Characteristics of the Liquid Film in Horizontal Two-Phase Flow

Horizontal annular two-phase flow is a very important condition in many common applications, including power generation, air-conditioning, and refrigeration. The purpose of this investigation is to present a broad survey of detailed liquid film thickness results for a variety of geometries for which film thickness data is not currently available. Measurements for round tubes, square tubes in two orientations, and a triangle tube in two orientations are presented. Beyond the fundamental importance of these data sets, the results are used to show that the base liquid layer tends to behave in a manner analogous to wall-bounded turbulent single-phase flow; that a limiting film thickness exists below which the wall dries out; and that pressure drop can be predicted for all geometries using vertical flow interfacial shear correlations. Flow through the square tubes shows strong evidence of the impact of secondary flows on the liquid profile, and a theory for the origins of secondary flows under two-phase annular conditions is developed. Another important class of tubing analyzed are tubes with 0.2 mm microgrooves of varying helix angles in the walls. Film thickness data indicate that these grooves act to rotate and redistribute the film, providing a basis for understanding the heat transfer enhancement these types of tubes exhibit. An experimental study of the effects of the angle and number of grooves is also presented. Key findings show that relatively few grooves are necessary to redistribute the film and that the grooves act on the film in such -away as to increase the surface fluctuations and lower the mean film thickness, both of which could have significant impact on heat transfer. Finally, a new diagnostic technique, thin film particle image velocimetry, is described with results providing evidence for strong upflows of liquid during the passage of waves and relatively smooth, draining flows in between. Together, the results of this study have enhanced the understanding of two-phase annular flow and have laid the groundwork for improved models to predict heat and mass transfer in systems important to daily life.Air Conditioning and Refrigeration Project 11