An Ultrasonic Instrumentation System for the Study of Vapour Bubble Formation and Collapse in Sodium

Abstract

An ultrasonic pulse-echo system has been developed for the study of the rapid growth and collapse of a single vapour bubble in a sodium pool under superheating and subcooling conditions up to 900°C. It is also capable of measuring the liquid level and detecting bulk boiling. The system controls the sequential operation of a cavitation device, which initiates bubble growth at a known site within the liquid body, and a specially-designed high-speed valve, which produces a high-pressure pulse to collapse the bubble. The ultrasound source is a pulse-excited 5 MHz piezoelectric transducer operating at a high p.r.f.; rapid electronic processing of the change in transit time of the bubble echoes follows the bubble growth. The transducer is coupled to the containment vessel through a steel diaphragm and a cooled liquid-filled tube; the former is part of the wall of the vessel and the latter limits the upper working temperature of the transducer to 280°C. Experiments with Woods metal at 400°C and liquid tin at 900°C encountered all the high-temperature problems anticipated with the sodium pool, and revealed that, provided the cooling tube is vertical, the controlled temperature gradient within it does not degrade the echo signal; distortion of the pulses by the diaphragm was avoided by choosing optimum thicknesses of odd multiples of the quarter wavelength, and by using a suitably shaped excitation pulse. The accuracy of bubble size measurement was verified by direct visual observation of air bubbles in a water-filled chamber identical to the sodium pool. Using high-speed ciné photography, the ultrasonic output was simultaneously superimposed on the film for direct comparison. The G@ results revealed that the system can give at least five observations of the bubble in 1 msec. A sodium pool has been built at A.E.R.E. Harwell. Specifications based on the results of the experiments described here have been applied to the pool.