Ultrasonic Bubble Imaging in Molten Salt Using a Multi-Mode Waveguide and Time Reversal

Direct ultrasound imaging of technical processes in harsh environments, such as high-temperature melts, is not possible beyond the destruction limits of ultrasound transducers. A waveguide between the measured medium and the sensitive ultrasound transducer allows acoustic access to the former while protecting the latter. State-of-the-art single-mode waveguides allow for acoustical coupling between a transducer and the measured medium but are unsuited for real-time imaging since mechanical scanning is necessary. In contrast, multimode waveguides can transmit complex 2-D signals to an imaged region. While multimode waveguides require adaptive wavefront correction to compensate for the complex propagation inside, they allow the reduction of mechanical complexity with the application of the time-reversal virtual array (TRVA) method. This calibration method takes advantage of the time invariance of sound propagation in linear media to identify and compensate for the propagation inside the waveguide. This work proposes a novel
in situ
calibration procedure for system identification based on a traversing passive point scatterer. We characterize the imaging capacity of such a TRVA inside a molten nitrate salt at about 300 °C, exceeding the array’s operating temperature by 250 °C. By real-time imaging rising gas bubbles in the melt for the first time, we demonstrate the method’s potential for monitoring dynamic processes, such as metallurgical bubble column reactors with hot and opaque melts.