Evidence for the relationship between the L/D cycle frequency and biomass productivity has been reported many times. On the other hand, the vortices generated by mixers improve the convection between the light and dark zones in light-limited cultures and, therefore, enhance the frequency of light/dark (L/D) cycles of algal cells. The reason why the introduction of mixers can increase biomass productivity can be understood as follows: On the one hand, the mixing effect caused by the mixer accelerates the gas exchange between the cells and the medium. These novel structures actually serve as mixers for enhancing PBR mixing performance. Apart from mixers, novel structures (e.g., the discrete double-inclined ribs in a tubular PBR and a wavy bottom in a pond-like PBR ) have also been introduced to PBRs to increase biomass productivity. ĭesigners are always seeking ways to increase algal biomass productivity in PBRs, and the way of introducing mixers to PBRs (including open ponds) has been well developed over the past several decades, such as the trapezoidal chamber mixer in a flat-plate PBR, the up–down chute baffles in an open pond, the Kenics mixer, the plate mixer with a V-groove at the top and a hole at the center, the inclined porous mixer and the helical mixer in tubular PBRs. Among these PBRs, tubular PBRs have been recognized for their potential to cultivate algae on a large scale because they can cultivate and harvest cells continuously. Enclosed PBRs can be generally divided into tubular PBRs, bubble columns, flat-plate PBRs and so on. Researchers invented enclosed photobioreactors (PBRs) to address these problems. However, open ponds consume much water, and algae in these ponds are easily contaminated. Currently, large-scale cultivation of microalgae occurs mainly in open ponds. Microalgae are used to produce pharmaceuticals, food and, looking toward the future, biofuels. The idea also provides useful rules to design and optimize a mixer for a higher L/D cycle frequency with a lower increase in pumping costs, and these rules will find widespread applications in PBR design.

This idea can be used to enhance our understanding of some known phenomena that emerge by mixer addition. The synergy idea may be stated as follows: the enhancement of L/D cycle frequency depends not only on the flow and light fields individually but also on their synergy. An effective method, i.e., decreasing the radial height of the helical mixer from the inner side, was found, by which the L/D cycle frequency was enhanced by 10.8% while the pumping cost was reduced by 23.8%. A helical mixer design is provided as a case study to demonstrate the importance and feasibility of the synergy idea. Second, and more importantly, the idea can help to develop new rules for mixer design. First, it can facilitate a better understanding of known numerical and experimental results about mixer addition. The synergy idea has two types of applications. Better synergy can be obtained if (a) the vortex core and L/D boundary are closer to each other and (b) the vortex whose core is too far from the L/D boundary is removed. Thus, the idea of synergy between flow and light fields is introduced to mixer design. In addition to the individual effects of flow and light fields, improving the synergy (coordination) between these fields may markedly enhance the L/D cycle frequency with a lower increase in pumping costs. This study aims to improve this situation. Moreover, a large pumping cost usually accompanies mixer introduction, and this cost is not favorable for practical applications. This approach is not knowledge-based, and thus little or no understanding of the underlying mechanisms of mixer design for mixing performance of photobioreactors is attained. However, existing mixer designs are usually developed using a trial-and-error approach that is largely based on the designer’s experience. Mixers are usually inserted into microalgal photobioreactors to generate vortices that can enhance light/dark cycles of algal cells and consequently enhance biomass productivity.

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