Measurement and Correlation of Pressure Drop for Gas-Liquid Two-phase Flow in Rectangular Microchannels

The pressure drop of gas-liquid two-phase flow in microchannel is of fundamental importance in heat and mass transfer processes. In this work,the pressure drop of gas-liquid two-phase flow in horizontal rectangular cross-section microchannels was measured by a pressure differential transducer system. Water,ethanol and n-propanol were used as liquid phase to study the effects of capillary number on pressure drop;air was used as the gas phase. Four microchannels with various dimensions of 100 μm× 200 μm,100 μm× 400 μm,100 μm× 800 μm and 100 μm× 2000 μm(depth × width) were used for determining the influence of configuration on the pressure drop. Experimental results showed that in micro-scale,the capillary number also affected the pressure drop remarkably,and in spite of only one-fold difference in aspect ratio,the variation of pressure drop reached up to near three times under the same experimental conditions. Taking the effects of aspect ratio and surface tension into account,a modi-fied correlation for Chisholm parameter C in the Chisholm model was proposed for predicting the frictional multi-plier,and the predicted values by the proposed correlation showed a satisfactory agreement with experimental data.

Cross-sectional distortion behaviors of thin-walled rectangular tube in rotary-draw bending process

The cross-sectional distortion usually appears during rotary-draw bending process of thin-walled rectangular tube with small bending radius.To study the cross-sectional distortion of the tube,a three-dimensional finite-element model of the process was developed based on ABAQUS/Explicit code and its reliability was validated by experiment.Then,the cross-sectional distortion behaviors of the tube were investigated.The results show that a zone of larger circumferential stress appears on the tube when bending angle reaches 30°.And in the larger circumferential stress zone,the sagging phenomenon is produced obviously.The maximum cross-sectional distortion is located in the larger circumferential stress zone and the angle between the plane of maximum cross-sectional distortion and the bending reference plane is about 50°.The position of the maximum cross-sectional distortion keeps almost unchanged with the variation of the clearances between dies and tube.

Effect of boundary slip on electroosmotic flow in a curved rectangular microchannel

The aim of this study is to numerically investigate the impact of boundary slip on electroosmotic flow(EOF) in curved rectangular microchannels. Navier slip boundary conditions were employed at the curved microchannel walls. The electric potential distribution was governed by the Poisson–Boltzmann equation, whereas the velocity distribution was determined by the Navier–Stokes equation. The finite-difference method was employed to solve these two equations. The detailed discussion focuses on the impact of the curvature ratio, electrokinetic width, aspect ratio and slip length on the velocity. The results indicate that the present problem is strongly dependent on these parameters. The results demonstrate that by varying the dimensionless slip length from 0.001 to 0.01 while maintaining a curvature ratio of 0.5 there is a twofold increase in the maximum velocity. Moreover, this increase becomes more pronounced at higher curvature ratios. In addition, the velocity difference between the inner and outer radial regions increases with increasing slip length. Therefore, the incorporation of the slip boundary condition results in an augmented velocity and a more non-uniform velocity distribution. The findings presented here offer valuable insights into the design and optimization of EOF performance in curved hydrophobic microchannels featuring rectangular cross-sections.

Electromagnetohydrodynamic flows and mass transport in curved rectangular microchannels

Curved microchannels are often encountered in lab-on-chip systems because the effective axial channel lengths of such channels are often larger than those of straight microchannels for a given per unit chip length.In this paper,the effective diffusivity of a neutral solute in an oscillating electromagnetohydrodynamic(EMHD)flow through a curved rectangular microchannel is investigated theoretically.The flow is assumed as a creeping flow due to the extremely low Reynolds number in such microflow systems.Through the theoretical analysis,we find that the effective diffusivity primarily depends on five dimensionless parameters,i.e.,the curvature ratio of the curved channel,the Schmidt number,the tidal displacement,the angular Reynolds number,and the dimensionless electric field strength parameter.Based on the obtained results,we can precisely control the mass transfer characteristics of the EMHD flow in a curved rectangular microchannel by appropriately altering the corresponding parameter values.

