Simultaneous Cavitation Susceptibility Meter and Holographic Measurements of Nuclei in Liquids

Abstract

Cavitation Susceptibility Meter (CSM) and holographic measurements of cavitation nuclei distributions are compared in this paper. The CSM optically detects cavitation in water samples flowing through a venturi and relates the unstable nuclei concentration to the applied tension in the fluid. A ruby laser holographic system measures the nuclei size distribution directly. Microbubbles have been used as the dominant nuclei source. The data from the two detection schemes are correlated by accounting for the dynamic response of the cavities in the venturi throat. The active nuclei distributions predicted by the holographic data compare favorably with those measured by the CSM. Both detection methods show that the nuclei concentration rises approximately exponentially as the applied tension is increased and then, with further reduction in the pressure, tends to a nearly constant maximum due to the shortage of remaining cavitatable nuclei. The CSM consistently underestimates the concentration of active cavitation nuclei, due to limited electro-optical resolution and mutual interference effects between cavities in the venturi. The good qualitative agreement of the two techniques supports the validity of the data correlation model and clearly indicates that any practical interpretation of measured nuclei size distributions for cavitations predictions is highly dependent of the specific flow conditions. Attempts to cavitate saturated water of the California Institute of Technology Low Turbulence Water Tunnel in the CSM were unsuccessful even at the lowest attainable CSM throat pressures (about −40kPa). This is thought to be due to insufficient throat tension and, at least partially, to the short time available for cavity growth in the CSM.