Bubble size measurement using wire-mesh sensors

A wire-mesh sensor with a time resolution of 1.2 kHz was used to measure bu bble 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 eac h, put in the flow direction behind each other. The local instantaneous ele ctrical conductivity is directly measured between all pairs of crossing wir es, a tomographic image reconstruction is not necessary. The resulting 16 x 16 sensitive points are equally distributed over the cross section. This r esolution is sufficient to detect individual bubbles, which are imaged in s everal successive frames during their transition through the measuring plan e. To investigate the influence on bubbles, a model of the sensor was teste d in a transparent channel with a rectangular cross section of 50 x 50 rum at liquid velocities between 0 and 0.8 m/s. A comparison with high-speed vi deo observations has shown that the sensor causes a significant fragmentati on of the bubbles. Nevertheless, the measured signals still represent the s tructure of the two-phase flow before it is disturbed by the sensor. Bubble sizes can therefore be determined by integrating local instantaneous gas f ractions over an area of the measuring points occupied by the bubble. Bubbl e size distributions are obtained by analysing large assemblies of bubbles. The method was applied to study the formation of slug flow along a vertica l tube. The bubble size distributions obtained show the effect of coalescen ce as well as bubble fragmentation. (C) 2001 Elsevier Science Ltd. All righ ts reserved.