Local Liquid Velocity Measurements in Horizontal, Annular Two-Phase Flow

Abstract

Two-phase annular flow is commonly used in both commercial and industrial heat transfer; however, we do not yet possess a thorough understanding of the nature of the fluid. Most analytical annular two-phase models are based on a relationship between the liquid film thickness, liquid film mass flux, and the axial pressure gradient or interfacial shear stress. The film thickness calculated from these models can then be utilized to determine the heat transfer coefficient of the flow. Although they are specific to certain flow regimes and fluids, empirical models remain more accurate than these analytical models. The key to understanding these flows lies with the liquid film. Therefore, to better understand the pressure drop and heat transfer of annular two-phase flow, this study involves the development of local, liquid velocity measurement techniques and their application to horizontal, wavy-annular two-phase flow. Two techniques, Bubble Streak Tracking (BST) and Thin Film Particle Image Velocimetry (TFPIV), have been developed in this study. Utilizing naturally occurring bubbles within the liquid film, the BST technique determines the liquid velocity by measuring reflected light streaks from the bubbles. A three-colored LED array creates directionally unambiguous streaks, while a strobe illuminates interfacial features that affect the liquid velocity. The TFPIV technique applies a typical micro-PIV system to a macroscopic flow with the addition of a non-trivial image processing algorithm. This algorithm successfully overcomes the image noise that occurs when applying PIV to a two-phase, thin film. Although difficulties arise when processing the BST data, the results of the BST and TFPIV methods are comparable, making BST an economical alternative to TFPIV for calculating liquid film velocities. In this study, these two techniques are applied to horizontal, two-phase flow. These measurements were made in the wavy, wavy-annular, and annular regimes to investigate the mechanism responsible for distributing the liquid film around the tube circumference. The data imply that two of the four major theories are incorrect. While experiments examining the remaining two mechanisms are inconclusive, images from both techniques suggest the waves are responsible for distributing the liquid film. Lastly, the TFPIV method was used to measure time-averaged velocity profiles within the liquid film of a wavy-annular flow: the first profile measurement of a liquid film at this scale. While the profile at the bottom of the tube is similar to the universal velocity profile utilized in annular two-phase models, the profile at the side and top of the tube exhibit a much different behavior.