Water disinfection by hydrodynamic cavitation in a rotor-stator device.

The efficiency of a rotor-stator device for water disinfection based on hydrodynamic cavitation is investigated. Water is infected with E. coli and E. faecalis with initial concentrations in the range 5 × 102-1.2 × 106 CFU/ml. Various geometries of the cavitation channel between rotor and stator are tested, achieving bacterial annihilation in less than 10 min of treatment times. Microorganism permanent elimination is verified via micro-seeding to discard viable non-culturable bacteria; micro-seeding was done for those samples displaying no CFU growth via normalized cultures on a Petri dish. TEM photographs are analyzed and the extent of bacterial damages is tentatively correlated with the various cavitation mechanisms. Rotor-stator cavitation assemblies used in the current research are between one and two orders of magnitude more energy efficient than those tested by other investigators. Acoustic pressure spectra are measured to assess the implosion intensity. Parametric analyses are conducted changing the rotor diameter (110-155 mm), the cavitation channel contraction ratio, Amax/Amin(4.56-5.0), and the number of contractions (Nr:58-80 rotor vanes; Ns:8-16 stator vanes).

Influence of the accommodation coefficient on nonlinear bubble oscillations.

This paper numerically investigates the effect of mass transfer processes on spherical single bubble dynamics using the Hertz-Langmuir-Knudsen approximation for the mass flux across the interface. Bubble behavior, with and without mass transfer, is studied for different values of pressure wave amplitude and frequency, as well as initial bubble radius. Whereas mass transfer processes do not seem to play a significant role on the bubble response for pressure amplitudes smaller than 0.9 atm, they appear to have an important effect when the amplitude is greater than or equal to 1 atm. For the later case, where the minimum liquid pressure reaches values around its vapor pressure, the importance of mass transfer depends on frequency. For frequencies in the 10(3)-10(5) Hz range and initial bubble radii of the order of tens of microns, bubble implosions with and with no mass transfer are significantly different; smaller radii display a lower sensitivity. In this regime, accurate model predictions must, therefore, carefully select the correct value of the accommodation coefficient. For frequencies greater than 10(5) Hz, as a first approximation mass transfer can be ignored.

Dynamics of a single cavitating and reacting bubble.

Some of the studies on the dynamics of cavitating bubbles often consider simplified submodels assuming uniform fluid properties within the gas bubbles, ignoring chemical reactions, or suppressing fluid transport phenomena across the bubble interface. Another group of works, to which the present contribution belongs, includes the radial dependence of the fluid variables. Important fluid processes that occur inside the gas bubble, such as chemical reactions, and across the bubble interface, such as heat and mass transfer phenomena, are here considered also. As a consequence, this model should yield more realistic results. In particular, it is found that water evaporation and condensation are fundamental transport phenomena in estimating the dissociation reactions of water into OH. The thermal and mass boundary layers and the radial variation of the chemical concentrations also seem essential for accurate predictions.

Local geometry of isoscalar surfaces.

An inert dynamically passive scalar in a constant density fluid forced by a statistically homogeneous field of turbulence has been investigated using the results of a 256(3) grid direct numerical simulation. Mixing characteristics are characterized in terms of either principal curvatures or mean and Gauss curvatures. The most probable small-scale scalar geometries are flat and tilelike isosurfaces. Preliminary correlations between flow and scalar small-scale structures associate highly curved saddle points with large-strain regions and elliptic points with vorticity-dominated zones. The concavity of the scalar profiles along the isosurface normal coordinate xn correlates well with negative mean curvatures, Gauss curvatures displaying any sign, which correspond to scalar minima, tiles, or saddle points; on the other hand, convexity along xn is associated with positive mean curvatures, Gauss curvatures ranging from negative to positive signs, featuring maxima, tiles, or saddle points; inflection points along xn correlate well with small values of the mean curvature and zero or negative values of kg, corresponding to plane isosurfaces or saddle points with curvatures of equal and opposite signs. Small values of the scalar gradient are associated with elliptic points, either concave or convex (kg>0) , for both concave and convex scalar profiles along xn. Large values of the scalar gradient (or, equivalently, scalar fluctuation dissipation rates) are generally connected with small values of the Gauss curvature (either flat or moderate-curvature tilelike local geometries), with both concave and convex scalar profiles along xn equally probable. Vortical local flow structures correlate well with small and moderate values of the scalar gradient, while strain-dominated regions are associated with large values.