微型热光电系统的新型微混合器的模拟研究

通过使用计算流体动力学软件FLUENT 6,研究了一种混合通道中带有不动凸块的新型T型微混合器,并与直通道不带不动凸块的相同大小的T型微混合器在相同的工况下进行模拟比较,观察到前者的混合效果比后者有显著改善,认为这是由于当混合气体通过这些不动凸块时产生漩涡和分层引起的.同时也研究了混合通道内不动凸块的数量对混合强度的影响,证明混合管道越长,不动凸块的数量越多,混合效果越明显.研究结果对微热光电系统中混合器的选用和制造具有指导意义.     

叶序排布展开结构微混合器混合特性模拟分析

为了提高液体混合的效率,根据生物科学中的叶序理论,设计柱状叶序排布展开结构的微混合器.利用计算机对混合效果进行仿真,获得了叶序系数、微圆柱直径对微混合器混合效果的影响规律,微圆柱的叶序排布、矩阵排布、错位排布的混合效果进行对比.结果表明:微圆柱叶序排布的混合器混合效率最高,混合液体在混合通道中沿着顺时针和逆时针叶列螺旋沟槽流动,液体能更好的交互扩散流动,有利于液体混合.     

一种基于多重涡流新型微混合器的设计与实验

设计了一种基于多重涡流的新型微混合器结构.该结构中多个呈反向分布的规则排列的扩张和收缩结构可产生迪恩流与扩张涡流,基于此,可进行高效率的被动式混合.首先阐述了该结构产生内部迪恩流和扩张涡流的机理,并利用COMSOL软件对该结构进行了液流混合过程的三维数值模拟验证,分析了不同流体流速对多重涡流强弱和混合程度的影响.最后制作了该微混合器,利用2种不同的染料进行了混合实验,通过分析样品的混合效果验证了该混合器的性能.仿真和实验结果均表明:该混合器在低输入流速(0.01 μL/s)和高输入流速(7μL/s)下均可实现高效快速的混合,在0.8cm的距离内即可实现完全混合,在输入流速为5 μL/s时最高混合效率可达98.6％.     

雷诺数对高负荷低速涡轮性能影响的试验研究

为了研究雷诺数对高负荷低压低速涡轮性能的影响规律,以某双级高负荷低压低速涡轮为研究对象,在涡轮部件试验器上开展了变雷诺数试验研究。控制其他相似参数保持一致,通过来流节流、出口引射的方式改变涡轮内雷诺数水平。共采集了6个雷诺数状态下的气动参数,分析了雷诺数对涡轮总特性及测量截面流场参数的变化规律。结果表明,该涡轮效率随着雷诺数的降低而显著降低,研究雷诺数范围内,效率降低了2.8%;在低雷诺数下,第二级导叶通道涡、尾缘涡等涡系尺度及强度均发展壮大,气动损失增加;第二级动叶轮周功减小,做功能力损失6.3%。     

一种新的纸基微流开关及其活跃方法

提出了一种新的、基于声表面波的纸基微流开关。通过软光刻技术制作内含两个微孔的聚二甲基硅氧烷(PDMS)微架,其上固定经折叠、长度可变的纸通道。PDMS微架贴附于压电基片之上,并在待连接的两微通道之下方,折叠纸通道最低端离压电基片间距为2 mm。压电基片上采用微电子工艺光刻一对叉指换能器和反射栅。当足够强度的电信号加到叉指换能器对时,激发两相向声表面波,使得压电基片上微流体输运到折叠纸通道,改变其长度,连接其上待连通的两纸基微通道,完成开关功能。对可编程微流器件提供了一种新的编程和开关控制方法。     

出流比对带侧流梯形扰流柱通道换热的影响

为了解侧向出流比对扰流柱通道端壁换热的影响,对简化后的涡轮叶片尾缘扰流柱冷气通道进行了数值仿真研究.结果表明:出流比对端壁表面的换热有很大影响.出流比为0时,梯形通道端壁换热系数呈不对称分布,靠近梯形长边处较大,窄边处较小;出流比大于0时,端壁靠近梯形窄边通道弦向出流口区域的换热系数随着出流比的增大而增大,而靠近梯形长边区域换热系数则随着出流比的增大而降低;出流比对通道端壁表面换热的影响随着雷诺数的增大而增大.     

