Ultrasonic Velocity Measurements in Liquid Metals Using Acoustic Wave Guides

The ultrasound Doppler velocimetry (UDV) is an innovative and non-invasive method to determine the velocity in liquid flows. Main advantages are the ability to investigate flows of opaque liquids and to deliver complete velocity profiles in real-time [1,2]. Therefore, the intention to apply this measuring technique to liquid metal flows is obvious with respect to the situation that until today no velocity measuring technique for liquid metal flows is available commercially. The feasibility of velocity profile measurements by UDV has already been reported for low temperature liquid metals as mercury [2,3] and gallium [4]. Recently, successful measurements for liquid sodium at a temperature of 150°C were published [5]. The wetting of the sensor by the liquid metal to achieve a sufficient acoustic coupling or the allocation of suitable tracer particles have to be considered as relevant problems for the application of UDV in liquid metal flows. However, the today's available ultrasonic technology reveals considerable limitations with respect to the practicability of measurements at high temperatures above 200°C. The conventional piezoelectric transducers using PZT based materials are usually restricted to temperatures of 150°C. A way to develop transducers for higher temperatures is to use piezoelectric materials with a higher Curie temperature, for instance GaPO4 or LiNbO3. Ultrasonic transducers based on LiNbO3 piezoelectric elements were used, for instance, as detector for the fluid level of liquid sodium in Liquid Metal Fast Breeder Reactors (LMFBR) [6]. Such measuring systems work up to temperatures of about 700°C, but, there is a significant loss in the piezoelectric properties at high temperatures. Such transducers show a low coupling factor resulting in a poor signal-to-noise ratio. The sensitivity of a sensor required for UDV cannot be achieved. Another approach is described here. An integrated ultrasonic sensor is designed consisting of the piezoelectric element and an acoustic wave guide made of stainless steel. The effect of the wave guide is the thermal and the chemical decoupling between the active transducer and the fluid. Therefore, the wave guide allows to extend the range of UDV applications to higher temperatures and measurements in chemically reactive fluids.