Numerical and experimental investigation of hydrodynamics and light transfer in open raceway ponds at various algal cell concentrations and medium depths

A spectral radiation-transport model was integrated with a three dimensional computational fluid dynamics model to simulate the hydrodynamics and light transfer in open raceway ponds (ORPs). The predicted three-dimensional velocity and light intensity agreed well with measured values collected on a lab-scale ORP. However, there was a slight difference in the predicted velocity profiles using two different types of boundaries for the paddlewheel, i.e., the moving zone boundary and inlet velocity boundary, with R² values between the predicted and measured velocities of 0.9947 and 0.9838, respectively. The R² value between the predicted and measured light intensity was 0.9939. Simulations were further conducted on a large-scale ORP with 100 m² surface area operated at total medium depths of 0.2 and 0.3 m, average cell concentration of 0.4 g/L, and inlet velocities of 0.1, 0.2 and 0.3 m/s from the paddlewheel. The increase of inlet flow velocity from 0.1 to 0.2 m/s resulted in a more uniform cell concentration profile. However, when the inlet velocity was further increased from 0.2 to 0.3 m/s, there was only a slight increase in the uniformity of the cell concentration. In addition, the simulation results showed that sedimentation of cells more likely occurred at the bottom of the ORP with a total medium depth of 0.2 m than at 0.3 m at the same inlet velocity. The increase of inlet velocity from the paddlewheel resulted in a uniformly distributed light intensity in the region near the medium surface (e.g., 0.05 m depth from the surface) owing to improved mixing. However, owing to a sudden drop in the light intensity after a few centimeters from the medium surface, the cell sedimentation that occurred at the bottom of the ORPs had negligible effects on the light penetration depth in the medium.