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 μm, 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.

超声多普勒测速 (UDV) 是一种强大的、广泛使用的技术，用于测量金属熔体中的流动。然而，金属熔体中的 UDV 可靠性不高，因为它的运行依赖于知之甚少的现象。在这项研究中，我们调查了金属熔体中大量回波的特征不佳的来源以及相应的超声信号劣化机制。我们提供了来自电子显微镜和超声波测量的证据，证明氧化物夹杂物是镓中大量回波的主要来源。通过测量它们的终端速度，我们估计镓中回波物体的平均尺寸为 58-64 μm，这意味着 Mie 散射是主要的散射机制。通过比较信号直接传输到流体中的 UDV 测量结果，对于信号通过血管壁传输的其他人，我们证明有两种不同的信号衰减机制：来自主体的回声物体的损失以及声耦合的恶化和换能器表面的润湿。我们建议大力搅拌并使用间接接触 UDV 测量策略来减轻金属熔体中的信号衰减。