3D-不对称菱形被动式微混合器混合特性

提高微混合器雷诺数的适用范围和混合强度是微混合器设计的发展趋势。本文基于非对称分离重组混合原理设计、制作了一种3D-不对称菱形被动式微混合器,并借助数值分析方法和可视化实验对混合强度和混合状态的变化进行了研究。研究发现:在低Re(0.01~10)范围内,两组分间的混合以扩散混合为主,随着Re的增加,流速对混合强度的影响有一定下降;在较高Re(10~200)范围内,受流速增加的影响,流体间不平衡微流惯性碰撞逐渐成为影响混合的主要因素。此时,混合强度随流速的增加逐渐增强并趋于平稳。对Re在0.01~200内的微混合器展开研究,分析了宽缝比Ws/S、分合角θ、宽厚比H/S等结构尺寸对混合强度的影响。通过综合考虑流体混合强度和通道压降的变化情况,确定最佳通道结构尺寸为Ws/S=0.2、θ=45°、H/S=0.5,此时微混合器的混合强度可维持在78%以上。与传统平面对称分合式混合器相比,设计制作的3D-不对称菱形被动式微混合器混合强度有较大的提高,验证了本文设计结构的有效性。     

基于拉格朗日跟踪法的微混合器内混沌混合特性

为了分析三维交叉导流式(Staggered oriented ridges,SOR)微混合器内混沌混合特性,在计算流体力学的基础上,采用拉格朗日跟踪技术数值模拟SOR混合器中流道截面上流体物质线和物质面的变形、拉伸情况,并通过平均对数线拉伸计算定量地评价SOR混合器的混合效果。研究结果表明：SOR微混合流道内可诱发混沌对流,并在其作用下可增加流体粒子的接触面积,提高混合效率。在小雷诺数时,线拉伸随雷诺数增大变化不大,但在雷诺数较大时,线拉伸随雷诺数增大而增大。     

主辅通道型微混合器的设计与制作

为了实现微量液体的快速均匀混合,设计了一种PDMS双层结构的新型微混合器。研究了混合器的制作方法以及几何尺寸和Re数对混合的影响。依据Fick第一定律介绍了主辅通道型微混合器的设计原理;采用有限元方法对不同几何尺寸及Re数下混合器中液体的速度流场及浓度场进行了数值模拟;数值分析显示,随着主辅通道出口宽度比的减小,通道长度的增加和雷诺数的减小,混合器的混合率增加。最后,依据仿真结果制作了主辅通道深度比为0.71,出口宽度比为1,通道长度为9mm的微混合器并进行了去离子水和红墨水的混合实验。实验结果表明:当Re5时,设计的混合器能实现液体的快速混合,并且混合率随着Re的减小而增大,基本满足低Re数下微量液体快速均匀混合的要求。     

基于混沌混合的分合式微混合器研究

按照混沌混合理论构建了Smale、Helical和Baker三种微混合器,为研究其混合机理与混合效果,对不同雷诺数下的对流扩散和混合进行了数值模拟,并用实验结果进行了验证。结果发现：实验结果与数值模拟结果吻合得很好。在低雷诺数下,由于Baker微混合器中流体的分层作用强,明显增大了流体的接触面积,也加快了流体混合速度,其混合效果要明显优于另外两种混合器;然而,在较大的雷诺数下,Smale微混合器中流体混合效果则优于Baker微混合器,其原因是Smale微混合器的分层效果虽然不如Baker微混合器,但其对流作用明显强于Baker微混合器;Helical微混合器因没有分层作用,其混合主要依赖于对流,所以在低雷诺数下混合性能较差,而高雷诺数下混合性能有所提高。     

