Coupled Electromagnetic-Thermal-Mechanical modeling on the damage mechanism of coaxial microwave heating granite

Microwave radiation is pioneered for weakening and cracking rock in the fields of tunneling and mining engineering. However, there has been limited research on the application of microwave heating for geothermal energy extraction, and the precise damage mechanism remains poorly understood. To address this gap, a fully coupled electromagnetic-thermal-mechanical model, incorporating damage variables, was established and then verified by lab tests. Simulation results of coaxial microwave heating granite show that the damage mechanism is different from that of conventional rectangular waveguide heating. Coaxial microwave heating leads to the formation of two distinct hot spots in the granite. The differential tensile stress between the interior and exterior of the borehole contributes to the damage in the upper hot spot, while the interior compressive and exterior tensile stress state induces damage in the lower hot spot. Consequently, the deformation pattern of granite assumes a vase shape, with the damaged area exhibiting a canopy shape. When the initial crack trend is 135 degrees, the crack connectivity efficiency is maximized to 61 % at 1.5 kW microwave power and 2.45 GHz frequency. These findings offer valuable theoretical insights for the potential application of microwave-assisted super-long gravity heat pipes in geothermal extraction.