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

In the Spallation Neutron Source, the mercury in the stainless steel target vessel experiences cavitation due to intense pressure waves produced by the almost instantaneous heating of the mercury following proton impact on the target. To protect the vessel walls from cavitation erosion, helium micro bubbles can be added to the mercury to act as attenuators of the pressure waves and to minimize cavitation effects. A diagnostic method is needed to monitor the gas injection technique and to measure the bubble size distribution. This project will develop an acoustic diagnostic tool that can meet the bubble size and void fraction requirements of the Spallation Neutron Source application. To enable use of larger amplitude acoustics waves and overcome excessive damping in the bubbly mixture, a non-linear acoustic theory and an intelligent artificial neural network using a priori knowledge from the acoustic theory will be used to extract bubble size distribution from acoustic data. Both acoustic wave transmission and reflection methods will be investigated and used to enable measurement at a range of void fractions that are often encountered in two-phase flow applications. During the Phase I study, significant progress was made in extending the capabilities of the acoustic bubble spectrometer hardware for a wider range of bubble sizes and void fractions, and in adapting it for the DOE mercury application. A non-linear bubble dynamics model that extends the linear theory utilized in the present spectrometer was developed and demonstrated. The feasibility of using a neural network based inverse problem solver to measure bubble size distribution was demonstrated using both synthetic and experimental data. The feasibility of using reflection waves to measure bubble size distribution and void fraction was also demonstrated; and theoretical formulations to interpret reflected sound wave signals to use in the inverse problem solver development were initiated. In Phase II the hardware of the spectrometer will be further improved to upgrade high frequency signal generation (for smaller bubble size measurements) and lower frequency hydrophones (for large bubble size measurement); and better designed integrated hydrophone for reflection tests. This will enable development of high fidelity databases for training and validating the neural network based spectrometer. The 3-D nonlinear acoustic model will be expanded to simulate practical geometries encountered in spectrometer operation and provide databases for training and testing the neural network in regimes where linear acoustic theory does not apply. Finally, the acoustic reflection method initialized in Phase I will be further developed, tested, and implemented in the new generation acoustic bubble spectrometer being developed. Commercial Applications and Other Benefits: A marketable acoustic instrument capable of measuring a wide range of bubble sizes and void fractions will be a valuable tool for diagnostics 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 Spallation Neutron Source application, the instrument will find applications in oceanographic, biological, chemical, pharmaceutical, energy, and other industries.