线性均衡和判决反馈均衡LMS算法仿真分析

介绍基于自适应最小均方线性均衡和判决反馈均衡算法的原理,并通过实验仿真比较两种算法在训练判决引导混合模式下的均衡性能,分析反馈滤波器长度对判决反馈均衡器性能的影响。结果表明:在训练阶段,最小均方线性均衡算法优于最小均方判决反馈均衡算法的性能;在判决阶段,良好信道条件下最小均方线性均衡具有比较理想的性能,当信道条件恶劣时,最小均方线性均衡算法性能变差,而最小均方判决反馈均衡算法随着反馈滤波器长度增加,均衡效果更优。     

二阶系统模糊自适应PID变阻尼控制仿真研究

为了改善二阶系统的动态性能,通常用速度反馈增大等效阻尼比的方法减小输出响应振荡和超调量,但阻尼比增大,系统响应迟钝,快速性变差.当无阻尼自然振荡角频率一定时,阻尼比是影响二阶系统性能的主要因素.在线调整速度反馈通道增益以改变阻尼比从而改善系统性能.由于很难准确建立系统跟踪偏差和偏差变化率与速度反馈通道增益之间的数学模型,所以,模糊自适应PID变阻尼控制结合PID控制和模糊控制的优点,运用模糊推理方法根据系统输出响应的不同阶段在线实时调整系统的阻尼比,从而提高系统快速性和稳定性.仿真结果表明该方法可行,系统动态性能得到明显改善,无超调现象,并具有良好的抗干扰性.     

预失真多合体功率放大器ACLR与反馈通道带宽的关系

针对TI公司数字预失真芯片GC5322和Xilinx公司的数字预失真知识产权核(IP Core),考虑输入九载波TD-SCDMA信号,测试了数字预失真多合体功率放大器的反馈通道带宽与邻道及次邻道泄漏功率比(ACLR)的关系。测试结果表明,当反馈通道带宽大于信号带宽1.7倍时,再增加反馈通道带宽,ACLR的改善量变化不明显,在±1.3 dB范围内波动;当反馈通道带宽小于信号带宽1.7倍以下时,再降低反馈通道带宽,ACLR的改善量恶化明显。该结果对工程实现时选取最低反馈通道带宽,降低系统对模数转换器(ADC)的指标要求和器件成本,具有重要的参考和应用价值。     

微小尺度射流流量传感器的设计与仿真分析

在许多新型传感器和微系统中均存在微量流体自动、精确地驱动和控制问题,而这有赖于微小尺度下对流量的精确测量。基于射流振荡原理设计了一种新型的无反馈通道微小尺度流量传感器,采用计算流体动力学(CFD)方法对该流量传感器的测量特性进行了仿真研究。通过观测振荡腔内部流场,分析了振荡器内部流动形态和射流振荡过程。通过对监测点压力变化曲线的分析,获得了不同入口速度下流体振荡频率,建立了流体流速与振荡频率的函数关系。研究结果表明,该微小尺度射流振荡器振荡平稳,主射流切换灵活,在较宽的流速范围内,流速与振荡频率具有线性关系,具有0.3%的较低的相对压力损失并可达到较小的测量下限,易于加工成型。     

Numerical investigation on the efficient mixing of overbridged split-and-recombine micromixer at low Reynolds number

It is promising to design a novel structured micromixer that can be easily processed but also exhibit high mixing efficiency as well as low pressure drop at a wide range of Reynolds numbers. The overbridged structure was introduced into the planner E-shape micromixers for the first time to construct a novel kind of bridge-street structure micromixer, in order to improve the mixing efficiency in the wide range of Reynolds number. We investigated numerically the mixing performance of six overbridged E-shape split-and-recombine micromixers via solving 3D Navier–Stokes equations and adopting species transfer model. It is indicated that at lower Reynolds number the tilted interface in the overbridged channel increases the interfacial area and improves the mass transfer efficiency, while at higher Reynolds number the overbridged channels tend to induce vortices and promote the convective diffusion. The results show that the optimal overbridged micromixer DBEM-3 has excellent mixing efficiency exceeding 95% in the range of Re = 0.5–100. The optimal structure of overbridged micromixer was studied further with different viscosity ratio and power law fluid. In addition, the pressure drop under various Reynolds number was calculated, and the pressure drop of the power law fluid was represented by Euler number to reflect the magnitude of the momentum loss rate. It is illustrated that DBEM-3 has excellent mixing efficiency in wide Reynolds number for three different fluid systems, which has promising applications in the biochemistry analysis or mixing systems.

