Design and multiphysics analysis of an active-passive thermal protection strategy for transverse flux induction heating of moving strips

Transverse flux induction heating (TFIH) offers distinct advantages in the field of metal strip heat treatment. However, optimizing its heating performance and ensuring the stable operation of the inductor requires a comprehensive multiphysics coupled analysis. To address the excessive inductor temperature rise caused by narrow air gaps in novel coil designs during high-temperature strip processing, this paper proposes a thermal protection strategy that combines active internal water cooling with passive composite thermal insulation. Forced water passages inside the hollow copper turns evacuate internal ohmic dissipation, while thin multi-layer ceramic-vacuum boards intercept intense radiative and convective heat fluxes from the moving strip. A finite element model of TFIH for moving strips is developed, accounting for strip motion, temperature-dependent material properties, water cooling, and thermal insulation. This modeling approach is experimentally validated using a low-power diamond-coil prototype. Subsequently, the validated method is applied to simulate a TFIH system featuring a novel dual-hexagonal-coil structure. Results demonstrate that the dual-hexagonal-coil structure significantly improves the thermal uniformity of the strip. Furthermore, the proposed protection measure not only prevents inductor overheating but also minimizes heat loss from the strip, producing a measurable rise in the delivered strip temperature without increasing input power. This study provides a valuable technical framework for the design and optimization of TFIH devices.