壁面润湿性对蛇形微通道内两相流流动特性影响的格子Boltzmann模拟

在90°Y形汇流的矩形截面蛇形微通道内,采用格子Boltzmann方法对不同接触角的蛇形微通道内气液两相流动进行了数值计算。首先以空气和水为工作流体对气液两相流动进行模拟研究并通过实验进行验证。验证模型合理性后,根据模拟计算结果,以气液相流速为坐标绘制了不同接触角下的流型图并分析其差异性及原因;同时深入研究了液相黏度和接触角对于弹状流流体力学性质的综合影响;比较了具有不同接触角壁面的蛇形微通道内两相流压降、摩擦因子、壁面摩擦系数和剪切应力的分布规律,并讨论了蛇形微通道内气液两相流动的影响因素。研究表明疏水壁面即接触角大于90°时,微通道内两相流压降、摩擦因子、壁面摩擦系数和剪切应力均低于亲水壁面微通道内相关参数,更利于流体流动。     

矩形截面螺旋通道气液两相流可视化研究与压降计算

对水平放置的矩形截面螺旋通道内空气-水两相流动进行了实验研究。通过高速摄影仪对其内部的流动进行可视化研究,实验中观察到了柱塞状流、弹状流、分散泡状流、环状流;实验测量了通道内两相流动摩擦压降,并与Chisholm模型下的预测值进行比较,实验结果表明:流型很大程度上制约着矩形截面螺旋通道内两相摩擦阻力,采用分流型计算矩形截面螺旋通道摩擦压降的方法可以有效提高预测精度;Chisholm模型适用于间歇流和环状流状态下的压降预测,对于分散泡状流状态下的压降预测偏差较大。     

小曲率蛇形微通道弯头处弹状流流动及传质特性的数值研究

采用CLSVOF（coupled level set and volume of fluid）方法,以空气和水为工作流体对小曲率矩形截面蛇形微通道内气液两相流动进行模拟研究。验证模型的合理性后,研究了曲率对弯通道内压降的影响,曲率及气相速度对弹状流气泡及液塞长度的综合影响;同时深入分析了弯管内气液两相流动的传质特性,包括不同曲率下气泡长度的变化,弯管内液侧体积传质系数与液膜体积传质系数的比较,曲率及气相速度对液相体积传质系数的影响。同时,对比了回转弯道与直微通道传质系数的差异,发现弯微通道可以强化传质。     

小曲率蛇形微通道弯头处弹状流流动及传质特性的数值研究

采用CLSVOF(coupled level set and volume of fluid)方法,以空气和水为工作流体对小曲率矩形截面蛇形微通道内气液两相流动进行模拟研究。验证模型的合理性后,研究了曲率对弯通道内压降的影响,曲率及气相速度对弹状流气泡及液塞长度的综合影响;同时深入分析了弯管内气液两相流动的传质特性,包括不同曲率下气泡长度的变化,弯管内液侧体积传质系数与液膜体积传质系数的比较,曲率及气相速度对液相体积传质系数的影响。同时,对比了回转弯道与直微通道传质系数的差异,发现弯微通道可以强化传质。     

小通道内两相流摩擦压降的实验研究

以氮气作为气相流体,分别采用纯水和乙醇为液相流体,对水平矩形小通道(dh=1.2 mm)内的两相流压力降开展实验研究,同时采集了两相流压降数据,采用高速摄影仪对两相流流型进行了拍摄.根据流型解释了表面张力对摩擦压降的影响,并将均相流模型和分相流模型的预测结果与实验值进行了对比.得出如下结论:在较低折算流速下,均相流模型...     