     

Interfacial configurations and mixing performances of fluids in staggered curved-channel micromixers

We present a parallel laminar micromixer with staggered curved channels for homogeneously mixing two fluids by Dean vortex. The secondary flows are produced in curved rectangular channels by the centrifugal forces; the diffusion distance of two fluids is reduced due to the staggered structures of the flow channels. The mixing strength is increased when one stream is injected into the other. Confocal microscopy and pH indicator have been used to study the mixing. Computational fluid dynamics simulations are utilized to examine the interfacial configurations and the mixing behaviors inside the channels. The interface of the two fluids is heavily distorted and increases the interfacial area because of the unique structures. The mixing index of the staggered curved-channel mixer with tapered channels is higher than those of the other curved-channel mixers. The effects of various Reynolds numbers and channel configurations on mixing performances are investigated in terms of the experimental mixing indices and the computational interfacial patterns. The comparison between the experimental data and numerical results shows a very similar trend.     

Effect of geometry on fluid mixing of the rhombic micromixers

A planar micromixer with rhombic microchannels and a converging-diverging element has been proposed for its effective mixing. Both CFD-ACE numerical simulations and experiments were used to design and investigate the effect of three parameters (number of rhombi, turning angle and absence or presence of the converging-diverging element) on mixing. Mixing efficiency is dependent upon Reynolds number and geometrical parameters. Through the results of numerical simulation, it is evident that smaller turning angle (α), higher Reynolds number and increasing number of rhombi will result in better fluid mixing due to the occurrence of larger recirculation. The large recirculation is beneficial for both the increased interfacial contact area between two species and the convective mixing. In the numerical simulations, mixing efficiency of 99% was achieved with a most efficient system consisting of three-rhombus mixer with a converging-diverging element at α = 30° and Re = 200. An experimental mixing efficiency of about 94% has been obtained with the same design parameters. As expected, it is lower than the theoretical efficiency but is still very effective. A micromixer with such design can be potentially useful in the future applications of rapid and high throughput mixing.     

A high-efficiency planar micromixer with convection and diffusion mixing over a wide Reynolds number range

Over a wide Reynolds number range (0.1 ≤ Re ≤ 40), the new planar obstacle micromixer has been demonstrated over 85% mixing efficiency covering the mixing improvement in both convection-enhanced (higher Re flow) and diffusion-enhanced (lower Re flow) mechanisms. Mixing behavior between two operation windows was investigated by numerical simulations and experiments. For the adaptive design, numerical simulations and Taguchi method were used to study the effect of four geometrical factors on sensitivity of mixing. The factors are gap ratio (H/W), number of mixing units, baffle width (W _b) and chamber ratio (W _m /W). The degree of sensitivity using the Taguchi method can be ranked as: Gap ratio > Number of mixing units > Baffle width > Chamber ratio. Micromixer performance is greatly influenced by the gap ratio and Reynolds number. Beside the wide Reynolds number range, good mixing efficiency can be obtained at short distance of a mixing channel and relatively low-pressure drop. This micromixer had improved both complex fabrication process of multi-layer or 3D micromixers and low mixing efficiency of planar micromixer at Re < 100. The trend of the verified experimental results is in agreement with the simulate results.     

An efficient micromixer actuated by induced-charge electroosmosis using asymmetrical floating electrodes

Efficient microfluid mixing is an important process for various microfluidic-based biological and chemical reactions. Herein we propose an efficient micromixer actuated by induced-charge electroosmosis (ICEO). The microchannel of this device is easy to fabricate for its simple straight channel structure. Importantly, unlike previous design featuring complicated three-dimensional conducting posts, we utilize the simpler asymmetrical planar floating-electrodes to induce asymmetrical microvortices. For evaluating the mixing performance of this micromixer, we conducted a series of simulations and experiments. The mixing performance was quantified using the mixing index, specifically, the mixing efficiency can reach 94.7% at a flow rate of 1500 µm/s under a sinusoidal wave with a peak voltage of 14 V and a frequency of 400 Hz. Finally, we compared this micromixer with different micromixing devices using a comparative mixing index, demonstrating that this micromixer remains competitive among these existing designs. Therefore, the method proposed herein can offer a simple solution for efficient fluids mixing in microfluidic systems.     

Investigation of surface roughness effects on fluid flow in passive micromixer

Surface roughness effects are dominant at microscale. In this study, microchannels are fabricated on Silicon substrate. The roughness morphology is modeled for the fabricated structure using Weierstrass-Mandelbrot function for self-similar fractals. A two dimensional model of hexagonal passive micromixer is analyzed with surface roughness present on inner walls of channels using parallel Lattice Boltzmann method, implemented on sixteen node cluster. The results are compared by simulating this micromixer structure using Navier–Stokes equations. The experimental results on the fabricated micromixers are also presented. The effects of relative roughness, fractal dimension and Reynolds number are discussed on laminar flow in hexagonal passive micromixers. The study concludes the importance of modeling surface roughness effect for better mixing efficiency.     

