Experimental characterization of flow patterns and flow stability in the bulk mercury flow field of the spallation neutron source mercury target /

by Pointer, W. David

The Spallation Neutron Source will provide an intense pulsed source of neutrons for neutron scattering research by focusing a high-energy pulsed proton beam on a liquid mercury target. Interactions between the protons and the nuclei of the mercury will result in the production of neutrons through a spallation reaction. The use of liquid mercury obviates concerns with radiation damage associated with solid targets, facilitates removal of the heat deposited in the target, and allows online processing of the target material to reduce the concentration of the products of the spallation reaction. These advantages allow the use of a much higher energy proton beam than in existing facilities with solid targets. While the use of liquid mercury has many advantages, the liquid mercury that flows through the target must provide sufficient heat transfer to maintain the temperature of the target structure within the thermal limits of the structural materials. Therefore, the liquid mercury flow field must be adequately characterized to provide an accurate evaluation of heat transfer in the SNS target. Since liquid mercury is completely opaque and corrosive to many materials, the use of liquid mercury as the working fluid makes characterization of the flow field by experiment difficult. Furthermore, the appearance of flow asymmetries and pseudo-periodic instabilities in the flow field is difficult to capture in Computational Fluid Dynamics models of the system using current technology. Thus, a thorough experimental program using well-scaled experiments is required to validate and tune the computational model for the evaluation of the SNS mercury target design. The Spallation Neutron Source experimental program uses two scaled experiments to evaluate the fluid dynamic behavior of the bulk mercury flow of the SNS mercury target. The first facility uses air as a surrogate fluid for the liquid mercury, and the second facility uses water as a surrogate fluid. Flow visualization studies provide a qualitative evaluation of the flow behavior. Velocity mapping through Laser Doppler Velocimetry and Ultrasonic Doppler Profilimetry provide a more quantitative measure of the behavior of the flow field. The evaluation of the pressure field in the test sections provides insight into the nature of instabilities in the flow field. The evaluation of diffusion and dissipation in the flow field provides insight into the possible fluctuations in the temperature distribution in the mercury target. In general the experimental program indicates that the computational model provides a conservative evaluation of the flow behavior in the SNS target. iv