Passive micromixer with baffles distributed on both sides of microchannels based on the Koch fractal principle

Since the beginning of the 21st Century, the development of microfluidic chip technology has been very rapid and has attracted the attention of more and more scholars. As an important part of the microfluidic chip, the performance of the micromixer is critical. The fractal structure in the microchannels helps to improve the mixing performance of the micromixer and improve the mixing efficiency of the micromixer. The research results of other scholars are of great significance to the research of the present paper, which mainly studies the effect of changing the baffle state on the mixing efficiency of the micromixer based on the Koch fractal principle. Through simulation analysis, it was found that the mixing efficiency of the baffles distributed on both sides of the microchannel was higher than the mixing efficiency of the baffles distributed on the microchannel side. When the distance between adjacent baffles was divided into 0.15, 0.25 and 0.35 mm, simulated data suggested that the baffle distance of 0.15 mm was best. Increasing the number of baffles from six to eight groups increased the mixing path of the fluid in the microchannel and improved mixing efficiency. A comparison of mixing efficiencies of the 0°, 15° and 30° baffle angles revealed that very significant improvement in mixing efficiency was obtained at 30°. © 2019 Society of Chemical Industry     

Frontiers in combustion techniques and burner designs for emissions control and CO2 capture: A review

The use of fossil fuel is expected to increase significantly by midcentury because of the large rise in the world energy demand despite the effective integration of renewable energies in the energy production sector. This increase, alongside with the development of stricter emission regulations, forced the manufacturers of combustion systems, especially gas turbines, to develop novel combustion techniques for the control of NO_x and CO₂ emissions, the latter being a greenhouse gas responsible for more than 60% to the global warming problem. The present review addresses different burner designs and combustion techniques for clean power production in gas turbines. Combustion and emission characteristics, flame instabilities, and solution techniques are presented, such as lean premixed air‐fuel (LPM) and premixed oxy‐fuel combustion techniques, and the combustor performance is compared for both cases. The fuel flexibility approach is also reviewed, as one of the combustion techniques for controlling emissions and reducing flame instabilities, focusing on the hydrogen‐enrichment and the integrated fuel‐flexible premixed oxy‐combustion approaches. State‐of‐the‐art burner designs for gas turbine combustion applications are reviewed in this study, including stagnation point reverse flow (SPRF) burner, dry low NO_x (DLN) and dry low‐emission (DLE) burners, EnVironmental burners (including EV, AEV, and SEV burners), perforated plate (PP) burner, and micromixer (MM) burner. Special emphasis is made on the MM combustor technology, as one of the most recent advances in gas turbines for stable premixed flame operation with wide turndown and effective control of NO_x emissions. Since the generation of pure oxygen is prerequisite to oxy‐combustion, oxygen‐separation membranes became of immense importance either for air separation for clean oxy‐combustion applications or for conversion/splitting of the effluent CO₂ into useful chemical and energy products. The different carbon‐capture technologies, along with the most recent carbon‐utilization approaches towards CO₂ emissions control, are also reviewed.     

Evaluation of mixing profiles for a new micromixer design strategy

The relationship between mixing history and reaction performance in microreactors using computational fluid dynamics (CFD) simulations is identified. In the idealized, simplified mixing model, mixing proceeds linearly and only the mixing time determined the reaction performance. However, in the case of realistic models where mixing proceeds unequally, the partial rapid progression of mixing, more than the mixing time, significantly impacts the reaction. The use of the fluid segment size distribution to capture this effect is proposed. The effective Damköhler number derived from the fluid segment size distribution predicted the reaction yield well. To demonstrate the utility of the mixing profile design strategy, we fabricated a novel micromixer with multiple partial rapid mixing zones. This micromixer achieved excellent results both in a CFD simulation and an experiment. © 2015 American Institute of Chemical Engineers AIChE J, 62: 1154–1161, 2016     

AC Electrokinetic Fast Mixing in Non-Parallel Microchannels

A novel ultra-fast micromixer of a quasi T-channel with electrically conductive sidewalls is presented here and some new phenomena in its mixing process are observed and reported. The mixing is about 10²–10³ times faster than that by purely molecular diffusion, and about 10² times faster than that in existing micromixers, which are based on the electrokinetic instability (EKI). Both parallel and non-parallel channel are investigated and compared by evaluating their mixing. Mixing behaviors in the microchannels are studied in terms of scalar concentration distributions. It is found that with a small angle (about 5° in this case) between the two electrodes sidewalls, mixing can be enhanced rapidly at even low AC voltage. The influence of the applied AC voltage phase shift between the two electrodes on the mixing process is also explored. The result reveals that the mixing is the strongest under a 180° signal phase shift. Fast mixing is also achieved under high AC frequency in this micromixer. Fluorescent micro particles are used to visualize the flow pattern for better understanding of the mixing enhancement mechanism. The design of this micromixer could provide new opportunity for applications where fast mixing is demanded.     