Chaotic mixing using periodic and aperiodic sequences of mixing protocols in a micromixer

We conducted a numerical study on mixing in a barrier embedded micromixer with an emphasis on the effect of periodic and aperiodic sequences of mixing protocols on mixing performance. A mapping method was employed to investigate mixing in various sequences, enabling us to qualitatively observe the progress of mixing and also to quantify both the rate and the final state of mixing. First, we introduce the design concept of the four mixing protocols and the route to achieve chaotic mixing of the mixer. Then, several periodic sequences consisting of the four mixing protocols are used to investigate the mixing performance depending on the sequence. Chaotic mixing was observed, but with different mixing rates and different final mixing states significantly influenced by the specific sequence of mixing protocols and inertia. As for the effect of inertia, the higher the Reynolds number the larger the rotational motion of the fluid leading to faster mixing. We found that a sequence showing the best mixing performance at a certain Reynolds number is not always superior to other sequences in a different Reynolds number regime. A properly chosen aperiodic sequence results in faster and more uniform mixing than periodic sequences.     

Flow feature analysis of an eye shaped split and collision (ES-SAC) element based micromixer for lab-on-a-chip application

An eye shaped split and collision micromixer having low-pressure drop is proposed, which works on the concept of unbalanced splits and cross-collisions of fluid streams. The 3-D Navier–Stokes equations in combination with an advection–diffusion model were solved for the analysis with water and ethanol as working fluids. The in-depth analysis of the flow features and the mixing performance parameters viz. mixing index and pressure drop of the micromixer has been carried out. The micromixer model is composed of two sub-channels of equal/unequal widths which repeatedly undergo splitting and collision of fluid streams along the flow direction. The numerical study has been carried out on the micromixer at Reynolds numbers ranging from 0.1 to 45. The difference between the mass flow rates in the two sub-channels creates an unbalanced collision of the two fluid streams. Mixing enhancement is mainly due to the effect of unbalanced collisions of the fluid streams. The micromixers show exciting flow features for different ratios of the widths of the sub-channels. The ratios of a width of subchannels viz. 1, 1.4 and 2 are considered. The highest mixing performance has been observed for the width ratio of 2, whereas poor mixing performance has been observed for the width ratio of 1.

     

Improved serpentine laminating micromixer with enhanced local advection

It is a complicated task to achieve high level of mixing inside a microchannel because the flow is characterized by low Reynolds number (Re). Recently, the serpentine laminating micromixer (SLM) was reported to achieve efficient chaotic mixing by introducing “F”-shape mixing units successively in two layers such that two mixing mechanisms, namely splitting/recombination and chaotic advection, enhance the mixing performance in combination. The present paper proposes an improved serpentine laminating micromixer (ISLM) with a novel redesign of the “F”-shape mixing unit: reduced cross-sectional area at the recombination region locally enhances advection effect which helps better vertical lamination, resulting in improved mixing performance. Flow characteristics and mixing performances of SLM and ISLM are investigated numerically and verified experimentally. Numerical analysis system is developed based on a finite element method and a colored particle tracking method, while mixing entropy is adopted as a mixing measure. Numerical analysis result confirms enhanced vertical lamination performance and consequently improved mixing performance of ISLM. SLM and ISLM were fabricated by polydimethylsiloxane (PDMS) casting against SU-8 patterned masters. Mixing performance is observed by normalized purple color intensity change of phenolphthalein along the downchannel. Flow characteristics of SLM and ISLM are investigated by tracing the purple interface of two streams via optical micrograph. The normalized mixing intensity behavior confirms improved mixing performance of ISLM, which is consistent with numerical analysis result.     

A high-efficiency three-dimensional helical micromixer in fused silica

True three-dimensional (3D) micromixers in fused silica are highly desirable for efficient and compact mixing in microfluidic applications. However, realization of such devices remains technically challenging. Here, we report high-quality fabrication of 3D helical microchannels in fused silica by taking the full advantage of an improved femtosecond laser irradiation followed by chemical etching process, and a glass-PDMS interface structure is introduced for assembling 3D helical micromixer. Highly efficient mixing is achieved in the helical micromixer at low Reynolds numbers, whose excellent mixing performance is approved by the experimental evaluation and computational fluid dynamics simulation.