Saint-Venant torsion analysis of bars with rectangular cross-section and effective coating layers

This paper investigates the torsion analysis of coated bars with a rectangular cross-section. Two opposite faces of a bar are coated by two isotropie layers with different materials of the original substrate that are perfectly bonded to the bar. With the Saint- Venant torsion theory, the governing equation of the problem in terms of the warping function is established and solved using the finite Fourier cosine transform. The state of stress on the cross-section, warping of the cross-section, and torsional rigidity of the bar are evaluated. Effects of thickness of the coating layers and material properties on these quantities are investigated. A set of graphs are provided that can be used to determine the coating thicknesses and material properties so as to keep the maximum von Mises stress on the cross-section below an allowable value for effective use of the coating layer.

Hydrothermal performance analysis of various surface roughness configurations in trapezoidal microchannels at slip flow regime

The effects of various surface roughness geometrical properties including roughness height(5%,10%,15%),number(3,6),and shape(rectangular and triangular)on the flow and heat transfer of slip-flow in trapezoidal microchannels were investigated.The effects of mentioned parameters on the heat transfer coefficient through the microchannel,average Nusselt number and pressure drop for Reynolds number of 5,10,15 and 20 were examined.The obtained results showed that increasing the roughness height and number increases the pressure drop due to higher stagnation effects before and after roughness elements and decreases the Nusselt number due to higher recirculation zones effects than obstruction effects.The most reduction in Nusselt number and the most increment in pressure drop occur at the roughness height of 15%,roughness number of 6 and Reynolds number of 20 by about 10.6%and 52.8%than the smooth microchannel respectively.

PLASTIC POST－BUCKLING OF A SIMPLY SUPPORTED COLUMN WITH A SOLID RECTANGULAR CROSS－SECTION

The plastic post-buckling of a simply supported column with a solid rectangularcross-section is analysed by a new approach. High order terms in the asymptotic post-buckling expansions are carried out. In some aspects, the method proposed in thispaper is similar io Hutchinson's. However, the computation is simple since theintroduction is avoided of stretched coordinates. The method can be used to analyseinitial post-bifurcation of plates and shells in the plastic range.

Cross-sectional deformation behavior of double-ridged rectangular tube with fillers in different stages of H-typed bending

The bent double-ridged rectangular tube(DRRT)with high forming quality is helpful to improve the microwave transmission accuracy.For reducing the cross-sectional deformation in the H-typed bending process,in addition to using rigid mandrel to support the inside of tube,ridge groove fillers are also added to restrict the deformation of ridge grooves.Because of the change of stress and strain state of bent tube in bending,rigid mandrel retracting and specially twicespringback stages,and the springback of fillers,the cross-sectional deformation of tube in each stage may be different.Therefore,based on the ABAQUS platform,the finite element models(FEM)for H-typed bending,mandrel retracting and twice-springback stages of H96 DRRT with fillers were established and validated.It is found that,for the height and width deformation of tube and spacing deformation of ridge grooves,retraction of mandrel can make the distribution of these deformations more uniform along the bending direction.The first springback can reduce these deformations significantly,which should be emphasized.But the second springback only increases them by less amount,which can be ignored.The smaller height deformation of ridge groove and filler can be neglected.

Cross-sectional deformation of H96 brass double-ridged rectangular tube in rotary draw bending process with different yield criteria

Different yield criterion has great difference in predicting the deformation of tube with different material.In order to improve the prediction accuracy of the cross-sectional deformation of the double-ridged rectangular tube(DRRT)during rotary draw bending(RDB)process,Mises isotropic yield criterion,Hill’48 and Barlat/Lian anisotropic yield criteria commonly used in practical engineering are introduced to simulate RDB of DRRT.The inverse method combining uniaxial tensile test of whole tube and response surface methodology was proposed to identify the parameters of Hill’48 and Barlat/Lian yield criteria of small-sized H96 brass extrusion DRRT as well.Then based on ABAQUS/Explicit platform,the FE models of RDB process of DRRT considering Mises,Hill’48 and Barlat/Lian yield criteria were built.The results show that:The variation trend of cross-sectional deformation ratio is same when using different yield criteria.The cross-sectional deformation ratio by using Mises yield criterion is close to that by using Hill’48 yield criterion.However,there is a quite difference between by using Barlat/Lian yield criterion and by using Mises or Hill’48 yield criteria.The prediction values of cross-sectional height deformation by using three yield criteria all underestimate the experiment ones,and the prediction values of cross-sectional width deformation overestimate the experiment ones.By comparing the simulation results of cross-sectional deformation of the DRRT with different yield criteria and experiment ones,Barlat/Lian yield criterion is found to be suitable for describing the RDB process of DRRT.