内置周期挡板的T-型微混合器

设计了一种流道内布置周期挡板结构的高效T-型微混合器来提高微流控系统的混合效率。该微混合器结构简单,周期布置的挡板可以有效地缩短流体混合所需的流道长度和时间,混合效率高。安排了正交实验组,利用计算流体力学软件ANSYS CFX研究了流道结构参数对混合效果的影响。采用静态田口分析法对数值模拟结果进行分析。结果表明:流道结构参数对混合效果的相对影响程度排列如下:挡板攻角(θ)流道高度(H)挡板宽度(L)相邻混合单元之间距离(D)。根据结构参数对混合效果的影响程度,得出研究参数范围内的最优组合为:θ=75°,H=0.4 Wm,L=0.7 Wm,D=0.6 Wm(这里Wm为流道宽度,等于200μm)。实验显示,结构参数符合最优参数组合的微混合器的混合效果提升显著,雷诺数Re=54时即可实现完全混合(混合指标M95%)。文中研究了流道结构对进出口压降的影响,结果显示,攻角θ对进出口压降的影响趋势在不同雷诺数下相同,参数H,D亦如此。     

RGB色彩模型在气动微混合芯片中的应用

研究提出一种基于气动薄膜的微混合器及基于数字图像的混合效率量化分析方法。给出了利用RGB(Red,Green,Blue)色彩模型、灰度转换模型和方差数学模型对微混合腔内不同试剂的混合程度量化分析方法,并利用聚二甲基硅氧烷(PDMS)材料和软刻蚀方法对微混合芯片进行封装。采用实验研究方式使微混合腔内两种不同颜色试剂充分混合,利用混合程度量化分析方法对微混合腔内混合效率进行量化分析,并与自然对流混合效率进行对比。与传统计算混合效率的方法相比,基于RGB色彩模型的混合效率量化方法更简单、直观、有效和方便。     

基于MEMS技术的微流体混合器及相关技术

介绍了基于MEMS技术的微流体混合器及相关技术,给出了各种微流体混合器的结构、原理和特点,同时对微管道中流体混合的仿真、实验技术以及微管道中微量流体混合程度的评价方法等作了概述。     

脉动流动强化微混合的研究

在微尺度下,由于流动处于层流状态,因此混合是一个巨大的难题.针对微尺度混合的特点,提出了一种新型微混合器.这种微混合器利用入口速度方波型脉动实现微通道内流体的有效混合.针对物理问题建立了通用的无量纲方程和边界条件,利用计算流体动力学(CFD)方法研究了微混合器的特性和脉动参数的影响.结果表明在脉动方波占空比为0.5,相位差为180度时,能达到较好的混合效果.当雷诺数为1.0,脉动频率为2Hz时,混合度能达到79%.研究表明:微混合器中流动具有混沌对流特征,脉动流动可极大地增加界面面积,从而实现快速混合.综合各参数的影响建立了参数-脉动体积,当其未超过混合通道体积时,其值越大,混合效果越好.     

基于硅结构的微混合器研究进展与应用

阐述了微混合器的一般概念,从不同角度对微混合器进行分类,综述了微混合器的国内外研究现状,从尺度效应、表面效应和多相流等3个方面分析了微混合器研究过程中存在的问题和今后的研究方向,就微混合器在生物芯片和微小型化学化工机械系统中的应用作了简要说明,展望了微混合器的研究前景.     

Dismt - Determination of mixing time through color changes

The DISMT ("Dual Indicator System for Mixing Time'') mixing time/distance diagnostic encodes semi-quantitative visualization of liquid/liquid mixing processes in color changes. In DISMT, two liquids, one red and one blue, are mixed to produce a yellow liquid. Through appropriate choice of the acid-base indicators used, and the initial pH's of the two solutions, the yellow liquid appears only in those regions where the mixing fraction is within a designated fractional deviation (say 5%) of the infinite time mixing fraction. Thus, the 95% mixing time/ distance can be defined, for the whole volume of the mixing system, as the time/distance for all of the liquid to become yellow. This paper describes: (1) the principles underlying and choices involved in the design of DISMT systems, (2) protocols for carrying out DISMT-based experiments, and (3) examples of results obtained with DISMT.