Development of an Acoustic Instrument for Bubble Size Distribution Measurement in Mercury

Intense pulsed pressure waves induced by proton beam impact on mercury in a spallation target propagate in the mercury and reflect from vessel walls as expansion waves resulting in cavitation damage to the walls. Injection of gas bubbles (few micrometer diameter range and void fraction of the order of 1%) into the mercury flow is one of the promising methods under investigation to mitigate the damage. Due to the opacity of mercury, a non-optical diagnostic tool is needed to quantify the injected bubble populations. The fact that bubbles have strong effects on acoustic wave propagation makes acoustic methods very good candidates for this application. This project will develop an acoustic diagnostic tool that can meet all the bubble sizing requirements for Spallation Neutron Source (SNS) applications. It will build on the technology of the present state-of-the-art acoustic bubble sizing instrument, the ABS ACOUSTIC BUBBLE SPECTROMETER, which works well for void fractions of the order of 0.1% and for bubbles between 20 and 500 m in diameter. The new instrument will use a nonlinear bubble dynamics model that extends the current linear theory utilized in the present ABS system to fully account for large bubble oscillations, bubble interactions, and strong acoustic damping. It will also use artificial intelligence (neural networks) to help solve the acoustic inverse problem and improve measurement accuracy and speed. Additionally, the proposed new method will investigate a wave reflection scheme which has the potential to outperform the wave transmission scheme currently used in ABS systems in high void fraction conditions. An acoustic instrument that is capable of measuring a wide range of bubble sizes at high void fraction will be a valuable tool for diagnostic and control of numerous multi-phase flow and liquid metal applications. Successful development of the proposed instrument will have wide commercial and scientific applications and benefits. In addition to the SNS application, the instrument will find application in oceanographic, biological, chemical, pharmaceutical, and other industrial fields.