Identifying the dominant physical processes for mixing in full-scale raceway tanks

Open racetrack flumes represent the most common reactor for algae cultivation in the biodiesel industry, however, despite numerous experimental and numerical studies into reactor modifications, conditions remain suboptimal for algal growth. In response, a full-scale racetrack flume was constructed at the Ecohydraulics and Ecomorphodynamics Laboratory at the University of Illinois. Experiments on the racetrack flume were conducted for various depth and velocity conditions, using acoustic doppler velocimetry and surface particle velocimetry to characterize mean and turbulent velocity statistics, and dissolved oxygen measurements to investigate the effect of turbulent structures on gas transfer at the water-air interface. Longitudinal bed modifications were introduced to induce secondary flows in the straight portions of the flume. Semicircular and triangular bars of two different sizes were tested in an effort to increase the transfer velocity at the free surface. A range of flow structures were observed including secondary currents of Prandtl's first and second kinds, vortex shedding off of bend vanes, and periodic oscillations in surface lateral currents. Findings indicate that bend dynamics introduce the strongest and most resilient flow structures, and any attempt to induce vertical mixing or accelerate transfer velocities at the free surface will need to utilize or overwhelm these existing structures.