Effect of subcooling on film boiling from a downward-facing curved surface

Pool boiling from inclined and downward-facing flat and curved surfaces is of interest to the passive cooling of the pressure vessel lower head of a Light Water Reactor following a core meltdown accident. The effect of water subcooling on film boiling from a downward-facing curved surface was investigated experimentally. The local and surface average Nusselt numbers were correlated for both saturation and subcooling (5, 10, and 14 K) conditions. Video images of boiling surface and analysis of data demonstrated that surface rewetting in saturation boiling was hydrodynamic in nature, but thermally driven in subcooled boiling. Results confirmed that lower minimum film boiling heat flux values were associated with longer rewetting times and vice versa. Surface rewetting occurred when vapor film reached a critical thickness, δc, first at the lowermost position, θ = 0 degree, then sequentially at higher inclinations. For saturation boiling, δc ∼ 85 microm and 180 microm at θ = 0 degree and 8.26 degree, respectively. For subcooled boiling, however, δc varied for ∼50 to 175 microm, depending on the values of local inclination and water subcooling