Abstract
Microwave hotspots or thermal runaway is a long-standing challenge that severely limits the uniformity of microwave heating. In particular, the evolvement process and suppression methods of focused hotspots in high permittivity dimers are still not clear. In this work, thermal imaging was used to in situ track the microwave hotspots of grape dimers with different gaps and shapes in a multi-mode microwave oven. Two thermal characteristics of microwave resonance-induced focused hotspots, gap sensitivity and shape selectivity, were found. Finite element simulation verified the experimental results and calculated the influence of different conditions on the heating uniformity and the enhanced electric field. The results indicated that gap can gradually weaken the enhanced electric field between grapes and rapidly improve the heating uniformity by approximately 30 times. For dimers of different shapes in contact, the sharper the contact area, the more pronounced the focused hotspots. Moreover, the heating uniformity raises with increasing shape factor n, and when n = ∞, it is improved by about 35 times compared with n = 1. In addition, the enhanced electric field, which exhibits complexity due to microwave scattering interaction between grapes, was analyzed. These findings provide two strategies for improving the heating homogeneity of dimers or particle systems.
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