Liqid PIV measurements around a single gas slug rising through stagnant liquid in vertical pipes

Abstract

In this work, the Particle Image Velocimetry (PIV) technique is used to characterise the flow around a single Taylor bubble rising through a vertical column of stagnant liquid. The experimental facility and its operating principle are described. The gas slug velocity is determined from the signals of two photocells that receive light from two laser diodes placed opposite them in the column. The use of fluorescent PIV particles together with an optical filter allows suppressing intense reflections at the liquid/gas interface. Details concerning the PCO CCD camera and Nd: YAG laser synchronisation are presented. The gas slug surface co-ordinates are not accurately determined from PIV images, due to spurious optical effects that occur at the bubble interface when cross illuminated by a laser sheet. These important optical effects are investigated in detail by means of a silicon model of the liquid around the bubble with the same fluorescent PIV particles embedded. An explanation for the spurious optical effects is formulated. It is argued that one of the methods that is suitable to reliably determine the bubble shape is the shadow detection technique. PIV results of the liquid flow in the rear and around the nose of the gas slug are presented. PIV measurements at the liquid film surrounding the gas slug, for laminar liquid film, are presented. The velocity profiles in the liquid film around the bubble are compared with theoretical values given by the falling film theory. The encountered differences are less than 5%. The shear stress profile in the liquid film around a gas slug is presented. Comparison between the theoretical and experimental results shows good agreement. The experimental and theoretical values match within 5 % in the liquid film at a distance from the wall of less than 2 mm; for higher distances the error grows until 15 %. Although optical problems are present near the gas/liquid interface, the results are encouraging.