Ultrasonic measurements of the motion of gas bubbles rising in a
liquid metal
The ultrasonic detection of argon bubbles rising in a liquid metal caused by buoyancy forces is the topic of this paper. The metal is an eutectic alloy of In, Ga and Sn having a melting point of about 283,7°K (+10,5°C). Therefore, it is liquid at normal laboratory temperatures. Bubbles can be generated by different sparkling facilities on the bottom of circular-cylindrical vessels consisting of stainless steel, mineral or plastical glass.
The aim of the investigations is the determination of the bubble detaching frequencies and the estimation of the void and bubble size distributions over the cross section of the vessels at different levels above the vessel bottom as functions of the typ of the sparkling facility.
For the experimental determination of these two-phase flow parameters the ultrasonic pulse-echo method well known from the non-destructive material testing technology is used. A piezoceramic narrow band straight-beam probe having an ultrasonic resonance frequency of 15 MHz transmits short bursts with regular intervalls of about 2 kHz and receives the echoes. The probe is clamped outside of the vessel by means of a special assembling device allowing a comfortable installation of the probe at different positions on the vessel wall. The ultrasonic waves penetrate the wall and the liquid metal as a slim sonic cone perpendicularely to the inner wall of the vessel and will be reflected by the phase boundary between liquid metal and a gas bubble moving inside the sound cone. The detectibility of the bubbles is limited by their minimal size, by the sound attenuation in the liquid metal, by the unavoidable signal noise and other effects.
The ultrasonic probe is triggered by an electronic device generating the transmitting pulses as well as receiving and amplifying the sonic echo waves reflected by the bubbles. The amplitude and the time of flight of the sonic echoes will be transfered into two analogue electric voltage signals. These signals can be pre-storaged and recorded as functions of time by a digital storage oscilloscope (DSO).
For further data processing using the software tool FAMOS and final storing the measured signal data are transfered to a PC using the software code TRANSITION. By means of different FAMOS functions like histogram, FFT and others the distribution of the echo amplitudes of bubbles, the detaching frequencies at the bubble generating process and the bubble distances from the vessel wall can be determined. From this results, to the relative bubble sizes, bubble shapes, bubble velocities, local bubble positions and void distributions in the vessel can be concluded.