Numerical parametric study on local dry zone in underwater laser cutting for thick steel with supersonic assist gas from a dual nozzle

초록

Underwater laser cutting has several advantages for nuclear decommissioning. A critical component for successful underwater laser cutting is to secure a local dry zone (LDZ) for effective laser irradiation. In spite of its huge importance, the mechanism of LDZ formation remains poorly understood due to limited optical accessibility and turbidity of bubbly flows in experiments. In this study, three-dimensional simulations of compressible turbulent two-phase flows are performed to investigate LDZ formation and supersonic flow characteristics near the cutting front, focusing on key operating parameters including stand-off distance (SOD), nozzle total pressure, and kerf width. The results show that nozzle total pressure has a dominant influence on LDZ formation, inducing distinct topological transition across a critical pressure range. Below this threshold, the cutting front is fully covered by LDZ, leading to a significant increase in cutting front temperature. In contrast, the effect of SOD on LDZ shape is relatively minor within the investigated range. At high nozzle pressures, the wall shear stress at the cutting front decreases beyond a SOD threshold due to the formation of strong normal shock prior to jet impingement. An increase in kerf width weakens the jet impingement effect and delays flow separation within the kerf, allowing both air and water to penetrate more easily into the kerf. This results in an expanded region of elevated temperature and wall shear stress. These findings provide physical insights into the interplay between supersonic jet dynamics and LDZ formation, offering guidance for optimizing operating conditions in underwater laser cutting applications.

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