Bubbly flows appear in many different industrial fields and the measurement of bubble sizes is crucial for understanding phase interactions. The ultrasound pulse echo can be used for a non-intrusive measurement of the bubble surface position in one dimension even when there is no optical access to the bubble. A simultaneous measurement from two opposing directions gives the bubble size but has been performed only on single bubbles. This work applies the tracking technique, which allows a simultaneous measurement of multiple bubbles. The performance of the bubble size measurement was tested experimentally by comparing ultrasonic results with the sizes measured by a high-speed camera and also with metal cylinders. Possible sources of measurement uncertainty were analysed and discussed. The tested range of bubble sizes was up to 10 mm and the void fraction was lower than 1.25%.
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