A number of thin liquid protection schemes involving a sacrificial thin liquid layer have been proposed to protect the first walls of inertial fusion energy reactor chambers from excessive radiation and energetic ion damage. The Prometheus study used a tangentially injected high-speed film of molten lead attached to the first wall to protect the upper endcap of the chamber reactor. Minimizing droplet formation and detachment from this film to avoid interference with beam propagation is a major design issue for such flows.

Experiments were conducted on turbulent films of water injected tangentially with a rectangular nozzle into ambient air onto the underside of a horizontal flat plate. Previous efforts were focused on the effect of various design and operational parameters on the film detachment distance. This study focuses on measurement of the "hydrodynamic source term," i.e., the rate of droplet formation due to primary turbulent breakup at the film surface. Droplet mass flux was measured using a simple collection technique at various standoff distances measured with respect to the plate surface and downstream distances measured from the nozzle exit. The data show that the ejected droplet mass flux increases as the standoff distance decreases and as both downstream distance and Weber number increase. Comparisons of the experimental data on the estimated ejected droplet mass flux with previously published correlations suggest that the correlations overpredict the ejected droplet mass flux by more than three orders of magnitude.