Ultrasound Imaging in Hot Melts with Time Reversal Virtual Arrays

Industrial processes, such as silicon crystal growth for the photovoltaics industry, continuous steel casting, plastics and aluminum extrusion, involve hot, opaque liquids. Noninvasive inline monitoring is desirable to improve the quality of the products and the resource efficiency of the process. However, ultrasound-based imaging methods are severely limited by the transducer's resistance to high temperatures.We propose a method for imaging hot liquids using an ultrasound phased-array probe attached to a multi-mode waveguide (MMWG) for thermal decoupling. The complex wave propagation through the MMWG leads to a strongly distorted image, which is addressed with a time reversal virtual array (TRVA). The TRVA method performs a system identification and compensates the distortions based on the time-reversal invariance of sound propagation. We demonstrate planar imaging with this method in liquid tin at ≈ 300°C, well above the destruction limit of the transducers. The characterization of the imaging properties of the system showed a spatial resolution better than 1.6 mm.The proposed method for imaging through MMWG may open a new window into a variety of technical and industrial processes involving hot, opaque liquids in harsh environments.