Laura De Masi

Overview

Saltgae represented by the DCU Water Institute participated at the 6th congress of the International Society for Applied Phycology held in Nantes (France) on the 18-23rd June 2017.  It presented updates on research performed on the hydrodynamic characterisation of new open raceway developed at Arava R&D facilities for the treatment of aquaculture effluents. The transient particle tracking models developed are providing detailed insight in the complex flow structures found in the ponds and are helping researchers interpret the operational response of the algae biomass.

 

Technical/Scientific Description

DCU presented work carried out in collaboration with Arava R&D at the 6th congress of the International Society for Applied Phycology (ISAP 2017, Nantes, France)[1]. The presentation focused on the evaluation of High Rate Algae Pond (HRAP) and paddle wheel designs by assessing the mixing, dead zones (stagnant zones) and shear stress with different pond sizes (0.91m3, 1.19m3 and 4.55m3). The HRAP is oval in shape and has a central wall separating two channels. A paddlewheel generates the fluid circulation and provides mixing which is essential to ensure that the micro-algae has a proper expose to light.  Numerical simulations were carried out with a large eddy simulation approach to account for the turbulent flow, and an immersed boundary method[2] for the paddle wheel and fluid interaction. The results show that mixing occurs mostly in the bends and the neighbourhood of the paddle wheel. When the ratio of length to width is high, the simulations show that the vertical mixing in the middle of the two channels is poor. A smaller ratio is to be preferred while the energy provided to generate the flow circulation stays below the productivity threshold. The percentage of dead zones are smaller in long ponds, and the maximum shear stress is below the shear stress capacity threshold of most micro-algae. Mixing and shear stress were also evaluated with two paddle wheel designs. One of them had two plates in each side to reduce shear stress. The simulations show that, as predicted, the shear stress is reduced with the latter design and the mixing is similar with both designs.

[1] L. Rueda Villegas, M. Specklin, G. Savary, Y. Kohn and Y. Delauré, Evaluation of mixing and shear stresses in High Rate Algae Ponds for different paddle wheel designs, 6th Congress of the International Society for Applied Phycology, Nantes, 18-23rd June 2017.

[2] M. Specklin and Y. Delauré, A sharp immersed boundary method based on penalization and its application to moving boundaries and turbulent flows, submitted to Computers & Fluids, May 2017.