Numerical prediction of algae cell mixing feature in raceway ponds using particle tracing methods

Abstract

In the present study, a novel technique, which involves numerical computation of the mixing length of algae particles in raceway ponds, was used to evaluate the mixing process. A value of mixing length that is higher than the maximum streamwise distance (MSD) of algae cells indicates that the cells experienced an adequate turbulent mixing in the pond. A coupling methodology was adapted to map the pulsating effects of a 2D paddle wheel on a 3D raceway pond in this study. The turbulent mixing was examined based on the computations of mixing length, residence time, and algae cell distribution in the pond. The results revealed that the use of particle tracing methodology is an improved approach to define the mixing phenomenon more effectively. Moreover, the algae cell distribution aided in identifying the degree of mixing in terms of mixing length and residence time.

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

“…A 2D paddle wheel with six blades (0.55 m × 0.04 m) and a diameter of 0.6 m was used to circulate water in the 3D raceway pond. The boundary-connected coupling method was employed to introduce the pulsatile flow of the 2D paddle wheel in the 3D raceway pond [24,[28][29][30]. The Reynolds number (Re h ) based on the hydraulic diameter (D h ) determined the turbulent flow in the raceway pond as follows:…”

“…Turbulence magnitude is high near the paddle wheel and low in the downstream bend of a raceway pond [24]. The microalgal cells (Chlorella) near the paddle wheel experience larger fluid stress and thus present more cell wall deformation than those in the downstream bend.…”

“…The microalgal cells must follow the water flow to effectively interact with nutrients and CO 2 before reaching the pond's outlet. The Stokes number (St) is a widely used non-dimensional number to predict the existence of relative velocity between a fluid and a particle [24,34]. The governing equation of St can be expressed as follows:…”

“…Microalgal cells (Chlorella) with the condition (St << 1) followed the water flow and successfully reached the outlet of the raceway pond [24]. Aspect ratio is a key parameter in open-channel flow systems because it significantly affects the flow characteristics of raceway ponds [35].…”

“…Therefore, the mechanical properties (wall deformation and von Misses stress) of the cell wall must be estimated to investigate the effects of turbulence on microalgal cell damage [23]. The authors previously used CFD particle tracing to investigate the turbulent mixing of microalgal cells in a raceway pond [24]. However, the study focused on the evaluation of mixing phenomena and disregarded the effects of turbulence on microalgal cell wall damage.…”