特斯拉型混合器中障碍物布局的优化

微混合器作为一种可以实现快速、高效的混合设备在生物分析、化学反应和检测等领域中应用,特斯拉阀因其简单的结构和特殊的流动机理常应用于微混合器的结构设计中。为提高混合效率,在特斯拉型混合器中添加菱形障碍物,并基于流场分析对其布局进行了优化和实验验证。以混合率和压降比值最大为优化目标对菱形障碍物在特斯拉型微混合器布局进行优化,经过优化得最优菱形障碍物尺寸为46.35 μm,最优横向偏移量为18.78 μm,最优竖向偏移量为20 μm。基于优化结果,设计并制作了添加菱形障碍物的特斯拉型微混合器并对微混合器的混合效果进行实验验证。结果表明设计优化的菱形障碍物特斯拉型微混合器与未添加障碍物时的特斯拉型微混合器相比在相同条件下具有更高的混合效率。     

壁面粗糙度效应对微流体流动特性的影响

壁面粗糙度对微流道流动特性有重要影响。分别用矩形、三角形和圆顶形粗糙元对壁面粗糙度进行模拟,详细讨论了雷诺数、粗糙元高度、粗糙元间距等因素对流速、压降及流动阻力的影响。结果表明:与光滑流道相比,粗糙度使壁面附近的流动发生明显改变,从而导致微流道内流速、压降及流阻高于经典理论预测值;微流道内流动阻力随着雷诺数及粗糙元高度的增大而增大,而随着粗糙元间距的增大,流动阻力逐渐减小。三种粗糙元相比,矩形粗糙元的影响最大,圆顶形次之,而三角形粗糙元的影响最小,可见在实际应用场合,确立合适的粗糙元形状对分析结果非常重要。     

基于分合式混合单元的微混合器设计与研究

为实现微尺度下流体的快速均匀混合,基于对流体的分割、合并实现流体间接触面积指数式增长的原理,提出了一种分合式(SAR)混合单元,通过仿真对其支路夹角进行了优化,并最终确定支路夹角θ=70°.利用成熟的标准SU-8光刻及PDMS制作工艺,制作了分合式与组合分合式两种微混合器.采用甘油-水按体积比5:95(0.000 97...     

ZigZag微通道内S-CO_2流动传热性能分析

以超临界二氧化碳(S-CO_2)为工质,对其在Zig Zag半圆形横截面微通道内湍流状态下流动传热性能进行数值计算,分析了Zig Zag角度θ、单位周期流道轴向长度P对传热与流动阻力的影响。结果表明,流体在Zig Zag微通道内流动,在流道拐弯处有旋涡产生,导致流通面积减少,流体主流速度急剧增大并冲刷换热壁面,使边界层减薄或破坏,并且该位置附近湍动能急剧增大,增强了流体的扰动与混合,促进了能量传递,强化了换热;随着Zig Zag角度θ增大,微通道内传热性能提高而流动阻力急剧增大;随着单位周期流道轴向长度P增大,传热性能与流动阻力均下降;在文中所述计算条件下,θ=15°,P=15 mm时Zig Zag微通道内S-CO_2耦合传热综合传热性能最优。     

低雷诺数高效类魔方结构微混合器

为了在雷诺数条件不定的小尺寸的芯片内部集成高效的混合功能,根据菲克定律和布朗运动的爱因斯坦关系式提出了一种通过匹配接触面提高浓度差的策略来设计微混合器,对科恩达效应进行了扩展,分析了流体在通道表面的流动方向,从特定微通道模块中抽象出4种具体功能。通过模块的功能来预测和调控浓度梯度并构建微混合器。使用4种功能模块来旋转并匹配流体界面,设计了两种三维结构的被动式微混合器。采用三维Navier-Stokes方程组进行了数值分析,并通过软光刻工艺制作微混合器进行了实验验证。实验和仿真结果表明,在雷诺数为0.1～100内,设计的微混合器在3.3 mm,即22倍水力直径长度处能稳定提供94%～99%的混合效率,在等水力直径条件下具有明显的优势,而且结构易于在芯片上集成,证明了模块化设计的优越性。     