微肋分布方式对矩形微通道散热器换热性能影响研究

以矩形微通道散热器作为研究对象,应用数值模拟技术研究了三种微肋分布方式(平行分布、平行-侧壁分布和交错分布)对矩形微通道散热器换热性能的影响。当入口速度为0.2 m/s时,模拟结果表明:采用交错分布的通道中流体漩涡强度最大;三种通道进出口压差值依次为ΔP_(1)=47.81 Pa、ΔP_(2)=60.11 Pa、ΔP_(3)=57.50 Pa,由于增加侧壁微肋采用平行-侧壁分布的通道,流体流动阻力增大,进出口压差最大,由于微肋的作用采用交错分布的中间区域流体其流动阻力增大,通道压差大于平行分布的通道压差;采用交错分布的微通道恒热流底面的平均温度为306.97 K,沿底面向上的固体区域温度逐渐降低,顶面的温度平均温度为296.37 K,通道中相同位置微肋的温度均低于Ⅰ、Ⅱ型,说明微肋交错分布对矩形微通道散热器换热性能影响最大。

矩形微通道内制冷剂流动沸腾传热特性及可视化研究

为了探究微通道内流动沸腾及传热现象的机理,以制冷剂R22为工质在矩形微通道内进行了流动沸腾及可视化实验。结果表明,在核态沸腾下传热系数受质量流率的影响较小,却随着热流密度的增加而快速增加;微通道的尺寸越小,传热效果越好,水力直径为0.92 mm和1.33 mm微通道内的传热系数比2 mm微通道内的传热系数分别提高约25%、12%;根据实验值与预测值的对比情况,在Oh H K等[15]和Yun R等[7]模型基础上拟合得到新的传热系数预测关联式,平均绝对误差降至8.8%;通过可视化实验发现,在临界热流密度下微通道内出现波浪式气体层的现象。

矩形微通道中环状冷凝的三维数值模拟

建立了恒热流边界条件下矩形微通道中环状冷凝过程的三维模型。通过求解气相和弯月面区动量和质量方程及薄液膜厚度方程,得到了弯月面毛细半径分布、冷凝液膜厚度分布,以及传热系数和壁面温度分布。薄液膜区液膜将沿程逐渐增厚,到达一极值后再逐渐变薄。在通道截面中,薄液膜区的传热系数大于弯月面,最大局部传热系数及壁面最高温度皆位于薄液膜区和弯月面的连接处。在冷凝起始段,通道横截面平均传热系数沿程急剧减小至一极值;在此之后的很长一段距离内,则基本保持不变;直至接近环状冷凝终点时又再次沿程减小。

矩形微通道内流体流动特性的数值研究

对矩形微通道实体模型进行简化处理,并建立微通道内流体流动的数学模型。设定矩形微通道水力直径Dh=120～480μm,入口雷诺数Re=11.9～3 817.1,以20℃蒸馏水为流动工质,借助FLUENT分别对不同水力直径的三组矩形微通道内流体流动特性展开数值模拟研究,并将数值模拟结果与理论预测值及其他学者的研究结论进行对比。结果表明:随着微通道水力直径的减小,摩擦阻力系数、速度梯度和压强梯度都呈现增大趋势;在微尺度下,矩形微通道内临界Re提前,而且水力直径越小,临界Re值越小。

微通道流体物性对流动与传热影响的数值模拟和实验研究

针对微矩形通道内单相层流流动和传热特性进行实验研究和三维数值模拟研究。微矩形通道宽为50μm,高为200μm,工质采用去离子水。讨论流体热物性随温度变化对流动传热特性的影响。并把实验数据、模拟结果和文献参考值比较。结果显示,恒热流边界条件下,由于热量加入导致流体温度沿流动方向升高,摩擦系数沿流动方向有递减趋势。进口处的Nu数有最大值,在L*=0.2处降到最小值,之后又攀升到充分状态时的Nu数值。在充分发展段,Nu数保持不变。黏性耗散对热起始段的作用不能忽略。

粗糙度对微通道内两相流摩擦压降的影响

以去离子水和质量分数为0.3%的水基Al_2O_3纳米流体为工质,对水力直径为1 241μm的矩形微通道内流动沸腾过程进行研究。为了探究微通道壁面粗糙度对两相流摩擦压降的影响情况,运用化学抛光处理手段来改变壁面粗糙度。研究结果表明:实验工况相同时,两相流摩擦压降随着微通道壁面粗糙度的增大而增大;纳米流体为工质时两相流摩擦压降高于去离子水为工质时两相流摩擦压降,高热流密度下更为差异明显;将实验值分别与3种分相模型的预测值对比来验证现有压降模型的准确性,结果偏差较大,而且壁面越粗糙预测效果越差。现有的压降计算模型需要进一步完善以增强其普适性。

