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As for the application of CFD to biological utilization of carbon dioxide for photosynthetic production of microalgae, the number of published studies is rather limited. One of the most recent and representative studies available is the work reported by Chatterjee [66]. This article is concerned with the internal hydrodynamics of a photo‐bioreactor (PBR) but ignores any other aspects such as the chemistry of the system or the distribution of light across the vessel, which is crucial to the efficiency of the process and depends on the shape of the PBR. Another assumption in this model is that the mass and weight of the microalgae formed in the process were neglected.

Chatterjee [66] carried out a comparative study between a bubble column reactor and a serpentine PBR configuration using ANSYS CFX‐14.0 (Euler–Euler multiphase approach) with time steps ranging from 0.1 to 2 seconds and grids with a number of nodes varying between 75k and 109k. The modeler needs to have special care with the development of the mesh, which must have an appropriate resolution relative to the size of the gas bubbles formed within the PBR. The other crucial aspect is the choice of the turbulence model. Given that the scope of this work is to assess the mixing parameters, the turbulent model must be selected carefully. Both the standard k–ε model and the Reynolds stress model (RSM) were tested. The latter option resulted in a better match between the CFD and the experimental data in terms of gas hold‐up and gas velocities throughout the reactor. The set‐up gave way to valuable maps of turbulence kinetic energy, velocity swirling strength, and gas hold‐up within the domain. The results show that the serpentine configuration gives way to more intense turbulence, which in turn should theoretically result in better microalgae growth rate.

Perhaps the most innovative CFD work concerning PBRs is, however, the article presented by Zhang et al. (2019), who carried out a CFD study of a bionic fractal‐inspired branch‐like PBR. Fractal shapes are the solution provided by Nature for those applications where a high area‐to‐volume ratio is required. They used a similar set‐up to that presented by Chatterjee [66], based on the Euler–Euler method for gas–liquid interface tracking combined with the k–ε turbulence model in order to compare the performance of fractal‐inspired branched geometries to that of common configurations such as multi‐tubular and serpentine. As for the spatial discretization, finer meshes were employed because of the tiny spaces formed in the successive branching generations of the fractal shape (5^th branching generation with diameters in the order of the millimeter). A grid independence study was conducted using three non‐structured meshes (a coarse mesh with 1.30 m nodes, a medium mesh with 2.48 m nodes, and a fine one composed of 3.5 m nodes). The prospective user can thus have an approximation of the degree of grid refinement required in these simulations given the diameter of the channels and the number of nodes, which need substantial computational resources. The results of their model suggested that better mixing occurs when using the fractal shape relative to the multi‐tubular and serpentine configurations, together with small pressure drops (Figure 2.6).

The literature confirms thus, CFD modeling as a valuable tool to carry out preliminary proof of concept of innovative PBR shapes. An interesting proposal for further work would be, however, to use a multispecies approach including mass source terms to account for the consumption of carbon dioxide and the growth of microalgae.