We have conducted a study of the potential use of drag-reducing biopolymers produced by marine microalgae for engineering applications. Several marine microalgae species were tested for their production of drag-reducing polysaccharides in large custom-designed plate bioreactors. Promising species (such as Porphyridium cruentum, Rhodella maculata, Schizochlamydella capsulata and Chlorella stigmatophora) were cultured for periods of time ranging from a few weeks to over 6 months. The basic drag-reducing ability of the polysaccharides was established by comparing their drag reduction effectiveness at various concentrations in water. The algal polysaccharide mass productivity was also measured per unit area of bioreactor's illuminated surface. Finally, an all-inclusive criterion, the volumetric production of drag-reducing water giving a set level of drag reduction was quantified, and led us to a ranking of the tested species in order of productivity relevant to implementation. Some aspects of polysaccharide production by aged cultures were investigated as well. We also quantified the drag-reducing effectiveness of intracellular polysaccharides, and visualized the presence of exopolymer particles in the medium.
We have investigated the intentional temporary degradation and the subsequent recovery of the drag-reducing properties of surfactant solutions. Degradation was achieved by exposing the fluid to mechanical stresses after which it showed significantly reduced drag reduction capability. The recovery time varies from a fraction of second to several minutes, depending on the surfactant concentration, the counter-ion concentration and the temperature. The recovery process generally showed a linear increase of the drag reduction level with time, with the rate of recovery being essentially independent of the flow velocity and the initial level of degradation, though the recovery at rest is considerably slower than the recovery under turbulent flow conditions. The recovery time decreases sharply with increasing concentration and temperature, and with increasing counter-ion concentration for the cationic surfactant. The mechanical degradation apparently affects not only the shear-induced structure but also the rod-like micelles themselves.
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