An effective dual-medium approach to simulate microwave heating in strongly heterogeneous rocks

Abstract: Microwave irradiation is widely applied as a heating method since this approach avoids the intrinsic limitations of heat transfer via conduction. However, microwave heating in highly heterogeneous materials, such as rocks, remains poorly understood. Current approaches applied to rocks typically ignore (i) state transformations of liquid and solid, (ii) impacts of the temperature-dependent dielectric permittivity and specific heat capacity, and (iii) innate microscale mineral heterogeneities in the evolution of temperature within mineral aggregates. We address these limitations with a dual-component effective-medium approach. In this approach, mineral aggregates in the shale matrix are separated into high- and low-transformative-capability materials (HTC and LTC systems), coupled by heat transfer. The temperature increase in the HTC and LTC systems is affected by both microwave irradiation and heat transfer. The temperature differential between these two systems increases with increasing irradiation time, and heat transfer acts to ameliorate this differential. A three-stage temperature-evolution profile is replicated for rocks comprising linearly increasing, stable and rapidly increasing stages. The peak in the specific heat capacity-temperature curve is the main contributor to the plateau stage. Additionally, in the case of a high heat transfer coefficient, all three stages can be observed in both systems, while in the case of a low heat transfer coefficient, not all three stages occur. The impact of the real part of the dielectric permittivity is not universal, while a higher value of the imaginary part results in a larger increase in temperature. This work proposes an alternative approach to simulate the microwave heating process in heterogeneous materials. Highlights: Mineral aggregates in the rock matrix are separated into high- and low-transformative capacity systems, coupled by heat transfer.A dual-component effective-medium approach considering the above two systems is proposed to simulate the microwave heating process in heterogeneous rocks.A three-stage temperature-evolution profile is replicated for rocks comprising linearly increasing, stable and rapidly increasing stages.