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.

Date of Award	1976
Original language	English