Experiments on High-Speed Liquid Films Over Downward-Facing Wetting and Nonwetting Surfaces

The fusion event in inertial fusion energy reactors can damage the chamber first walls. The Prometheus design study used a high-speed tangentially injected thin film of molten lead to protect the upper endcap of the reactor chamber. To assure full chamber coverage, the film must remain attached. Film detachment due to gravitational effects is most likely to occur on downward-facing surfaces.

Experiments were therefore conducted on turbulent water films with initial thicknessess and speeds up to 2 mm and 11 m/s, respectively, onto the downward-facing surface of a flat plate 0-45° below the horizontal. Average film detachment and lateral extent along the plate were measured. Detachment length appears to be a linear function of Froude number. Results for film flows over wetting and nonwetting surfaces show that surface wettability has a major impact. The data are used to establish conservative "design windows" for film detachment. Film flow around cylindrical obstacles, modeling protective dams around chamber penetrations, was also studied. The results suggest that cylindrical dams cannot be used to protect penetrations, and that new chamber penetration geometries that avoid flow separation are a major design issue for this type of thin liquid protection.