Flow Characteristics in a Scaled-up Multi-inlet Vortex Nanoprecipitation Reactor

Abstract

The microscale multi-inlet vortex reactor (MIVR) has been developed for use in flash nanoprecipitation, a technique to generate functional nanoparticles. A scaled-up MIVR is motivated by the desire for a higher output of nanoparticles than the microscale reactor can provide. As the first step of this scaling process, the flow characteristics in a macro-scale MIVR have been investigated by stereoscopic particle image velocimetry. The studied Reynolds numbers based on the inlet geometry range from 3290 to 8225, resulting in a turbulent swirling flow within the reactor. The flow in the mixing chamber is found to be unstable with a wandering vortex center. The vortex wandering is constrained to a small area near the center of the reactor and has little effect on the mean velocity field. However, the measured turbulence kinetic energy and Reynolds stresses are found to be sensitive to the vortex wandering. The flow characteristics of the macro-scale MIVR are compared with the microscale MIVR in terms of swirl ratio and micromixing time. It is found that the swirl ratio and micromixing time of the flow increases as the MIVR is scaled up, indicating a flow with stronger swirl yet less mixing effectiveness in the scaled-up reactor. ■ INTRODUCTIONFunctional nanoparticles are of great scientific and industrial interest for their unique size-related properties and have a wide application in various areas, such as dyes, pesticides, and pharmaceuticals. 1 However, it is still challenging to produce functional nanoparticles in a relatively easy and inexpensive way. Flash nanoprecipitation (FNP) has been developed to produce functional nanoparticles with a narrow particle size distribution. 2 In the FNP technique, functional nanoparticles are formed by rapidly mixing a supersaturated organic active and a copolymer antisolvent, resulting in the organic active precipitation and particle growth where the growing particle size of the organic active is frozen by deposition of a block copolymer on its surface. 2 Mixing time in the FNP technique should be short enough to provide a uniform starting time for the precipitation. Two mixer geometries, the confined impinging jet reactor (CIJR) 3 and the multi-inlet vortex reactor (MIVR) 4 have been developed to meet the high demand of rapid mixing in FNP. While the CIJR is limited by the requirement of equal momenta of solvent and antisolvent streams, the MIVR is insensitive to the equality of the momentum from each stream, allowing the final fluid phase to be antisolvent dominant, which increases the stability of nanoparticles by depressing the rate of Ostwald Ripening. 4 Thus far, there have been many applications using the MIVR to produce functional nanoparticles. 5−9 To help understand the nanoprecipitation mechanism within the MIVR, mixing performance and flow characterization have been investigated in previous studies. Liu et al. 4 evaluated the mixing performance of a microscale MIVR by using a competitive reaction and computational fluid dynamics (CFD). Cheng et al. 10 measured fl...

Turbulent mixing in the confined swirling flow of a multi‐inlet vortex reactor

“… depends on the inlet flow rate, and Based on the mean velocity fields, the turbulent swirling flow in the MIVR can be divided into two regions, i.e., a free‐vortex region (r/R 0 > 0.2) and a forced‐vortex region (r/R 0 < 0.1) where R 0 is the radius of MIVR chamber and r is the distance from the reactor center . The turbulence intensity in the forced‐vortex region is much higher than in the free‐vortex region . As passive scalar transport is strongly dependent on the turbulence field, it is natural to describe the scalar field separately in these two regions.…”

“…In the MIVR, swirling flow is formed by four tangential jets entering a short cylindrical reactor, and high turbulence intensity is generated near the reactor center by confining outflow with a small outlet . Unlike the mixing in most combustion applications, mixing of the swirling flow in the MIVR occurs in a contracted geometry without a sudden expansion section . Thus, the commonly observed vortex breakdown in combustion applications does not exist in the swirling flow of the MIVR, and random vortex wandering is found near the reactor center .…”

“…Unlike the mixing in most combustion applications, mixing of the swirling flow in the MIVR occurs in a contracted geometry without a sudden expansion section . Thus, the commonly observed vortex breakdown in combustion applications does not exist in the swirling flow of the MIVR, and random vortex wandering is found near the reactor center . Mixing of the swirling flow in the MIVR occurs in the liquid phase, which has a much higher Schmidt number than for gas flow; thus requiring much higher resolution for fully resolving its smallest length scales.…”

“…14 Thus, the commonly observed vortex breakdown in combustion applications does not exist in the swirling flow of the MIVR, and random vortex wandering is found near the reactor center. 11,14 Mixing of the swirling flow in the MIVR occurs in the liquid phase, which has a much higher Schmidt number than for gas flow; thus requiring much higher resolution for fully resolving its smallest length scales. Investigating turbulent mixing of swirling flow in the MIVR can benefit its application in flash nanoprecipitation (FNP) 15 as the MIVR has been widely used in the FNP process to produce functional nanoparticles due to its ability to provide rapid mixing of solvent and anti-solvent.…”

“…The investigated Reynolds number is defined based on the bulk velocity of one inlet, as all four inlets have the same flow rate. 14 24 The Kolmogorov length scale g can be estimated based on previous stereoscopic Particle Image Velocimetry measurements. 14 Figure 3 shows the estimated Batchelor scale at the 1 = 2 plane for different Reynolds numbers.…”