Multiphysics modelling of millimetre-wave ablation of geological materials

This study presents a multiphysics model and the corresponding algorithm for the numerical simulation of millimetre-wave ablation of geological materials. Simulations of this process are particularly challenging due to the low thermal conductivity and limited melt mobility of rock, as well as the high latent heat of evaporation and melt-to-vapour density ratio (exceeding 2500:1), which induce strong velocity divergence effects at the melt–vapour interface. Specifically, the model introduced in this work addresses the low-intensity, volumetric heating of rocks, taking place over significantly larger spatial and temporal scales (approximately 300 and 100 times greater, respectively) compared to similar processes encountered in laser drilling of metals. A comprehensive description is provided of the mathematical formulation, beam model, and numerical algorithm employed to perform robust, fully three-dimensional simulations lasting up to 25 s. These demanding computations are made tractable through MPI parallelisation and hierarchical Adaptive Mesh Refinement. Following the presentation of the model and solution methodology, an experiment is introduced which was devised specifically to produce data for model calibration and validation purposes. Comparison of the numerical results against the experimental reference test case and a parametric study of varying beam intensity demonstrate that the model can make good predictions of the penetration depth and the volume of material removed. The bounds and limitations of the model are also explored through the parametric study, with desirable future extensions identified to improve the accuracy of the model predictions.