Abstract
Investigations about the role of nuclei and nucleation for the inception and formation of cavitation have been part of cavitation research since Harvey et al. (J. Cell. Physiol., vol. 24 (1), 1944, pp. 1–22) postulated the existence of gas filled crevices on surfaces and particles in liquids. In a supersaturated liquid, surface nuclei produce small gas bubbles due to mass transfer of gas or themselves work as weak spots in the liquid that are necessary for a phase change under technically relevant static pressures. Although various theories and models about nuclei and nucleation have found their way into standard literature, there is a lack of experimentally validated theories that describe the process of diffusion-driven nucleation in hydrodynamic cavitation. In order to close this gap we give new theoretical insights into the physics of this nucleation mechanism at technically relevant low supersaturations validated with extensive experimental results. The nucleation rate, the number of produced bubbles per second, is proportional to the supersaturation of the liquid and shows a nonlinear dependence on the shear rate at the surface nucleus. A model for the Strouhal number as dimensionless nucleation rate is derived allowing the estimation of nucleation rates from surface nuclei in hydrodynamic cavitation. The model provides three asymptotes, being a function of Péclet number, Weber number, the supersaturation of the liquid$\unicode[STIX]{x1D701}$and gas solubility$\unicode[STIX]{x1D6EC}$for three different detachment mechanisms,$Sr\propto \unicode[STIX]{x1D701}\unicode[STIX]{x1D6EC}We^{n}Pe^{1/3}$with$n=1/3,3/4,1$. The theoretical findings are in good agreement with experimental results, leading to a new assessment of the role of diffusion in cavitating flows.
Highlights
The occurrence of cavitation in hydraulic machines and maritime applications is usually accompanied by negative aspects
In the present paper the focus is on hydrodynamic cavitation
In cavitation research it is widely accepted that nucleation is triggered by the growth of minute gas bubbles which are entrapped in crevices when the pressure falls below a critical threshold pressure
Summary
The occurrence of cavitation in hydraulic machines and maritime applications is usually accompanied by negative aspects. Diffusion-driven nucleation from surface nuclei in hydrodynamic cavitation 139 the removal of bacteria and germs in water (Sarc, Oder & Dular 2016), the fragmentation of kidney stones in the human body by acoustic cavitation called lithotripsy, the targeted release of pharmacological agents (both Brennen 2015), and the enhancement of the efficiency of mixing processes (Spiridonov 2015) are only examples for the far-reaching application of hydrodynamic and acoustic cavitation. The supersaturation of the liquid ζ is the relevant quantity that allows the quantification of gas being solved in the liquid in relation to the equilibrium state and is important to describe diffusion processes. In cavitation research the focus is mainly on equilibrium processes, whereas experiments indicate that non-equilibrium processes, i.e. diffusion processes, are relevant as well (Pelz, Keil & Groß 2017)
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