Hydrothermal spallation drilling (HSD) is a new method, especially suitable for drilling hard brittle rocks. HSD is promising for drilling deep hard formations. Recent studies of HSD have mainly focused on the effect of non-uniform thermal stresses during rock-breaking processes, with a nozzle exit velocity of usually less than 20 m/s. Inspired by high-pressure water jet technology, a new technology is proposed for HSD in this study. The technology, named hydrothermal jet spallation drilling (HJSD), which refers to a high-speed jet with high temperature at the nozzle outlet, should be more efficient for rock-breaking because the rock breaks due to not only thermal spallation but also the impact force of the jet. In this paper, numerical simulation of the high pressure high temperature jet in HJSD (hereafter referred to as the hydrothermal jet) is performed to analyze the distributions of impact pressure and cross-flow velocity of the bottom-hole flow field. The influence of the inlet temperature and pressure on the flow field is also analyzed. In addition, sensitivity analysis is performed to determine which parameter is more influential on the impact pressure and drag force. The results show that compared with the conventional water jet, under conditions of the same nozzle structure and pressure difference, the hydrothermal jet can generate higher impact velocity, cross-flow velocity and impact pressure; however, its attenuation is less obvious. Impact pressure, cross-flow velocity and drag force increase with the inlet pressure. With increasing temperature, the impact pressure and drag force remain almost constant, whereas the cross-flow velocity increases. In terms of the impact effect, a higher pressure in the chamber is preferred.

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