A model for focused-beam microwave heating on rock fracturing

Abstract: Microwave-assisted rock fracturing has been widely applied to satisfy increasing demands for safety, high efficiency and energy saving. During microwave irradiation, rock damage occurs as a result of the irregular distribution of induced temperatures and resulting thermal stresses. This non-uniform temperature distribution may develop from either (1) intrinsic rock heterogeneity, or (2) from the impingement of a heterogeneous or focused electromagnetic field density. However, prior models have rarely been concerned with the impact of the latter phenomenon and no rational methods to define the accurate location of the initial fracturing are available. A nested spherical mechanical model is proposed to address this deficiency where the heated volume is divided into three zones: an interior high temperature zone (H zone), a transition zone (T zone) and a low temperature external zone (L zone). The thermal stress coupling coefficient is defined. The proposed model is verified against experiments conducted in this work and by others. As a result of this mechanical model, the geometric dimensions of the three regions have an important influence on the variation of the maximum compressive/tensile stress. The thermal stress coupling coefficient is closely related to the location of the maximum compressive/tensile stress and the thermal stress distribution. Fractures develop first in the extensional T zone. The proposed model enables the prediction of the likelihood, location and severity of fracturing to develop and therefore enables the potential optimization of the fracturing process. Article Highlights: We develop a nested spherical mechanical model to address this deficiency where the heated volume is divided into three zones: H zone, T zone and L zone.The stress distribution is shown to be uniquely related to both the thermal stress coupling coefficients and the geometric dimensions of the three zones.The maximum tensile stress appears in the inner boundary of T zone, or a small distance from H zone.