Ultrasonic Spiral-Strip Waveguide with Distributed Temperature Sensors Designed for Measuring the Surface Temperatures

This study introduces an ultrasonic spiral-strip waveguide sensor system with distributed reflectors, specifically designed to enhance surface contact over the measurement region through a flat sensing interface – addressing limitations associated with traditional cylindrical or wire waveguides. A thin metallic strip was configured into a spiral shape, and strategically positioned notch reflectors were integrated along its length to generate distinct reference echoes. Finite element (FE) simulations were initially employed to optimize the spiral geometry and reflector layout. The fabricated waveguide incorporated four pairs of convergent-divergent notches at calibrated radial intervals (directions), enabling seamless placement on a flat heated surface. A shear-mode ultrasonic transducer, aligned at 0° to the waveguide axis, was used to excite and receive the S0 Lamb wave mode. Reference echoes collected at ambient temperature served as a baseline to evaluate the time-of-flight (ToF) variation as the structure was exposed to elevated temperatures. The sensor was calibrated using co-located thermocouples to establish a correlation between surface temperature and ToF shifts (δToF). The proposed sensor system addresses the need for accurate and continuous temperature monitoring in high-temperature industrial environments, including power plants, metallurgical operations, and thermal processing units in food and chemical industries. Experimental validation demonstrated that the spiral-strip waveguide could reliably measure surface temperature at multiple distributed locations, showing good agreement with thermocouple data.