Changes in velocity distribution produced by paddlewheels in algae raceway ponds

Abstract

Open raceway ponds (RWPs) are widely used in large-scale algae production. To improve the performance of these systems, it is crucial to understand the flow structure throughout the ponds. Computational fluid dynamics (CFD) simulations have been widely used to study the flow field in RWPs. However, such studies typically require assumptions about the distribution of velocities at the inlet boundary of the domain (which is usually taken as the cross-section a short distance downstream from the paddlewheel). In this study, laboratory measurements were carried out to study the flow structure in the vicinity of a paddlewheel in a raceway pond model. Results showed that the paddlewheel may induce complex changes to the distribution of depth-averaged velocities, including the complex inversion of the velocity gradient across the channel. The measured non-uniform distribution of velocities was then used as the boundary condition of a CFD simulation to understand the effects of common assumptions on the accuracy of CFD simulations of flows in RWPs. Furthermore, observations in a field (full-scale) installation corroborated that the presence of a paddlewheel significantly affects the velocity distribution in its vicinity, further strengthening the argument that such changes to the flow should not be disregarded in numerical simulations of RWPs.