SOME CONSEQUENCES FOR TURBULENT MIXING OF FINE-SCALE INTERMITTENCY

Even in homogeneous turbulence, energy dissipation does not occur uniformly at a constant rate, but the fine-scale structure is intermittent and spotty. Equations traditionally used to calculate the rate of micromixing and the maximum stable drop size in a dispersion take no account of intermittency. Its effect should be to decrease both these quantities, relative to the values predicted by available equations. When scaling-up, intermittency modifies the fine-scale turbulence even when the power per unit volume is held constant. The oversimplified beta-model was employed to approximately quantify these effects. Correction factors for the traditional equations used for micromixing and drop dispersion were easily derived from the beta-model. The fine structure is not space-filling, but has a fractal dimension < 3, so that the correction factors deviate from unity. Examples of such corrections are given for micromixing and drop dispersion as well as for the scale-up of these operations. For dispersing drops having negligible viscosities the beta-model predicts an exponent on the Weber number of - 0.65, whereas a recent and extensive correlation of measurements gives - 0.66. It is suggested that more attention should be given to the influence of intermittency on certain chemical engineering operations.