Qin Chao, Wu Jing, Wang Jing
School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan, 430074 China.
Biotechnol Biofuels. 2019 Apr 23;12:93. doi: 10.1186/s13068-019-1430-y. eCollection 2019.
Mixers are usually inserted into microalgal photobioreactors to generate vortices that can enhance light/dark cycles of algal cells and consequently enhance biomass productivity. However, existing mixer designs are usually developed using a trial-and-error approach that is largely based on the designer's experience. 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. Moreover, a large pumping cost usually accompanies mixer introduction, and this cost is not favorable for practical applications. This study aims to improve this situation.
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. Thus, the idea of synergy between flow and light fields is introduced to mixer design. 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. The synergy idea has two types of applications. First, it can facilitate a better understanding of known numerical and experimental results about mixer addition. Second, and more importantly, the idea can help to develop new rules for mixer design. A helical mixer design is provided as a case study to demonstrate the importance and feasibility of the synergy idea. 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%.
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. This idea can be used to enhance our understanding of some known phenomena that emerge by mixer addition. 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.
搅拌器通常被插入微藻光生物反应器中以产生涡流，这些涡流可以增强藻类细胞的光/暗循环，从而提高生物质生产力。然而，现有的搅拌器设计通常采用试错法，这在很大程度上基于设计者的经验。这种方法不是基于知识的，因此对于光生物反应器混合性能的搅拌器设计的潜在机制几乎没有理解。此外，引入搅拌器通常伴随着高昂的泵送成本，而这种成本对于实际应用来说是不利的。本研究旨在改善这种情况。
除了流场和光场的单独作用外，改善这些场之间的协同作用（协调性）可能会显著提高光/暗循环频率，同时泵送成本的增加幅度较小。因此，将流场和光场之间的协同作用概念引入搅拌器设计。如果（a）涡核与光/暗边界彼此更接近，并且（b）去除其核心离光/暗边界太远的涡流，则可以获得更好的协同作用。协同作用概念有两种应用类型。首先，它有助于更好地理解关于添加搅拌器的已知数值和实验结果。其次，更重要的是，该概念有助于制定搅拌器设计的新规则。作为案例研究提供了一种螺旋搅拌器设计，以证明协同作用概念的重要性和可行性。发现了一种有效的方法，即从内侧减小螺旋搅拌器的径向高度，通过这种方法，光/暗循环频率提高了10.8%，而泵送成本降低了23.8%。
协同作用概念可以表述如下：光/暗循环频率的提高不仅取决于流场和光场各自的作用，还取决于它们之间的协同作用。这个概念可以用来增强我们对添加搅拌器后出现的一些已知现象的理解。该概念还提供了有用的规则，以设计和优化搅拌器，从而在泵送成本增加较小的情况下实现更高的光/暗循环频率，并且这些规则将在光生物反应器设计中得到广泛应用。
