Experimental method and diagnostics

The experimental apparatus consists of a vertical channel flow into which bubbles are injected. A vertical liquid flow was introduced to ensure that a hot-film system could be used to measure velocity at points. The vertical channel is of height 3.1 m and 0.3 x 0.15 m2 cross-sectional area. A 576 capillary tube network was placed at the entrance of the channel to generate bubbles. The tubes are uniformly spaced on a grid and are distributed across the whole section of the channel. 

Measurements of the vertical velocity, far from the injection site, were made at a fixed position using a hot-film system (e.g. [108]), from which the bubble diameter and rise speed could be calculated. The accuracy of the measurements have been extensively tested at low void fractions using LDV and optical methods. The time-series of the velocity measurements were converted into spatial information using a Taylor hypothesis that a mean flow advects disturbances past the probe. Examples of the velocity time-series is shown in Fig. 7.5. A more thorough description of the experimental technique and diagnostic tools applied to the data is given in [358], 

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