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In industrial processes, such as crystal growth of photovoltaic silicon or continuous steel casting, the quality of the end-products strongly depends on the melt flow. Therefore, in situ flow imaging of opaque melts under harsh conditions is important. However, ultrasound transducer arrays usually can not be operated at temperatures beyond 100 C. An approach is to a use multimode waveguide, to spatially seperate the sensor from the measurement fluid. To overcome the complex ultrasound propagation through a multimode waveguide and enable imaging of the measurement volume, the time reversal virtual array (TRVA) method is used. Time invariance of wave equation in a linear medium allows refocusing on a limited set of calibrated points, which form the TRVA. In this paper, the TRVA has been characterized theoretically, numerically and experimentally. It has been shown that the virtual array can focus the ultrasound beam in transmit and receive modes. The achieved spatial resolution varies from 2 mm to 4 mm in experiments with water and 3 MHz central transmit frequency. A planar velocity measurement of rotating flow in gallium-indium-tin is demonstrated through a 68 mm borosilicate waveguide. A comparison with a reference measurement showed a typical relative deviation smaller than 5,3%.

C2.1 - Ultrasound imaging through multimode waveguides using time reversal virtual array method