斜截半椭圆柱面涡流发生器强化换热和压降特性的试验研究

在矩形通道内布置一对斜截半椭圆柱面涡流发生器(高宽比h/b=1/2),通过改变斜边倾角α、来流攻角β、前沿间距S、布置方式以及顺列、错列两排涡流发生器研究不同工况下的换热和压降特性.试验雷诺数范围为Re=700～26 800.结果表明,在试验条件下α=12°时的换热效果要好于α=20°,Re=1 300时两者的最高局部对流换热系数分别比平直通道高19.7%和10.7%,Re=26 800时分别为23.2%和18.33%.斜截半椭圆柱面涡流发生器(α=12°)的最优攻角为β=60°,最优前沿间距为S= 20 mm.当Re=26 800,β=60°,S=20 mm时的最高局部对流换热系数约比平直通道高23.2%,压力损失约比平直通道高33.7%.雷诺数较小时,由于回流滞止区的影响,布置两排涡流发生器后,换热效果反而弱于单排.随着雷诺数增大到4 000,双排涡流发生器的换热效果要明显优于单排,Re=26 800时,最高局部对流换热系数约比单排涡流发生器高14.4%,比平直通道高33.4%.通过试验得出斜截半椭圆柱面涡流发生器在β=60°,α=12°时的涡旋作用距离大约在250～300 mm.     

Numerical modeling of internal flow in a fluidic oscillator

A numerical investigation of a fluidic oscillator was performed to understand the unsteady internal flow field and geometrical effects on the performance using three-dimensional unsteady Reynolds-averaged Navier-Stokes equations. Effects of the two geometrical parameters, i.e., the inlet width of the mixing chamber and outlet throat width, on peak jet velocity ratio at the exit and pressure drop through the oscillator, were evaluated. The unsteady simulation was performed using shear stress transport turbulence model with air as working fluid at Reynolds number 30000. Computational results showed good qualitative and quantitative agreements with available experimental results for the flow structure and frequency of the oscillating jet. Results of the parametric study suggested that the inlet width affected significantly the flow in the fluidic oscillator, while effects of the throat width on the performance parameters were not remarkable.     

混沌流微混合器的性能优化

为提高低雷诺数层流条件下被动式微混合器的效率,依据"三维马蹄变换"数学模型,对流体进行"挤压拉伸""弯曲折叠""二次折叠"和"逆变换-交集"操作,得到了一款包含6个混合单元,总长度为15 mm的混沌流微混合器.仿真研究表明:在"低流速-扩散主导"阶段u=2×10-4 m/s时,t=300 s后混合器进入稳定混合状态,出...     

Evaluation of surrogate models for optimization of herringbone groove micromixer

Surrogate models have been applied to shape optimizations of a micromixer with the aim of assessing the performance of the models. The surrogate models considered include polynomial response surface approximation, Kriging, and radial basis neural network. In addition, a weighted average model based on global error measures is constructed. A mixing index at the exit of the micromixer is used as the objective function. The mixing index is calculated based on Navier-Stokes equations. Two cases of optimization, one with two design variables and the other with three design variables, have been tested. The design variables are selected among the ratio of the groove depth to channel height, the angle of groove, and the ratio of groove width to groove pitch. D-Optimal design generated sampling points are used for sampling. It is found that although the weighted average model does not predict the best optimal point, it does show consistent and reliable performance.     

Forced air cooling by using manifold microchannel heat sinks

A study of manifold microchannel (MMC) heat sinks for forced air cooling was performed experimentally. The manifold microchannel heat sink differs from a traditional microchannel (TMC) heat sink in that the flow length is greatly reduced to a small fraction of the total length of the heat sink. In other words, the MMC heat sink features many inlet and outlet channels, alternating at a periodic distance along the length of the microchannels while the TMC heat sink features one inlet and one outlet channels. The present study primarily focused to investigate the effects of geometrical parameters on the thermal performance of the manifold microchannel heat sinks for optimal design. Also, the thermal resistances of the MMC heat sinks were compared with those of the TMC heat sinks. Experimental results showed the thermal resistances of MMC heat sinks were affected strongly by the pumping power, the microchannel width and the manifold inlet/outlet channel width, but weakly by the microchannel thickness-width ratio and the microchannel depth coorporated with the manifold inlet/outlet channel width. However, it was found that there existed the optimum values of the latter parameters. Under the optimum condition of geometrical parameters in the present study, the thermal resistance of the MMC heat sink was approximately 35% lower than that of a TMC heat sink, which clearly demonstrated the effectiveness of using a manifold.     

