Laminar Mixing in Stirred Tank Agitated by an Impeller Inclined

The mixing performance in a vessel agitated by an impeller that inclined itself, which is considered one of the typical ways to promote mixing performance by the spatial chaotic mixing, has been investigated experimentally and numerically. The mixing time was measured by the decolorization method and it was found that the inclined impeller could reduce mixing time compared to that obtained by the vertically located impeller in laminar flow region. The effect of eccentric position of inclined impeller on mixing time was also studied and a significant reduction of mixing time was observed. To confirm the experimental results, the velocity profiles were calculated numerically and two novel numerical simulation methods were proposed. 1. Introduction In recent years, many theoretical and experimental studies have been carried out for laminar chaotic mixing and have provided much beneficial information on how laminar mixing can be enhanced. However, it has also been recognized that the mixing in periodic flows is not necessarily complete, because such systems often display coexisting chaotic and nonchaotic regions. Fluids can neither penetrate nor leave these islands of unmixed fluids by regular motion. Therefore, the isolated mixing regions may become barriers to mixing. In the pioneering studies in this field, Lamberto et al. [1] first attempted to solve this problem using time-dependent rotational velocity to enhance the mixing in a stirred tank equipped with ordinary small impellers. After that, several experimental studies have been also undertaken [2, 3] to demonstrate that the mixing performance can be improved markedly by increasing the chaotic degree in temporal terms. While the chaotic degree can be increased effectively by temporal terms, the approach is restricted to practical applications because of the restriction of the motor and the speed reducer machine to drive the impeller. In practice, the temporal approach has rarely been employed in the mixing industry except for washing machines. In contrast, the chaotic degree can also be increased by spatial terms, for example, by reducing the circumferential symmetry and shifting the complexity in normal mixing equipment. While this spatial method does not improve the mixing performance as impressively as the temporal method, it places less demanding requirements on the machinery. Consequently, this approach has been widely used in industry. The special measures employed in an agitated vessel related to spatial chaotic mixing include baffles, off-center impeller mounting, and uncircumferentially