Bubble size measurement using wire-mesh sensors

A wire-mesh sensor with a time resolution of 1.2 kHz is used to measure bubble size distributions in a gas-liquid flow. The sensor consists of two electrode grids with 16 electrodes each, put in 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 x 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 its transition through the measuring plane of the sensor. A comparison with high-speed video observations has shown that the sensor causes a significant fragmentation of 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 measuring points occupied by the bubble. Bubble size distributions can be obtained if large assemblies of bubbles are analysed. The method was applied to study the establishing slug flow along a vertical tube. The obtained bubble size distributions show the effect of coalescence as well as bubble fragmentation.