Interrogating the effect of 90° bends on air–silicone oil flows using advanced instrumentation

When gas/liquid mixtures flow around a bend they are subjected to forces additional to those encountered in a straight pipe. The behaviour of the flows at the inlet and outlet of the bend depends on the orientation of the pipes. Air/silicone oil flows around a 90° bend have been investigated using advanced instrumentation: Electrical Capacitance Tomography (ECT), Wire Mesh Sensor Tomography (WMS) and high-speed video. The first two provide time and cross-sectionally resolved data on void fraction. ECT probes were mounted 10 diameters upstream of the bend whilst WMS was positioned either immediately upstream or immediately downstream of the bend. The downstream pipe was maintained horizontal whilst the upstream pipe was mounted either vertically or horizontally. The bend (R/D=2.3) was made of transparent acrylic resin. From an analysis of the output from the tomography equipment, flow patterns were identified using both the reconstructed images as well as the characteristic signatures of Probability Density Function (PDF) plots of the time series of cross-sectionally averaged void fraction as suggested by Costigan and Whalley (1996). The superficial velocities of the air ranged from 0.05 to 4.73 m/s and for the silicone oil from 0.05 to 0.38 m/s. Bubble/spherical cap, slug, unstable slug and churn flows were observed before the bend for the vertical pipe and plug, slug, stratified wavy and annular flows when the pipe was horizontal. Bubble, stratified wavy, slug, semi-annular and annular flows are seen after the bend for the vertical 90° bend whilst for the horizontal 90° bend, the flow patterns remained the same as before the bend. Flow patterns for the vertical and horizontal 90° bends are shown on the diagram of the gas superficial velocity versus liquid superficial velocity. These results are confirmed by the high-speed videos taken around the bend. A previously proposed criterion, to determine stratification after the bend, based on a modified Froude number have been shown to be valid for a liquid different from that tested in the original paper.