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

Raceway;Algae Biofuel;Mixing;Gas Transfer;Secondary Currents

Leman, Andrew S. ; Tinoco ; Rafael O.

关键词: Raceway;    Algae Biofuel;    Mixing;    Gas Transfer;    Secondary Currents;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/100952/LEMAN-THESIS-2018.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Biodiesels from microalgae are a promising alternative to fossil fuels with a number of unique benefits over other alternative fuel sources. However, to date, their viability in the fuels market remains infeasible due in part to a number of inefficiencies in the cultivation process. This has led to a growing interest in the hydrodynamics of open raceway tank (RWT) reactors, and cultivating conditions that are favorable to algae growth. In particular, two impediments require attention: the lack of vertical mixing in the straight portions of the reactor, and the need for efficient introduction of atmospheric carbon into the growth medium. It is proposed that the promotion of cellular, secondary currents by introduction of longitudinal ridges along the bed of the reactor can both promote vertical mixing and enhance gas transfer across the free surface of the growth medium. Experiments are performed in a full-scale open RWT at the University of Illinois, Urbana-Champaign, Ecohydraulics and Ecomorphodynamics Laboratory (EEL), in which the bed of the RWT is modified with longitudinal ridges of two different sizes, triangular and semi-circular cross-sections, and variable spacing. The 3-dimensional velocity components are monitored at significant locations in the reactor by means of acoustic doppler velocimetry (ADV) and surface particle image velocimetry (sPIV), and the gas transfer across the free-surface is measured by re-aeration curves of dissolved oxygen (DO). It is found that the cellular, secondary cells can be visualized in the velocity data after the introduction of longitudinal ridges. However, the impacts at the free surface are limited and consequently so is the gas transfer. A range of flow structures are observed that impact the relative significance of these cellular currents, including: vortex shedding from bend vanes, secondary currents of Prandtl’s first kind induced by the bends, paddlewheel pulsation, and boundary layer development. These structures are found to overwhelm the presence of the cellular currents, and indicate that flows in the straight portions of the RWT reactor are a complex composite of boundary layer flow, secondary currents, resonance structures, energy input conditions, and shear flows. If cellular, secondary currents are to be used to promote the ideal algae growth conditions, further developments on this method will be required to overwhelm or work cooperatively with the dominant bend-induced dynamics.