Microalgae are seen as the most promising flexible feedstock, being considered the crop of the future They grow fast, just needing sunlight, carbon dioxide and minerals They contain high value ingredients, such as proteins, carbohydrates, lipids, nucleic acids and others (carotenoids and polymers) Thus, they can be produced for a wide range of markets, including human and animal nutrition, cosmetics, pharmaceuticals and biofuels However, the production cost is still high, limiting their commercial applications to high-valued compounds The reduction of these costs can be obtained with efficient bioreactor designs, which are able to achieve high areal biomass productivities In this context, Computational Fluid Dynamics (CFD) may play an important role in the optimization of bioreactor design, analysing the interaction of hydrodynamics, light supply, heat and mass transfer and biological kinetics This study addresses the recent advances in CFD modelling of both open pond and closed bioreactors

Photobioreactor design for microalgae production through computational fluid dynamics: A review

We numerically analyze the mixing and pressure drop characteristics for flow through wavy micromixer of two geometrical configurations, namely raccoon and serpentine for different values of amplitude of the waviness of the mixer (α), wavelength of the waviness (λ), Reynolds number(Re) and Schmidt number(Sc). Three different flow regimes are identified depending on the parameters influencing the mixing index. The mixing index for both the raccoon and serpentine mixer is very close to unity in the first regime (0.1

Numerical study of mixing in wavy micromixers: comparison between raccoon and serpentine mixer

In this numerical study, a new type of serpentine micromixer involving mixing units with a non-rectangular cross-section is investigated. Similar to other serpentine/spiral shaped micromixers, the design exploits the formation of transversal vortices (Dean flows) in pressure-driven systems, associated with the centrifugal forces experienced by the fluid as it is confined to move along curved geometries. In contrast with other previous designs, though, the use of non-rectangular cross-sections that change orientation between mixing units is exploited to control the center of rotation of the transversal flows formed. The associated extensional flows that thus develop between the mixing segments complement the existent rotational flows, leading to a more complex fluid motion. The fluid flow characteristics and associated mixing are determined numerically from computational solutions to Navier–Stokes equations and the concentration-diffusion equation. It is found that the performance of the investigated mixers exceeds that of simple serpentine channels with a more consistent behavior at low and high Reynolds numbers. An analysis of the mixing quality using an entropic mixing index indicates that maximum mixing can be achieved at Reynolds numbers as small as 20 in less than four serpentine mixing units.

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Mixing Enhancement in Serpentine Micromixers with a Non-Rectangular Cross-Section.

Nanofluids have opened the doors towards the enhancement of many of today’s existing thermal applications performance. This is because these advanced working fluids exhibit exceptional thermophysical properties, and thus making them excellent candidates for replacing conventional working fluids. On the other hand, nanomaterials of carbon-base were proven throughout the literature to have the highest thermal conductivity among all other types of nanoscaled materials. Therefore, when these materials are homogeneously dispersed in a base fluid, the resulting suspension will theoretically attain orders of magnitude higher effective thermal conductivity than its counterpart. Despite this fact, there are still some challenges that are associated with these types of fluids. The main obstacle is the dispersion stability of the nanomaterials, which can lead the attractive properties of the nanofluid to degrade with time, up to the point where they lose their effectiveness. For such reason, this work has been devoted towards providing a systematic review on nanofluids of carbon-base, precisely; carbon nanotubes, graphene, and nanodiamonds, and their employment in thermal systems commonly used in the energy sectors. Firstly, this work reviews the synthesis approaches of the carbon-based feedstock. Then, it explains the different nanofluids fabrication methods. The dispersion stability is also discussed in terms of measuring techniques, enhancement methods, and its effect on the suspension thermophysical properties. The study summarizes the development in the correlations used to predict the thermophysical properties of the dispersion. Furthermore, it assesses the influence of these advanced working fluids on parabolic trough solar collectors, nuclear reactor systems, and air conditioning and refrigeration systems. Lastly, the current gap in scientific knowledge is provided to set up future research directions.

Carbon-Based Nanofluids and Their Advances towards Heat Transfer Applications-A Review.

The aim of our work is the numerical modeling of two dimensional mechanical–thermal material mixing observed in stir welding process using a high order algorithm. This algorithm is based on coupling a meshless approach, a time discretization, a homotopy transformation, a development in Taylor series and a continuation method. The performance of the proposed model is the consideration of large deformations in the formulation of the posed problem. For the spatial treatment, we use the moving least squares approximation which will be applied directly to the strong form formulation of conservation equations. Each collocation point holds mechanical–thermal variables. The high order algorithm and the homotopy transformation allow reducing the number of tangent matrices to decompose and to avoid iterative procedure. Comparisons with the classical iterative solver (Jamal et al. in J Comput Mech 28:375–380, 2002) are performed. Numerical results reveal that a few number of matrix factorization is needed with the proposed approach, decreasing the computation time.

A 2D mechanical–thermal coupled model to simulate material mixing observed in friction stir welding process

Sample mixing is difficult in microfluidic devices because of laminar flow. Micromixers are designed to ensure the optimal use of miniaturized devices. The present study aims to design a chaotic-advection-based passive micromixer with enhanced mixing efficiency. A serpentine-shaped microchannel with sinusoidal side walls was designed, and three cases, with amplitude to wavelength (A/λ) ratios of 0.1, 0.15, and 0.2 were investigated. Numerical simulations were conducted using the Navier–Stokes equations, to determine the flow field. The flow was then coupled with the convection–diffusion equation to obtain the species concentration distribution. The mixing performance of sinusoidal walled channels was compared with that of a simple serpentine channel for Reynolds numbers ranging from 0.1 to 50. Secondary flows were observed at high Reynolds numbers that mixed the fluid streams. These flows were dominant in the proposed sinusoidal walled channels, thereby showing better mixing performance than the simple serpentine channel at similar or less mixing cost. Higher mixing efficiency was obtained by increasing the A/λ ratio.

