Stability analysis of a high velocity liquid lithium jet over a concave surface

The International Fusion Materials Irradiation Test Facility (IFMIF) is being developed to provide information on the behavior of materials subject to radiation environments typical of a fusion power plant. The design for the IFMIF involves a liquid lithium jet flowing at 17 m/s on a concave surface of radius 0.025 m in a vacuum. This jet is the target for a 35-MeV deuteron beam which reacts with the liquid lithium target to produce high energy neutrons with an energy spectrum peaked around 14.7 MeV which is typical of that produced during a deuterium-tritium fusion reaction. The 19-mm thick liquid lithium jet is placed on a concave surface in order to elevate the pressure and saturation temperature of the flow near the supporting surface. This is necessary since the volumetric power generation is maximum for the 35-MeV deuteron beam 16 mm below the jet surface. The jet velocity is also chosen to limit the rise in fluid temperature associated with the energy deposition. The fluid dynamic behavior of the jet must be well characterized because the nuclear and thermal performance of the system is very sensitive to the thickness of the jet in the target region.

In the present stability study, surface tension, gravitational and centrifugal forces are modeled using potential flow. The stability limits of the jet and the wavelengths of the free surface oscillations are determined using linear stability analysis. The flow velocity profile changes in the flow development region following the nozzle are not considered in this analysis. Also, the relaxation of the boundary layer that must occur on the free surface when the jet leaves the nozzle is not explicitly considered. However, boundary layer relaxation is expected to be the source of the initial perturbation.

This study shows that the wavelength of surface oscillations is inversely proportional to the flow velocity and square root of the liquid density, but directly proportional to the square root of the surface tension. A hydraulic experiment is scaled based on the results of the fluid dynamic linear stability analysis in order to provide experimental verification of the predicted stability limits. The scaled experiment uses water as the working fluid. The water jet is 2.87 cm deep and flows on a surface with a radius of curvature 43.5 cm. The velocity of the water is varied between 2 and 5 m/s and the temperature of the water is varied from 20 to 80 Celsius degrees to obtain data on the stability limit of the jet for varying Froude, Weber and Reynolds numbers. The experimental test section is designed to minimize effects of flow noise and flow induced vibrations. Flow turbulence in the delivery nozzle is monitored. Structural vibration is also monitored to assure this is not responsible for disturbing the water jet.