纳米粒子浓度对纳米流体流动沸腾传热及压降影响综合评价

探究纳米粒子浓度对纳米流体在微细通道内流动沸腾传热和压降特性的综合影响,运用超声波振动法制备质量分数为0.05%、0.1%、0.2%、0.3%、0.4%均匀、稳定的Al_2O_3/R141b纳米流体制冷剂,在直接激光烧结(DMLS)微型换热器内,设计系统压力为176k Pa,纳米流体制冷剂入口温度为40℃、热流密度24~42k W/m~2和质量流率183.13~457.83kg/(m~2·s)工况下,进行单因素实验及正交实验,运用方差齐性检验法及多指标综合评价法研究粒子浓度对纳米流体在微细通道内流动沸腾传热和压降特性的综合影响。研究得出:纳米粒子浓度对纳米流体沸腾传热有显著性影响,对总压降没有显著性影响;纳米流体流动沸腾总压降随浓度的增加而减少,传热性能随纳米颗粒浓度增加成非线性变化,质量分数为0.05%~0.1%之间,传热系数随颗粒浓度的增加而增加,当质量分数大于0.1%时,传热系数随颗粒浓度的增加而减少;综合考虑纳米颗粒浓度对传热及压降的影响,运用熵值法得出纳米颗粒对传热及压降影响的权重分别为0.285、0.715,基于多指标综合评价法得出纳米流体颗粒质量分数为0.2%时,纳米流体的传热系数最佳,总压降最小。     

环空缩径和扩径圆管压降计算方法研究

根据缩径、扩径圆管压降公式和压力损失规律推导出环空缩径和扩径圆管的压降公式;建立了缩径和扩径圆管数值模型,验证了数值模型的准确性,而后开展了不同环空变径圆管的流速和压力分布研究,确定了环空缩径和扩径圆管内流体产生不可恢复压降的位置。根据数值模拟结果验证和完善了所推导的环空缩径和扩径圆管压降公式。     

入口角度及壁面性质对蛇形微通道内弹状流流动特性影响的格子Boltzmann模拟

在Y型汇流的矩形截面蛇形微通道内,采用格子Boltzmann方法对不同壁面性质的蛇形微通道内弹状流流动进行了数值计算。首先以空气和水为工作流体对气液两相流动进行模拟研究并通过实验进行验证。通过验证实验后,模拟计算了气相速度,Y型夹角和壁面性质对气泡长度的影响,以及Y型夹角对微通道内弹状流压降和流动阻力的影响;探讨了粗糙度与壁面润湿性对流动阻力的影响;同时,针对蛇形微通道弯管部分,分析了角度和壁面性质对弹状流流动的影响。通过计算,发现当壁面接触角及Y型夹角为90?时,气泡长度最大;当直微通道为亲水性光滑壁面,回转弯道为粗糙度较大的疏水壁面时,Po数较小。     

板翅式换热器二相流的分配特性

为了提高板翅式换热器内部二相流体分配的均匀性,对二相流体在结构改进前后的换热器内的分布特性进行了实验研究。实验结果表明,由于现有换热器结构设计的不合理,使得二相流体分布极不均匀。而且,由于二相流体之间复杂的相互作用,二相流体在换热器内的分布较之单相流体更复杂且更不均匀。而改进型的孔板封头结构可有效提高换热器外围通道的流体分配,从整体上改善板束单元体截面二相流体的分布,其二相分布的不均匀系数均有效降低。而且,其分配稳定性好,不易受雷诺数变化的影响。     

对翼几何参数对半圆螺旋通道内流体流动特性的影响

为了深入考察渐缩方式布置的三角对翼几何参数对半圆截面螺旋通道内流体流动特性与涡旋特性的影响,采用实验和数值模拟方法进行了研究.通道曲率δ在0.05～0.125之间,其中δ=0.067通道内三维速度场的模拟结果与激光多普勒测速仪测量结果吻合较好.结果 表明,对翼诱导产生的纵向涡旋螺旋发展并强化离心涡旋,雷诺数Re越大,复...     

