Development of a Two-Phase Bubble Generator for SNS Cavitation Mitigation

The Oak Ridge National Laboratory (ORNL) has recognized that the generation of strong shock waves in the Spallation Neutron Source (SNS) can lead to cavitation and significant erosion on the vessel wall containing the liquid mercury target. Based on preliminary numerical and experimental work at ORNL and other laboratories, it has been proposed that a cloud of small gas bubbles in the mercury target could absorb and deflect the shock waves and protect the walls from cavitation erosion. In order to maximize effectiveness, a bubble generator would have to be able to produce a large quantity (order of 1% void fraction) of small bubbles of the order of 10 µm. This project will develop a two-phase-flow bubble generator which combines air injection, water and air shear, and a fine separator mesh. Liquid will be entrained by gas injection near a free surface, resulting in a two-phase mixture impacting on the mesh. The mixture flowing through the mesh will be composed of a large number of small bubbles whose size is a function of the pore size of the mesh. In Phase I, this two-phase-flow bubble generator will be adapted and characterized for the mercury application, scaled appropriately, and adapted for application in the ORNL SNS target. Bubble sizes will be characterized using the Acoustic Bubble Spectrometer and optical measurements. Commercial Applications and other Benefits as described by the awardee: Besides the intended application to the spallation neutron source, the bubble generation technology should have several commercial applications: liquid metal heat transfer assemblies for microelectronics using gallium, cooling hard drives, and other electronic components; removal of impurities during refining; manufacturing of ultrasonic imaging contrast agents; production of chemicals (slurry bubble columns); separation of slurries using flotation; aeration applications in aquaculture and wastewater treatment; shellfish depuration; bubble curtains to acoustically shield a certain region under water; and hydrodynamic studies of microbubble drag reduction; flow monitoring, mass transfer studies, and chemical synthesis reactions.