Improvement of vertical mixing in a raceway pond using internal structures

Abstract

Open raceway pond (RWP) is the most common cultivation system for the mass production of microalgae. The algal productivity in the RWP depend on the light intensity. In the RWP, the region near the surface is light zone and the region near the bottom is dark zone. The microalgal cells that remain in the dark region cannot receive sufficient light to grow. This thesis aims to improve vertical mixing so that microalgae can move into the light regions. To enhance mixing efficiency, various designs of internal structure were installed to improve vertical mixing and the fluid flow was compared by experiments and CFD simulations. First, we adopted a suitable structure to decrease of algal settling. The fluid velocity near the bottom increased more than twice compared with inlet velocity. By experiment for microalgae settlement, it was confirmed that less settling occurred in the RWP with the structure than the conventional RWP. Second, another design was proposed to prevent settling and enhance vertical flow. The size and gap of the structure were changed and the flow velocity and vertical velocity increased as the size increased and the gap decreased. Several sets of the largest structures with the small gap were installed along the RWP. As a result, regular vertical flow occurred and average vertical velocity increased by 29 % as compared to the conventional RWP.