Bubble size measurement using wire-mesh sensors

Abstract

A wire-mesh sensor with a time resolution of 1.2 kHz was used to measure bubble size distributions in a gas-liquid flow. It is designed for a pipe of 51.2 mm diameter and consists of two electrode grids with 16 electrodes each, put in the flow direction behind each other. The local instantaneous electrical conductivity is directly measured between all pairs of crossing wires, a tomographic image reconstruction is not necessary. The resulting 16 × 16 sensitive points are equally distributed over the cross section. This resolution is sufficient to detect individual bubbles, which are imaged in several successive frames during their transition through the measuring plane. To investigate the influence on bubbles, a model of the sensor was tested in a transparent channel with a rectangular cross section of 50 × 50 mm at liquid velocities between 0 and 0.8 m/s. A comparison with high-speed video observations has shown that the sensor causes a significant fragmentation of the bubbles. Nevertheless, the measured signals still represent the structure of the two-phase flow before it is disturbed by the sensor. Bubble sizes can therefore be determined by integrating local instantaneous gas fractions over an area of the measuring points occupied by the bubble. Bubble size distributions are obtained by analysing large assemblies of bubbles. The method was applied to study the formation of slug flow along a vertical tube. The bubble size distributions obtained show the effect of coalescence as well as bubble fragmentation.