表面张力对矩形微通道汽液两相流动换热的影响

为探究表面张力对微通道内水溶液流动沸腾的影响,针对0.2 mm×20.0 mm的矩形微通道,底面恒热流(q=200 k W·m-2)加热的工况进行数值模拟.采取VOF多相流模型和用户自定义函数建模,应用几何重构的方法追踪汽液两相界面迁移的变化.通过计算得到表面张力σ分别为0.035,0.045,0.059 N·m-1(纯水)时,微通道内水溶液流动沸腾过程中汽泡沿流动方向的成长、聚并等演变行为和相应的汽液两相流的流型发展状况(泡状流、弹状流和拉伸汽泡流).结果表明:与σ=0.059 N·m-1相比,σ=0.035 N·m-1时,单个汽泡尺寸减小约1/2,流型转变滞后,且通道进出口压降Δp波动幅度减小约2.1 k Pa,加热壁面温度超过400 K的区域减少3/4,最高过热温度从1 600 K降到1 000 K;不同的表面张力,微通道内流动方向上的汽泡演变过程有所差别,但均会依次出现泡状流、弹状流和拉伸汽泡流的流型变化规律;减小表面张力,有助于提高汽液两相流的流动稳定性,进而保证微通道内相变换热的可靠性.

矩形微流道内电渗流影响因素的数值模拟

针对电渗泵的设计参数优化问题,建立电渗驱动流的数学模型,应用有限控制容积法对矩形流道断面上双电层场和速度场的耦合控制方程进行数值求解。详尽分析流道断面深宽比、流道当量直径、电解质溶液浓度和外加电场强度对电势、速度和流量的影响,并给出电渗流量和平均速度与影响因素之间的拟合公式。仿真结果表明:流量随流道深宽比的增大而减小,当流道深宽比为1时,流量最小;流量随流道当量直径的平方增加;平均速度与电解质溶液浓度和外加电场强度有关、与流道深宽比和当量直径的大小基本无关。

PDMS微流控芯片中焦耳热效应的数值研究

用数值模拟的方法对矩形通道微流控芯片中由于焦耳热效应而产生的温度场分布进行了分析。通过求解耦合的动量以及能量方程,得到了PDMS/PDMS芯片中温度场的瞬态发展过程。对稳定状态下不同缓冲溶液浓度,底片材质以及厚度的芯片中的温度场分布也做了比较和分析。结果表明,相比于PDMS/PDMS芯片,相同条件下PDMS/玻璃芯片中的焦耳热效应会减小很多,而缓冲溶液浓度以及芯片厚度对缓冲溶液的出口平均温度也有显著影响。最后提出了减小焦耳热效应的方法。

微小矩形通道内气体层流换热的数值模拟

在基于贴壁层概念和已经求得的贴壁层内气体热导率变化规律的基础上 ,数值求解了微小矩形通道内气体层流已充分发展时 ,在壁面等热流边界条件下的温度分布和换热 ,得到了矩形微通道在不同截面长宽比 ζ和不同Knudsen数下的量纲一的温度分布和 Nu 值 ,并进行了相应的讨论。

微机电系统中的矩形通道内微气泡控制生长

采用微机电系统(MEMS)硅加工工艺,设计、加工出了6种不同规格的实验用微气泡控制生长MEMS器件;构建了MEMS器件中微气泡控制生长实验系统并完成了实验,讨论了热负荷、微加热元宽度、微通道截面参数、工质流速及物性参数等对微气泡生长的影响。结果表明:同等实验条件下,加热电压幅值越高,微气泡生长速率越快;加热脉冲宽度仅对微气泡形成后的进一步生长有影响;加热条件相同的前提下,微加热元宽度越大,气泡成核所需的时间越短、微气泡生长速率越快;微通道宽度一定且高宽比大于1的条件下,高宽比越小,后期微气泡生长速率越慢;微流体的流速越高,微气泡生长始点越晚、生长速率也越低。相同实验条件下,R113、FC-72、去离子水三者中,R113中微气泡生长始点最靠前、生长速率最快,去离子水中微气泡生长最靠后、生长速率最慢。