Ind. Eng. Chem. Res. 1989,28,825-830
825
GENERAL RESEARCH Rapid Micromixing by the Impingement of Thin Liquid Sheets. 1. A Photographic Study of the Flow Pattern Robert J. Demyanovich* and John R. Bourne Technisch-Chemisches Laboratorium, ETH, 8092 Zurich, Switzerland
The impingement of thin liquid sheets has been previously shown to produce rapid mixing on the molecular scale (micromixing). The observed rapid mixing was assumed to be the result of an inelastic collision that produced high local rates of energy dissipation. Verification of this assumption employing a double-flash photographic technique is presented here. By use of two separate flashes of 10-ps duration, sheet velocities were determined by photographing the movement of holes formed within milk sheets. Since the sheet velocity is constant, a comparison of single-sheet velocities to mixed-sheet velocities indicates whether energy has been lost as a result of the impingement. If the mixed- and single-sheet velocities are equal, the collision is elastic; if the ratio of these velocities is equal to the cosine of the half-angle of impingement, the collision is inelastic. The comparison showed that the collision of thin sheets is inelastic a t impingement angles less than 60°,confirming that the collision produces high-energy dissipation rates which lead to rapid mixing on the molecular scale. The impingement of thin sheets of liquids proposed by Demyanovich (1988)is an effective method for producing high local rates of energy dissipation, leading to rapid mixing of relatively large flow rates. The liquids are initially spread out into two continuous, thin sheets (of the order of 100 pm in thickness) and then impinged at one another. The devices that form the sheets are similar to atomizers and operate at relatively low pressure drops (0.3-1.0 bar). Demyanovich and Bourne (1988)have shown that, in the range of total flow rates between 2.0 and 3.7 L/min, liquids of 1.0 CPcan be mixed on a timescale of 1 ms to several milliseconds. Since the impingement zone contains a mass of less than 0.01 g, collision of the sheets produces high rates of energy dissipation. These rates were calculated to lie in the range 20 OOO to almost 90OOO W/ kg. The assumption was made that the impingement of the thin sheets resulted in an inelastic collision. The loss of kinetic energy yielded high-energy dissipation rates that led to rapid mixing. An elastic collision would not result in a conversion of kinetic energy, and the mixing would be relatively slow (on the order of 0.1-1 5). Although this assumption appeared to be supported by the mixing results, an independent verification is desirable. In part 1 we present photographic evidence that the impingement of thin liquid sheets results in an inelastic collision (up to an impingement angle of SO0). Part 2 presents the results of a mixing study using the competitive, consecutive reaction scheme developed by Bourne et al. (1981).
Characteristics of Liquid Sheets Figure 1 is a photograph of a thin sheet of milk formed in free space in the shape of an arc segment or fan. Dombrowski et al. (1960)have shown that liquid flows from the center of the sheet outward along radial lines with no crossover of liquid from one radial line to the next. Further, once the sheet is formed in free space, the radial velocity of the fluid within the sheet remains constant
(typical sheet velocities are of the order of 10 m/s and higher). At low ambient gas pressures (
