Abstract
Due to their ability to provide efficient mixing at small scales, confined impinging jet mixers (CIJMs) are employed widely in nanoparticle assembly processes such as flash nanoprecipitation and flash nanocomplexation, which require rapid mixing. In this mixing device, two jets from opposite directions impinge directly on each other forming a thin shear layer that breaks down rapidly into small flow structures. This enables effective mixing of the species transported by each jet by drastically reducing the diffusion distance. In the present study, the mixing performance of a commonly used cylindrical CIJM is examined by direct numerical simulations. Analysis of the simulation results indicates that the interaction of the shear layer with the inner walls of the CIJM is critical in inducing a range of instabilities in the impinging jet flow. By examining flow structures, statistical quantities, and metrics, we have characterized and quantified the mixing quality of a binary mixture in the CIJM. Product uniformity in processes such as precipitation and complexation is expected to depend on the residence time of the constituents, and this quantity is also calculated and compared for the cases with different jet Reynolds numbers. The jet Reynolds numbers of Re = 200, 600, and 1000 are considered, and the simulation results show that the CIJM achieves very good mixing for the Re = 600 and Re = 1000 cases. It is also found that the Re = 600 case performs slightly better than the other cases in terms of uniformity of the residence time. These quantitative analyses offer useful insights into the mechanism of nanoparticle size control and uniformity afforded by the unique flow physics and mixing characteristics in the CIJMs.
Highlights
In a multi-component reaction or assembly system, effective mixing of different components is critical in controlling the characteristics and uniformity of the manufactured products
Analysis of the simulation results indicates that the interaction of the shear layer with the inner walls of the confined impinging jet mixers (CIJMs) is critical in inducing a range of instabilities in the impinging jet flow
The fluid dynamics in the CIJM is characterized by a number of identifiable features, the first of which is a “disk” like shear layer that is formed by the impinging jets
Summary
In a multi-component reaction or assembly system, effective mixing of different components is critical in controlling the characteristics and uniformity of the manufactured products. Rapid breakdown of the shear layer resulting from the impinged jets and transition to turbulence can induce a high degree of mixing of the chemical species even in smallscale mixers, where Reynolds numbers are relatively low [O(1000) or lower].10. Wood et al. investigated the flow field in a CIJM experimentally and numerically They quantified the Strouhal number as a function of Reynolds number and observed a low frequency fluctuation pattern of the flow at a Strouhal number based on the inlet jet diameter and inlet velocity of ∼0.01. We have performed the direct numerical simulations for the flow and mixing in the CIJM, and the mixing quality is investigated by quantifying the cross correlation of chemical species within the mixer.
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