Jet phase transition behavior driven by pressure and temperature difference during subcooled water injection into liquid pool

• Innovative vertical/horizontal water injection into the high-temperature oil pool. • Revealed dual driving mechanisms: flash evaporation & heat transfer vaporization • Gravity effect on phase transition extent was negligible. • Developed a predictive model for jet volume expansion in oil–water system. Jet phase transition behavior during subcooled water injection into high-temperature liquid pool is a ubiquitous phenomenon in steam generator tube rupture (SGTR) accidents in Lead-cooled Fast Reactors (LFRs). In this paper, a visualization experimental platform was conducted for studying the jet volume expansion and associated heat transfer and pressurization characteristics, with parameters including upward and horizontal injection directions, nozzle diameter (1–4 mm), water pressure (0.4–1.0 MPa), oil temperature (100–190 ℃), and gas volume fraction (16.7–50 %). High-speed imaging revealed three distinct injection stages: initial jet cavity formation, quasi-steady injection, and steam cloud with entrained droplets diffusion after flow reversal. Jet phase transition was driven by two mechanisms: rapid flash evaporation upon depressurization (dominated by water superheat) and subsequent heat transfer vaporization (governed by oil–water temperature difference). Key results indicated large nozzle diameter and injection pressure amplified steam generation, while gas volume fraction affected jet expansion, and smaller gas space suppressed jet expansion under high-temperature oil conditions. Meanwhile, gravity effects were negligible at jet phase transition due to rapid flash evaporation kinetics. A jet volume expansion model was established by using 4 dimensionless thermophysical parameters. The model successfully predicted the average jet volume and angle with mean relative errors of 7.14 % and 5.16 % respectively. This work elucidates the interfacial thermal hydraulics interaction under SGTR accidents, providing mechanistic insights for safety analysis.