Abstract

The discharge of a two-phase flow from a stratified region through single or multiple branches is an important process in many industrial applications including the pumping of fluid from storage tanks, shell-and-tube heat exchangers, and the fluid flow through small breaks in cooling channels of nuclear reactors. Knowledge of the flow phenomena and flow structure involved during the onset of gas entrainments (OGE) in branches is essential for the design and/or performance prediction of such thermal systems. In the present investigation, extensive data were generated for the two-phase flow structure at the onset of gas entrainment from an air-water stratified region through small branches (d = 6.35 mm) over a wide range of Froude numbers (0 to 100). The test sections were in close dimensional resemblance with that of a CANDU header-feeder system, with branches mounted at orientation angles of 0, 45 and 90 degrees from the horizontal. Three groups of new data were generated for single discharge, dual discharge and triple discharge configurations. The Particle Image Velocimetry (PIV) was used to provide detailed measurements of the two-phase flow field. In each of these measurements, the critical height at the onset of entrainment was first achieved, and the volume of interest close to the branch-header junctions was then determined and divided by a number of horizontal image planes. Each image plane required a separate spatial and temporal calibration for PIV measurements. The vorticity profile, stream lines, flow field development and coherent structure, were presented over a wide range of operating conditions. A theoretical analysis for the onset of gas entrainment in a single downward discharge, from a stratified gas-liquid region, was developed. The discharge was modeled as a point-sink and Kelvin-Laplace's equation was used to incorporate surface tension effects. The dip geometry was experimentally investigated and a correlation was developed relating the dip radius of curvature to the discharge Froude number. The correlation was used in conjunction with the theoretical model. It was found that the predicted critical height demonstrated a good agreement with experimental data. The inclusion of surface tension improved the model's capability to predict the critical height, particularly at discharge Froude numbers below one. The single-discharge model was then extended to dual and triple discharge cases, with considering the branches as point sinks and two-dimensional slots. The results of dual and triple discharges were found to be a function of mass flow rate through the branches, and the position of the secondary branch (maintaining liquid phase flow only) with respect to the primary branch position (at which OGE occurs) and the angle between the branches. The present analysis applies to any two immiscible fluids with the term "gas entrainment" referring to the appearance of the lighter fluid through the upper branch.