Exploiting scattering of ultrasonic guided waves for exploring metallic melts

Abstract

Abstract In industrial processes, such as crystal growth of photovoltaic silicon or continuous steel casting, the quality of the end products depends on the melt flow. Therefore, in situ flow imaging of opaque melts under harsh conditions is important. Conventional ultrasound measurement systems can not be operated at temperatures beyond the Curie temperature of the ultrasound transducer. An approach is to use a multimode waveguide to spatially separate the sensor from the hot measurement fluid. To overcome the complex ultra.sound propagation through the waveguide and enable imaging of the measurement volume, the time reversal virtual array (TRVA) method is used. The time invariance of the wave equation in an unknown, linear medium allows the refocusing on a limited set of calibrated points, which form the virtual array. This virtual array can concepttually be treated as a conventional phased array to image the flow field behind it. In this paper, the TRVA has been characterized theoretically, numerically and experimentally. It has been shown, that the virtual array is able to focus the ultrasound beam in transmit and receive modes. A planar velocity measurement of rotating flow in gallium-indium-tin is demonstrated using a 68mm borosilicate waveguide and 3 MHz central transmit frequency. A comparison with a reference measurement showed a relative deviation smaller than 25%.