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

The generation of a strong shock wave 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 various 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 produce a relatively large quantity (order of 1% void fraction) of micron sized bubbles. A method for generating a large quantity of tiny bubbles is to combine a classical bubble generation scheme, based on gas injection from nozzles, with a technique for screening unwanted large-size bubbles with a fine mesh. The liquid entrained by the gas injection contributes to a two-phase mixture impacting on the mesh. This two-phase bubble generator could be made more efficient by adding side liquid jets to apply a localized shearing force at the gas nozzle. In this project, this two-phase flow bubble generator will be further developed, adapted to large flow rate, characterized for the SNS mercury application, and scaled up for application in the SNS target. Phase I examined the physics of operation of simple, small-scale, two-phase-flow bubble generators. Techniques to test and analyze bubbles in mercury were developed, and the bubble generator was adapted to mercury applications and characterized. In Phase II, these concepts will be extended to a larger scale set-up that can be used in the mercury SNS application, and a prototype bubble generator will be developed and built for the mercury flow of the SNS facility. The prototype then will be tested at the facility, and the bubble sizes will be characterized using an acoustic bubble spectrometer and radiography. Commercial Applications and Other Benefits as described by the awardee: Besides the SNS application, the bubble generation technology has many potential commercial applications: heat transfer in liquid metal assemblies for microelectronics using gallium, cooling of hard drives and other electronic components, removal impurities during refining, manufacturing of ultrasonic imaging contrast agents, production of chemicals in slurry bubble columns, separation of slurries using flotation, aeration in aquaculture and wastewater treatment, shellfish depuration, bubble curtains to acoustically shield a certain region under water, hydrodynamic studies of micro-bubble drag reduction, flow monitoring, mass transfer studies, and chemical synthesis reactions.