Analysis of Passive Mixing in a Serpentine Microchannel with Sinusoidal Side Walls

In the present study, a novel technique, which involves numerical computation of the mixing length of algae particles in raceway ponds, was used to evaluate the mixing process. A value of mixing length that is higher than the maximum streamwise distance (MSD) of algae cells indicates that the cells experienced an adequate turbulent mixing in the pond. A coupling methodology was adapted to map the pulsating effects of a 2D paddle wheel on a 3D raceway pond in this study. The turbulent mixing was examined based on the computations of mixing length, residence time, and algae cell distribution in the pond. The results revealed that the use of particle tracing methodology is an improved approach to define the mixing phenomenon more effectively. Moreover, the algae cell distribution aided in identifying the degree of mixing in terms of mixing length and residence time. Biotechnol. Bioeng. 2015;112: 297–307. © 2014 Wiley Periodicals, Inc.

Numerical prediction of algae cell mixing feature in raceway ponds using particle tracing methods

The practical implication of nanofluids is essentially dependent on their accurate modelling, particularly in comparison with the high cost of experimental investigations, yet the accuracy of different computational approaches to simulate nanofluids remains controversial to this day. Therefore, the present study is aimed at analysing the homogenous, multiphase Eulerian–Eulerian (volume of fluid, mixture, Eulerian) and Lagrangian–Eulerian approximation of nanofluids containing nonspherical nanoparticles. The heat transfer and pressure drop characteristics of the multiwalled carbon nanotubes (MWCNT)-based and multiwalled carbon nanotubes/graphene nanoplatelets (MWCNT/GNP)-based nanofluids are computed by incorporating the influence of several physical mechanisms, including interfacial nanolayering. The accuracy of tested computational approaches is evaluated by considering particle concentration and Reynolds number ranges of 0.075–0.25 wt% and 200–470, respectively. The results demonstrate that for all nanofluid combinations and operational conditions, the Lagrangian–Eulerian approximation provides the most accurate convective heat transfer coefficient values with a maximum deviation of 5.34% for 0.25 wt% of MWCNT–water nanofluid at the largest Reynolds number, while single-phase and Eulerian–Eulerian multiphase models accurately estimate the thermal fields of the diluted nanofluids at low Reynolds numbers, but overestimate the results for denser nanofluids at high Reynolds numbers.

Homogeneous and Multiphase Analysis of Nanofluids Containing Nonspherical MWCNT and GNP Nanoparticles Considering the Influence of Interfacial Layering.

In this study, we reported continuous partial curing and tip-shaped modification methods for continuous production of dry adhesive with microscale mushroom-shaped structures. Typical fabrication methods of dry adhesive with mushroom-shaped structures are less productive due to the failure of large tips on pillar during demolding. To solve this problem, a typical pillar structure was fabricated through partial curing, and tip widening was realized through applying the proper pressure. Polyurethane acrylate was used in making the mushroom structure using two-step UV-assisted capillary force lithography (CFL). To make the mushroom structure, partial curing was performed on the micropillar, followed by tip widening. Dry adhesives with properties similar to those of typical mushroom-shaped dry adhesives were fabricated with reasonable adhesion force using the two-step UV-assisted CFL. This production technology was applied to the roll-to-roll process to improve productivity, thereby realizing continuous production without any defects. Such a technology is expected to be applied to various fields by achieving the productivity improvement of dry adhesives, which is essential for various applications.

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Continuous Tip Widening Technique for Roll-to-Roll Fabrication of Dry Adhesives

Thermal performance enhancement in microchannel heat sinks has recently become a challenge due to advancements in modern microelectronics, which demand compatibility with heat sinks able to dissipate ever-increasing amounts of heat. Recent advancements in manufacturing techniques, such as additive manufacturing, have made the modification of the microchannel heat sink geometry possible well beyond the conventional rectangular model to improve the cooling capacity of these devices. One such modification in microchannel geometry includes the introduction of secondary flow channels in the walls between adjacent mainstream microchannels. The present study computationally models secondary flow channels in regular trapezoidal and parallel orientations for fluid circulation through the microchannel walls in a heat sink design. The heat sink is made of silicon wafer, and water is used as the circulating fluid in this study. Continuity, momentum, and energy equations are solved for the fluid flow through the regular trapezoidal secondary flow and parallel secondary flow designs in the heat sink with I-type, C-type, and Z-type inlet–outlet configurations. Plots of velocity contours show that I-type geometry creates optimal flow disruption in the heat sink. Therefore, for this design, the pressure drop and base plate temperatures are plotted for a volumetric flow rate range, and corresponding contour plots are obtained. The results are compared with corresponding trends for the conventional rectangular microchannel design, and associated trends are explained. The study suggests that the flow phenomena such as flow impingement onto the microchannel walls and formation of vortices inside the secondary flow passages coupled with an increase in heat transfer area due to secondary flow passages may significantly improve the heat sink performance.

https://www.mdpi.com/1996-1073/13/21/5616/pdf

Cheol Woo Park

Papers

Analysis of Passive Mixing in a Serpentine Microchannel with Sinusoidal Side Walls

Numerical prediction of algae cell mixing feature in raceway ponds using particle tracing methods

Homogeneous and Multiphase Analysis of Nanofluids Containing Nonspherical MWCNT and GNP Nanoparticles Considering the Influence of Interfacial Layering.

Continuous Tip Widening Technique for Roll-to-Roll Fabrication of Dry Adhesives

Hydrothermal Investigation of a Microchannel Heat Sink Using Secondary Flows in Trapezoidal and Parallel Orientations