Thermal characteristics and removal mechanism of high energy plasma jet rock-breaking

High-energy plasma jet rock-breaking technology is regarded as a very promising new drilling approach for deep hard rock,attributed to its high energy density,high rock-breaking efficiency,absence of mechanical wear,and capability to drill high-hardness rocks.However,the thermal characteristics and rockbreaking mechanism of plasma jet remains unclear.This study thoroughly investigates the internal temperature distribution characteristics of granite and the thermal removal mechanism of plasma jet with combined experimental and numerical approaches.The spallation temperature of granite is calculated based on the Weibull statistical theory of tensile failure.A numerical model of the thermal melting process of granite is developed to obtain the erosion morphology and temperature distribution characteristics during the rock-breaking process.The results indicate that the spallation temperature induced by the plasma jet is approximately 557℃,and the experimentally obtained hole profile on the upper surface coincides with the isotherm corresponding to the spallation temperature from the simulation.The temperature gradients of granite in the radial and axial directions of plasma arc operation can reach up to 38.79 and 66.13℃/mm,respectively.And the heat-affected region expands with increasing current.The optimal removal efficiency can be achieved between 20 and 30 s under various plasma current conditions,with the maximum value of 1188 mm3/s at a current of 300 A.The plasma jet rock-breaking process can be characterized into three stages:dominant spalling in the early rockbreaking stage,followed by the coexistence of hot melting and spalling in the middle stage,and dominant high-temperature melting removal in the later stage.The results of this study provide theoretical guidance for engineering application of high-energy plasma jet rock-breaking drilling.