Enhancing Gas Transfer At An Air-Water Interface Through Strengthened Secondary Flows Motivated By Algal Biofuel Production

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Interest in algal biofuel production has increased significantly in recent years as alternatives to fossil fuels develop. Motivated by the desire to improve gas transfer efficiencies of open-air algae raceway ponds for large-scale algae production, we investigate a low energy, hydro-mechanical approach to enhance the direct air capture (DAC) of CO2 at the air-water interface. This hydro-mechanical DAC approach has the potential to increase algal productivity rates by not only enhancing the flux of CO2 into the solution, but by also enhancing the flux of O2 out of the raceway pond, thus eliminating supersaturation conditions. Secondary flows, namely, streamwise counter-rotating vortices, are known to exist in nature in wide, open channels and scale with the flow depth, H (Nezu and Nakagawa, 1984). Experiments are conducted in the wide, open channel, recirculating flume (test section Lf = 15.0 m long, B = 2.0 m wide, H = 0.1 m deep) in the DeFrees Hydraulics Laboratory to study the effect of counter-rotating vortices on interfacial gas transfer rates. Longitudinal half-sections of PVC pipes are used to form ridges, spaced at 2H, to stabilize and strengthen the streamwise vortices. In situ acoustic Doppler velocimeter (ADV) and surface particle image velocimetry (PIV) measurements are used to verify their existence and characterize their strength. Surface turbulence and surface divergence measurements are obtained for three flow cases ( ReH = U s H / [nu] > 17500, where U s is the bulk streamwise surface velocity) with N=10 ridges and are compared to three control flow cases without ridges. Oxygen transfer velocities, k, are determined for each flow case from dissolved oxygen i reaeration curves that are collected in accordance with ASCE (1993) guidelines. Our findings suggest that the increased transport of fresh water parcels to the surface by these cellular vortices (shown by elevated surface turbulence and divergence levels) thins the concentration boundary layer, increases the concentration gradient, and results in 9-15% higher k values for the flow cases with secondary flows. ii
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Gas transfer at an air-water interface; Surface turbulence and divergence; Algal biofuel production
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Cowen III,Edwin Alfred
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Bisogni Jr,James John
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Civil and Environmental Engineering
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M.S., Civil and Environmental Engineering
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Master of Science
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Government Document
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dissertation or thesis
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