Physical Basis of Hydrodynamic Cavitation Generation

2.4 Nucleation, Gas Content, and Liquid Properties

The same nominal hydraulic condition can produce markedly different cavitation fields in filtered water, gas saturated liquids, emulsions, and high solids slurries. Cavitation nuclei may be trapped in surface crevices, transported on particles, stabilized by surfactants, or intentionally introduced. Dissolved gas can diffuse into growing cavities and cushion final collapse because noncondensable gas remains during compression. Conversely, an increased nuclei population can raise the number of collapse events and broaden the spatial coverage of treatment.

Viscosity attenuates radial bubble motion and small scale turbulence, while surface tension resists cavity growth and deformation of droplets or particle interfaces. Vapor pressure increases strongly with temperature, which generally promotes inception but can reduce collapse severity because a larger vapor mass must condense. Temperature also affects reaction kinetics, gas solubility, viscosity, and product stability. A complete process description should therefore report, at minimum, temperature, viscosity, density, surface or interfacial tension when relevant, gas content or degassing condition, solids concentration, and surfactant content.

Table 2.2. Dimensionless groups and engineering quantities used in cavitation processing.

Quantity	Definition	Primary use	Important qualification
Cavitation number $\sigma$	$(p_{ref}-p_v)/\left(\rho U_{ref}^2/2\right)$	Inception and relative cavitation intensity	State pressure and velocity reference locations
Reynolds number $Re$	$\rho UD/\mu$	Dynamic similarity and turbulence	Does not capture nuclei or two phase effects
Weber number $We$	$\rho U^{2}D/\gamma$	Droplet and interface deformation	Use the appropriate interfacial tension
Euler number $Eu$	$\mathrm{\Delta }p/\left(\rho U^2/2\right)$	Pressure loss and geometric comparison	Sensitive to velocity definition
Pressure recovery ratio ${\mathrm{\Pi }}_r$	$p_2/p_1$	Collapse driving condition	Does not provide minimum pressure
Specific energy consumption	$P_{el}/\dot{m}$ or $\mathrm{\Delta }pQ/\dot{m}$	Energy normalized process comparison	State whether electrical or hydraulic power is used
Cavitational yield	Product change divided by input energy	Useful outcome per energy input	The product metric must be explicitly defined