Microscale mechanisms of ultrasound velocity measurement in metal melts

Ultrasound Doppler velocimetry (UDV) is a powerful, widely used technique for measuring flow in metal melts. However, UDV in metal melts suffers from substandard reliability because its operation depends on phenomena that are poorly understood. In this study, we investigate the poorly characterized source of bulk echoes in metal melts and the corresponding mechanisms of ultrasound signal deterioration. We present evidence from electron microscopy and ultrasound measurements that oxide inclusions are the main source of bulk echoes in gallium. By measuring their terminal velocity, we estimate the mean size of echoing objects in gallium to be 58-64 mu m, implying that Mie scattering is the dominant scattering mechanism. By comparing UDV measurements in which signals were transmitted directly into the fluid, to others in which signals were transmitted through a vessel wall, we show evidence that there are two distinct mechanisms for signal degradation: the loss of echoing objects from the bulk and the deterioration of acoustic coupling and wetting at the transducer surface. We suggest stirring vigorously and using indirect-contact UDV measurement strategy to mitigate the signal degradation in metal melts.