Rapid Transient Boiling Heat-Transfer from a Coated Strip Horizontally Placed in Liquid Helium-I.

To make a thermal stability analysis of superconducting magnets, fundamental data on rapid transient boiling heat-transfer giving consideration to the influence of electrically insulated film are needed. In order to simulate an insulated superconductor in practical use, a 20-μm-thick stainless steel strip was applied as the test heater surface in this study. Both surfaces of the strip were coated with a PVF and carbon-paste films. Steady-state and rapid transient boiling heat-transfer were studied utilizing stepwise heat input from the test piece, which was placed horizontally in saturated liquid helium-4. In steady-state boiling, the thermal resistance of the coatings suppressed burn-out and quick transition. Consequently, the heat-transfer data correlated well with a single boiling curve having smooth and continuous transition from nucleate boiling to film boiling. The boiling heat-transfer characteristics are separated into three regions by the steady-state critical flux (CHF) of upward and downward-facing heaters. The nucleate boiling heat-transfer was almost the same as that of the vertically-placed heater reported previously by the authors, but the transition and film boiling heat-transfer were lower. When a stepwise heat flux higher than the steady-state CHF for the downward-facing heater was applied, the wall heat flux sharply dipped once just after the transition from transient nucleate boiling to film boiling. The heat conduction and transient nucleate boiling heat-transfer were almost the same as that of the vertically-placed heater. In contrast, heat-transfer after the transition to film boiling was lower.