Evaluation of the mixing performance of three passive micromixers

This work presents a numerical investigation on mixing and flow structures in microchannels with different geometries: zig-zag; square-wave; and curved. To conduct the investigation, geometric parameters, such as the cross-section of the channel, channel height, axial length of the channel, and number of pitches, are kept constant for all three cases. Analyses of mixing and flow fields have been carried out for a wide range – 0.267–267 – of the Reynolds number. Mixing in the channels has been analyzed by using Navier–Stokes equations with two working fluids, water and ethanol. The results show that the square-wave microchannel yields the best mixing performance, and the curved and the zig-zag microchannels show nearly the same performance for most Reynolds number. For all three cases, the pressure drop has been calculated for channels with equal streamwise lengths. The curved channel exhibits the smallest pressure drop among the microchannels, while the pressure drops in the square-wave and zig-zag channels are approximately the same.     

矩形截面微通道内气-液两相流压力降的实验测定及关联(英文)

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.     

壁面润湿性对微细通道内R141b流动沸腾不稳定性的影响

为了探究壁面润湿性对制冷剂R141b流动沸腾不稳定性的影响，设计微细通道流动沸腾实验平台，制备3种不同润湿性的矩形微细通道，其壁面接触角分别为62.3°、接近0°和158.7°。以R141b为实验工质，在截面宽×高为1mm×2mm的矩形微细通道内进行流动沸腾换热实验，研究了沿程测点压力波动情况以及影响进出口总压降波动的因素，最后对总压降波动信号进行Hurst指数分析，结果表明：微细通道沿程测点波动方差最大的位置正处于沸腾起始点（ONB）附近，热流密度的减小以及质量通量的增大均会使沸腾起始点推后；进出口总压降波动受热流密度、质量通量和壁面润湿性的影响，相同工况下，热流密度增大和质量通量的减小都会引起系统不稳定性增强，超疏水表面微细通道的总压降波动方差均比其他两种表面的大，是波动方差最小的超亲水表面的1.35～1.84倍；利用Hurst指数分析，表明系统具有混沌现象，超疏水表面微细通道的Hurst指数最大，表现出更强烈的不稳定性。     

Liquid–liquid two-phase flow and mass transfer characteristics in packed microchannels

In this work, the flow hydrodynamic characteristics and the mass transfer performance of immiscible fluids in the packed microchannels are investigated experimentally. Water–kerosene system is used for visually identifying the flow hydrodynamic characteristics in PMMA microchannels, and water–succinic acid–n-butanol is chosen for investigating mass transfer performance in stainless steel microchannels. Quartz sand micro-particles are used as packing particles. In packed microchannels, high liquid–liquid dispersions can be obtained, and the diameter of droplets produced in the packed microchannel can be even less than 10 μm. It ensures better mixing performance and larger effective interfacial area of two immiscible fluids, and improves the mass transfer performance obviously. Compared to the extraction efficiency (46–61%) in the non-packed microchannel, it can reach 81–96% in the packed microchannel. The effects of packing length, micro-particle size on liquid–liquid dispersions and extraction efficiency are investigated. The pressure drop and the specific energy dissipation in the packed microchannels are also discussed.     

Gas-liquid two-phase flow in serpentine microchannel with different wall wettability

Gas-liquid flow in serpentine microchannel with different surface properties exhibits drastically different flow behavior.With water and air as working fluids,the method of numerical simulation was adopted in this paper based on CLSVOF (coupled level set and volume of fluid method) multiphase model.After verifing the reasonability of the model through experiment,by changing wall properties and Re number (Re ＜ 1500),the influences of contact angle and surface roughness on flow regime and Po number were discussed.Moreover,the difference of pressure drop between curve and straight microchannel was also calculated.Beyond that,the combined effect of curve channel and wall properties on flow resistance was analyzed.This paper finds that wall properties have great influence on gasliquid flow in microchannels not only on flow regime but also flow characteristics.Meanwhile,the pressure drop in curve microchannels is larger than straight.It is more beneficial for fluid flowing when the straight part of microchannel is hydrophilic smooth wall and curve part is hydrophobic with large roughness.     

