Experimental studies on air-water two-phase flow through a 90-degree vertical elbow

Geometric effects of a 90-Degree vertical elbow and flow configuration in two-phase flow are investigated. It is shown that the elbow makes a significant effect on the transport characteristics of two-phase flow, which includes the changes in interfacial structures, bubble interaction mechanisms and flow regime transition. The present study is performed in a test section consisting of vertical and horizontal sections made out of 50.8 mm inner diameter acrylic pipes connected via 90-degree vertical elbows. The length of each of the vertical and horizontal sections is 4 meters and 10 meters, yielding a development length of 66 and 188 respectively. There are a total of 24 local measurement locations available along the axial direction of the test section. The pressure measurements are made over a wide range of flow conditions to characterize the frictional pressure drop and minor loss across the vertical elbows in both single-phase and two-phase flow conditions. A modified two-phase flow regime map accounting for the vertical elbow in the horizontal two-phase flow are obtained based on extensive flow visualization study performed using a high speed camera. Based on the modified flow regime maps, a set of test conditions, all within or near bubbly flow regime are chosen. The four-sensor conductivity probe is used to measure the local two-phase flow parameters along pipe radius at various axial locations across the elbow. The measured local data include: void fraction, bubble velocity, interfacial area concentration, bubble frequency and bubble Sauter mean diameter. The bubble distribution in vertical upward flow is found to be axisymmetric and hence local data is collected only along the half diameter of the tube cross-section. However, at measurement locations in the horizontal section, downstream of the elbow, the flow becomes asymmetric. In order to capture the elbow effect, local data is taken along the entire diameter of the tube cross-section and at eight different azimuthal angles with an interval of 22.5 degree by rotating the measurement port. This yields a total of 120 local data points across the tube cross-section at the given axial position. It is found, at measurement location immediately downstream of the elbow, the bubbles are distributed in two distinct streams along the horizontal radius of the tube cross-section. This leads to a dual-peak in the local void fraction and interfacial area concentration profiles along the horizontal radius of the tube cross-section. The elbow effect decays further downstream of the elbow and bubbles rise up due to buoyancy causing the local profiles of void fraction and interfacial area concentration to peak in the top half of the tube cross-section. The data for axial transport of the area-averaged void fraction and interfacial area concentration indicates that the 90-degree vertical elbow promotes bubble disintegration, which confirms the flow visualization study.