Title: Rapid Transient Boiling Heat-Transfer from a Coated Strip Vertically Placed in Liquid Helium-I.
Abstract: In order to simulate an insulated superconductor in practical use, a 20-μm-thick stainless steel strip, both surfaces of which were coated with a PVF film and a carbon-paste film, was used as a test piece in the present experiment. Rapid transient (up to approx. 5, 000K/s) and steady-state boiling heat transfer from this vertical test strip, which was heated stepwise in saturated liquid helium-4 at atmospheric pressure, has been studied. In steady-state boiling, thermal resistance of the coating shifts the nucleate boiling curve to a higher wall-superheat region and suppresses burnout and a quick transition; consequently, heat transfer data were well correlated with a single boiling curve with smooth and continuous transition from nucleate boiling to film boiling. In the case of stepwise heating with a heat flux below the steady-state CHF, the wall superheat suddenly increases to a value corresponding to the Kapitza resistance just as the thermal wave reaches the test piece surface; steady-state nucleate boiling is attained within a very short time after rapid increase in the wall temperature. In the case of a heat flux above the steady-state CHF, just after the transition from transient nucleate boiling to film boiling, the wall heat flux sharply dips once; then steady-state film boiling is attained within a very short time. A rapid transient boiling curve has also been obtained.