Determination of the Structural Characteristics of Microalgal Cells Walls under the Influence of Turbulent Mixing Energy in Open Raceway Ponds

Abstract

Turbulent flow mixing is essential in optimizing microalgal cultivation in raceway ponds. Microalgal cells are however highly sensitive to hydrodynamic stresses produced by turbulent mixing because of their small size. The mechanical properties (wall deformation and von Misses stress) of the microalgal cell wall structure under the influence of turbulent mixing are yet to be explored. High turbulence magnitudes damage microalgal cell walls by adversely affecting their mechanical properties which consequently destroy the microalgal cells and reduce the biofuel production. Therefore, such a study is required to improve the biofuel productivity of microalgal cells before their cell wall damage in raceway pond. This study developed a novel fluid-structure interaction (FSI)-based numerical model to investigate the effects of turbulent mixing on the cell wall damage of microalgal cells in raceway ponds. The study investigated microalgal cell wall damage at four different locations in a raceway pond in consideration of the effects of pond's hydrodynamic and geometric properties. An experiment was conducted with a laboratory-scale raceway pond to compare and validate the numerical results by using time-dependent water velocities. Microalgal cell wall shear stress, cell wall deformation, and von Misses stress in the raceway pond were investigated by considering the effects of aspect ratios, water depths, and paddle wheel rotational speeds. Results showed that the proposed numerical model can be used as a prerequisite method for the selection of appropriate turbulent mixing. Microalgal cell wall damage is high in shallow and narrow raceway ponds with high paddle rotational speeds.

The Role of Turbulent Coherent Structures on Microalgal Mixing for Nutrient Removal in Jet and Paddlewheel Raceway Ponds

“…Measurements were used in this study to examine the microalgal mixing processes due to turbulent coherent structures. To date, extensive investigation has been conducted to link turbulent coherent structures to describe the near-bed non-organic material resuspension process [21,23,49]; however, no one investigated the organic material mixing…”

“…Measurements were used in this study to examine the microalgal mixing processes due to turbulent coherent structures. To date, extensive investigation has been conducted to link turbulent coherent structures to describe the near-bed non-organic material resuspension process [21,23,49]; however, no one investigated the organic material mixing mechanism (microalgae in our case) in such detail towards the development of a precise 'microalgae mixing model' merging turbulence features. Two contrasting race ponds, one powered by a paddlewheel and the other by jet, were inspected in order to look at the mechanisms involved in the mixing process of microalgae, where it has been observed that turbulent bursting events have a significant contribution towards microalgal mixing.…”

“…Therefore, the overarching aim of this paper is to signify the importance of instantaneous events on microalgae mixing, which were not considered in the traditional raceway pond mixing approach that uses a time-averaged fluid velocity only to define the Reynolds number [22,23]. We aimed to simulate the hydrodynamic processes involved in two contrasting turbulent environments, i.e., jet and paddlewheel raceway ponds.…”

Two-Phase Flow Modeling of Solid Dissolution in Liquid for Nutrient Mixing Improvement in Algal Raceway Ponds

“…This study used a large-scale 3D raceway pond with a length (L) of 23 m and a channel width (W) of 2.25 m (Figure 1a). The 2D paddle wheel comprises six blades (0.55 m × 0.04 m) with a diameter of 0.6 m. The boundary-connected coupling technique was applied to import the flow effects of the 2D paddle wheel in the 3D raceway pond [24][25][26][27][28]. The Reynolds number (Re) based on hydraulic diameter (D h ) was utilized to characterize the flow in the raceway pond.…”

“…The transport of diluted species interface of COMSOL-Multiphysics was utilized to simulate the concentration of nutrients in the raceway pond. A 2D paddle wheel was modeled and coupled with the 3D raceway pond using the boundary-connected coupling methodology to save the computational memory and time [24][25][26][27][28]. The 2D paddle wheel was simulated with the rotating machinery interface of the COMSOL-Multiphysics.…”