Visualization of bubble behavior and heat transfer characteristics in liquid hydrogen pool boiling

Visualization and evaluation of the heat transfer characteristics of liquid hydrogen boiling are essential for establishing a hydrogen supply chain. In this study, pool boiling experiments were conducted under 0.1–0.3 MPa to investigate the effects of pressure on cryogenic boiling using high-speed visualization. The results indicated that pressurized conditions yielded higher values of the boiling heat transfer coefficient and critical heat flux in this pressure range. The measured boiling heat transfer coefficients were compared with existing correlations, suggesting that the Rohsenow correlation demonstrated good agreement with our findings. Specifically, the empirical coefficient of the Rohsenow correlation was determined to be 0.021 for the liquid hydrogen‑copper combination within the pressure range of 0.1–0.3 MPa. Furthermore, the boiling characteristics of liquid hydrogen were successfully visualized under steady-state nucleate and film boiling regimes. High-speed visualization of the isolated liquid hydrogen bubbles demonstrated that their growth is governed by heat conduction within both the fluid and solid. While vertical oscillations associated with capillary waves were observed, we hypothesize that, in the low buoyancy environment, minute bubbles accumulate and coalesce into a larger mass before undergoing macroscopic departure.