面向催化剂载体应用的TiO2多孔陶瓷结构优化设计

周期性点阵结构的多孔陶瓷采用增材制造可有效解决传统粉末催化剂可回收性差和块状催化剂催化效率低的问题。由于结构特征参数与应用性能之间的复杂关系,周期性点阵结构的精确与快速设计仍是一个巨大的挑战。针对该挑战,构建了一套用于多孔陶瓷的仿真模型,采用3D打印技术成功制备了具有精细多孔结构的TiO2陶瓷样件,并通过实验实现了对仿真模型的验证和修正。在此基础上,采用单因素优化分析方法进一步探究了结构构成要素对性能的影响机制,选取角度a=153.4°、b=90°、c=45°作为最优参数设计了一种新型周期性点阵结构并进行了优化。对比结果显示,优化设计后的多孔结构的压降减小了57.2%,表面积增大了25.3%。该工作为扩展多孔陶瓷在催化剂载体领域的应用提供了一种新的结构设计方法。     

Numerical and experimental study on a channel mixer with a periodic array of cross baffles

In this study we show an enhanced mixing effect with a simple channel having a periodic array of cross baffles. We performed numerical compulation to obtain the steady flow field within the channel at low Reynolds numbers by using a commercial code, ANSYS CFX 10.0. A visualization experiment was also conducted to validate our numerical results qualitatively. In evaluating the mixing performance, we employed the Lyapunov exponent. It was shown that the visualized mixing pattern was in a good agreement with that numerically given. Our Liapunov exponent distribution in the space also demonstrates that the proposed channel design indeed exhibits a chaotic stirring at low Reynolds numbers. Our design is thus assumed to be applicable to designing a microchannel mixer with enhanced mixing effect     

立式捏合机桨叶间隙、螺旋角对搅拌扭矩和功率特性的影响

以1 L两桨叶立式捏合机为研究对象,采用CFD方法系统性地研究立式捏合机桨叶结构参数（间隙、螺旋角）对搅拌牛顿流体（玉米糖浆）的搅拌扭矩和功率消耗的影响关系。立式捏合机仿真模型与试验数据进行对比验证,确保了模型的可信度。研究结果表明,减小桨叶间隙或增大桨叶螺旋角可增大桨叶搅拌扭矩和功率消耗。减小桨叶螺旋角可增加桨叶捏合作用时间,减小桨桨间隙或增大桨叶螺旋角可增大桨叶捏合作用强度。一定范围内,桨桨间隙c₁=3.0 mm、螺旋角β_k=35°时,空心桨平均扭矩取得最小值0.046 93 N·m;桨桨间隙c₁=1.0 mm、螺旋角β_k=55°,空心桨平均扭矩取得最大值0.068 73 N·m。搅拌牛顿流体时,立式捏合机功率准数N_pM与雷诺数R_eM为线性关系,并且桨桨间隙对立式捏合机功率特性曲线的影响大于桨叶螺旋角的影响。     

Incompressible SPH modeling and analysis of non-Newtonian power-law fluids,mixing in a microchannel with an oscillating stirrer

In the present study, a robust Incompressible smoothed particle hydrodynamics (ISPH) method, based on an advanced Smoothed particle hydrodynamics (SPH) discretization, is introduced to study the effects of the non-Newtonian power-law index and stirrer frequency on fluid mixing in an active micromixer that uses an oscillating stir-bar. Two Reynolds numbers (20 and 72) are considered, and more than 70 SPH simulations are carried out, in order to investigate the effects of the power-law index and stirrer frequency on fluid mixing. The results show that this active micromixer is more efficient at the lower power-law indices.