness profile from successive images. Using these unique temporally varying film profiles, quantitative identification measures were developed for the primary flow regimes, including the ability to explain and quantify the more subtle transitions that exist between dominant regimes as the two-phase flows progress from saturated liquid to qualities of 0.32. Application of this methodology has shown the Taitel–Dukler, Ullmann–Brauner, and Wojtan et al. phenomenological flow regime maps to capture the salient features and transition boundaries, with varying accuracy, for small-diameter two-phase flow in a mass flux range of 15 to 230 kg/m2-s.