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The demand for rapid, high-quality, and controlled mixing at the microscale has led to the development of various types of micromixers. Micromixers are commonly categorised as active, or passive based on whether they utilise external energy to enhance mixing. Passive micromixers utilise a complex geometry to enhance the diffusion coefficient at lower Reynolds numbers and induce chaotic advection at higher Reynolds numbers for effectively mixing fluids without external energy. Active micromixers, on the other hand, achieve precise, fast, and controllable mixing by employing external energy sources such as pressure, electric, magnetic, or acoustic fields. Some active methods such as magnetic field-driven micromixers need fluids with specific properties. Others, such as acoustic field-driven micromixers apply to various types of fluids. Bubbles can be used as membranes or stirrers in microfluidic devices for both passive and active micromixers. They are easy to use, compatible with microfluidic systems, low cost, and effective. Improvements in manufacturing methods, notably, 3D printing have emerged as promising methods for the development of new micromixer designs. In this paper, a wide range of micromixer types is reviewed and the main mechanism for enhanced mixing is investigated. This study aims to guide researchers proposing innovative designs. Furthermore, it is shown that combining different methods can lead to the development of more effective micromixers, promising further advancements in microscale mixing technology. [ABSTRACT FROM AUTHOR]

Many designs for natural-convection heat sinks and semi-empirical correlations have been proposed in the recent years, but they are only valid in a limited range of Elenbaas numbers E l and were mostly tested for laminar flows. To alleviate those limits, parametric studies with 2D and quasi-3D models were carried out, in ranges of Grashof numbers up to 1.55 × 10 11 and Elenbaas numbers up to 3.42 × 10 7 . Ansys Fluent's laminar, transition-SST, SST k- ω and k- ϵ models were applied. In addition, when used in this valid range, i.e., mean Elenbaas numbers, with the simplified quasi-3D model, the transition-SST model could predict better results, overestimating the heat flux by 10 to 15 % compared to semi-empirical correlations. The 2D model was not deemed satisfying, regarding turbulence models. Consequently, a quasi-3D model was developed: it appeared to be an efficient trade-off between computational time and prediction accuracy, in particular for turbulence models. New grouping factors were also found, to ensure proper dimensioning of natural-convection heat sinks. They corresponded to non-dimensional parameters that dictated the physical behaviour of the heat sink with respect to the semi-empirical correlations. Typically, the ratio of the spacing to the optimal spacing predicted by Bar-Cohen's correlation turned out to be an appropriate grouping factor with a threshold of 1, above which the fins could safely be considered as isolated, thus greatly simplifying all further calculations. [ABSTRACT FROM AUTHOR]

Enhancing both permeability and capillary pumping in porous structures has emerged as a key focus for researchers, leading to the development of biporous media. While experimental studies on these structures have been conducted recently, there is a lack of numerical simulations due to difficulties in describing the geometry. To address this gap, the present study explores pore-scale numerical simulation of two-phase capillary flow in biporous media. A new simplified biporous structure is proposed, featuring a staggered arrangement of clusters, with each cluster composed of closely packed solid particles. For comparison, a monoporous media case is contrasted and represented using a conventional staggered arrangement of solid particles. Both passive and active capillary flow modes are considered in the present study. The numerical results align well with previous experimental findings on biporous media, indicating that the proposed biporous geometry effectively models two-phase flow in complex structures at a reasonable computational cost. The results show that capillary effects in biporous media are up to two times more effective than in monoporous structures. Simultaneously, permeability is enhanced by a factor of four in biporous media under similar circumstances, with most of the mass flow rate (more than 95%) passing through the larger pores between clusters. This combined impact of increased capillary action and higher permeability leads to enhanced wicking performance in biporous structures. The outcomes can help to understand two-phase flow physics in the biporous structure and develop reliable models for the simulation of biporous media on a macroscopic scale. Numerical modeling and comprehension of capillary structures play a crucial role in designing optimized geometries to enhance their performance. [ABSTRACT FROM AUTHOR]

Designing an exhaust manifold with close-coupled catalyst (CCC) relies heavily on time-consuming transient computional fluid dynamics. The current paper provides experimental validation of the addition principle for pulsating flow in CCC manifolds. The addition principle states that the time-averaged catalyst velocity distribution in pulsating flow equals a linear combination .of velocity distributions obtained for steady flow through each of the exhaust runners. A charged motored engine flow rig provides cold pulsating flow in the exhaust manifold featuring blow down and displacement phases, typical of fired engine conditions. Oscillating hot-wire anemometry is used to measure the bidirectional velocity, with a maximum measurable negative velocity of-1 m/s. In part load and zero load conditions, instantaneous reverse flow occurs following the blow-down phase. The two-stage nature of the exhaust stroke combined with strong Helmholtz resonances results in strong fluctuations of the time-resolved mean catalyst velocity. The validity of the addition principle is quantified based on the shape and magnitude similarity between steady and pulsating flow distributions. Appropriate nondimensional groups are used to characterize the flow and quantify the similarity. Statistical significances are provided for the addition principle's validity. The addition principle is valid when the nondimensional scavenging number S exceeds a critical value [S.sub.crit], corresponding to cases of low engine speed and/or high flow rate. This study suggests that the CCC manifold efficiency with respect to catalyst flow uniformity could be quantified using a single scalar parameter, i.e., [S.sub.crit]. The results from the current study are discussed with respect to previously reported results. The combined results are in good agreement and provide a thorough statistically founded experimental validation of the addition principle, based on a broad applicability range. [DOI: 10.1115/1.2201646] Keywords: exhaust system, close-coupled catalyst, pulsating flow, addition principle, oscillating hot-wire anemometer, bidirectional velocity