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

Abstract

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 ine ciencies 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 e cient 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 di↵erent 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 ii 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.iii To Jarah.

From Substrate to Surface: A Turbulence‐Based Model for Gas Transfer Across Sediment‐Water‐Air Interfaces in Vegetated Streams

“…Equation was widely used for experimental designs to measure the gas transfer rate, k L , during the re‐aeration process (Leman et al., 2018; Stenstrom, 2007; Tseng & Tinoco, 2020), which we will follow in Section 3.3.…”

Canopy Randomness, Scale, and Stem Size Effects on the Interfacial Transfer Process in Vegetated Flows

“…Based on the Diffusive Boundary Layer theory (Danckwerts, 1951) with Fick's diffusion law: $$$F=-D{\left.\frac{\partial {C}_{w}}{\partial z}\right\vert }_{z=0}$$$ where C w is the local instantaneous DO concentration in water (see Figure 1a), the recovery curve of DO concentration under different scenarios can be used to fit the corresponding air‐water surface gas transfer rates, k L , by solving the above first order ordinary differential equation with an assumption that $${C}_{D{O}_{t=0}}=0$$: $$${C}_{DO}={C}_{\mathit{sat}}\left[1-\mathrm{exp}\left(-\frac{1}{H}{k}_{L}t\right)\right]$$$ where H is the averaged water depth and C sat is the saturated DO concentration. The above re‐aeration equation was widely used for experimental designs to measure the gas transfer rate, k L , from the re‐aeration process (Leman et al., 2018; Stenstrom, 2007; Tseng & Tinoco, 2020, 2022).…”

“…where H is the averaged water depth and C sat is the saturated DO concentration. The above re-aeration equation was widely used for experimental designs to measure the gas transfer rate, k L , from the re-aeration process (Leman et al, 2018;Stenstrom, 2007;Tseng & Tinoco, 2020.…”

Use of Airfoils for Enhancement of Photosynthesis Rate of Microalgae in Raceway

“…In the case of Free flow 𝜈 𝑎𝑣 is very low, presenting a maximum of 0.1 Hz at the minimum concentration (𝐶 𝑏 = 0.4 g•L− 1 ) and at the minimum linear displacement of 1m. 𝜈 𝑎𝑣 decreases exponentially due to the increase in concentration (𝐶 𝑏 ) and also due to the linear displacement length of the particles, which results in a limitation of the operation and explains why raceways reactors are constantly treated with limited productivity (Inostroza et al, 2021) , This is explained by the fact that a large number of particles have slight changes in their vertical position, according to the literature (Amini et al, 2016;Barceló-Villalobos et al, 2019a;Leman et al, 2018). The analysis has been carried out to a velocity of 0.22 m•s −1 , despite the fact that authors such as Barceló-Villalobos et al, 2019;Brindley et al, 2016 recommend increasing the velocity of the liquid to increase the frequency.…”