微细通道纳米制冷剂流动沸腾阻力特性

分别以质量分数为0.2%、0.5%和0.8%的Al₂O₃-R141b纳米制冷剂和纯制冷剂R141b为工质,在水力直径为1333μm的矩形微细通道内进行了流动沸腾实验,分析了纳米颗粒浓度对工质两相摩擦压降的影响,对比了实验前后换热壁面的表面能。研究结果表明：实验工况相同时,质量分数为0.2%、0.5%和0.8%的纳米制冷剂的两相摩擦压降均比纯制冷剂低,降低的最大幅度分别约为11.6%、14.8%和19.2%;实验后纳米颗粒在换热壁面附着,使壁面表面能增大,质量分数为0.2%、0.5%和0.8%的纳米制冷剂实验后换热壁面表面能比实验前分别增大了1.26倍、1.44倍和1.91倍,减小了换热表面的粗糙度和提高其润湿性,使得工质两相摩擦压降减小;根据实验值与模型预测值的对比情况,对Qu-Mudawar模型进行修正,拟合得到的关联式能很好预测实验值,平均绝对误差为9.78%。     

仿蜂巢分形微管道网络中的流动与换热

受自然界中蜂巢结构分形特征的启发,设计和加工了仿蜂巢分形微管道网络,并进行了参数优化.在微管道截面参数、对流传热系数、传热温差均相同的条件下,对流动与换热特性的理论分析表明：加热底面积相同时,仿蜂巢分形微管道网络所能带走的热量可达平行阵列微管道网络的5倍以上;不计分流、合流效应,总换热量一定时,仿蜂巢分形微管道网络所需的泵送功率约为传统平行阵列微管道网络的1/10.恒定热流条件下的去离子水层流对流换热实验也证明：仿蜂巢分形微管道网络比传统的平行阵列微管道网络具有更高的Nusselt数和更低的流动压降.这种分形微管道网络除用于电子器件冷却,还可用于微燃料电池极板、微混合器、微生化反应器等微化工系统结构设计.     

Laminar Drag Reduction in Hydrophobic Microchannels

The apparent slip effects of laminar water flow in smooth hydrophobic microchannels and patterned hydrophobic microchannels were investigated. A series of experiments were performed to demonstrate the drag reductions for laminar water flow in hydrophobic microchannels. These microchannels were fabricated from silicon wafers using photolithography and were coated with hydrophobic octadecyltrichlorosilane (OTS). To generate a larger drag reduction, the patterned hydrophobic microchannels were fabricated to allow the liquid to flow over a region of trapped air in the cavity between the microridges. With the geometrical dimensions used, pressure drop reductions ranging from 10 to 30 % were found in the smooth microchannels and patterned microchannels. The pressure drop reduction was shown to increase with increasing microridge spacing and decreasing microchannel width. Using micro‐particle image velocimetry (PIV), we measured an apparent slip velocity at the wall of approximately 8 % of the centerline velocity, yielding a slip length of approximately 2 μm in the smooth hydrophobic microchannel. Theoretically, the analytical solution derived for three‐dimensional flow in a rectangular duct is presented to predict the slip velocity and slip length at the wall based on the pressure drop measurement. These results are in agreement with the experimental data obtained using micro‐PIV.     

基于抛物线形气-液界面的超疏水微通道减阻特性

针对超疏水表面微通道中的流动减阻特性,基于抛物线形气-液界面假设,采用VOF模型模拟了微通道中的二维层流流动,分析了流动和结构参数对减阻效果的影响。结果表明,含矩形微坑的超疏水表面微通道具有显著减阻作用,fRe随Reynolds数增大而略有提高,量纲1压降比随入口速度增大而略有下降。当增大微坑面积比或减小微通道高度时,fRe减小,量纲1压降比增大;且微通道高度越小,微坑面积比对fRe的影响越显著。随抛物线形高度增加,压降比和滑移长度均线性减小,而fRe则线性增加。当微坑深度大于其宽度的40%时,压降比和滑移长度趋于定值。微坑形状对减阻效果的影响依次是燕尾形、矩形、梯形和三角形。