微尺度下H形分布流道充模工艺参数对流动不平衡的影响分析

为探究微尺度下的工艺参数对聚合物熔体充填不平衡的影响,考虑模具温度、熔体温度、注射速率以及微流道尺寸,采用聚甲醛(POM),对H形对称分布的半圆形截面微流道系统进行了充模流动数值模拟实验,分析了浇口处对称点间的温差数据.结果表明,微注塑过程中流道系统内存在温度不对称,随着剪切速率的增加,对称点间的温度差也逐渐增加;而熔...     

聚合物熔体全三维非等温数值模拟

采用基于交错网格的有限体积法(FVM)离散了4大方程,给出了能量方程的全三维离散格式。运用SIMPLE算法求解了矩形截面流道内熔体的速度场和压力场,通过耦合动量方程和能量方程,进而得到整个机头流道内温度的分布。计算中采用了Carreau流变模型,并给出了作为温度函数的流动指数n的解析表达式。模拟结果表明:在入口区,熔体从近壁面区域向流道的中心区域汇集,进入全展流区后,熔体的流场不再变化;熔体内温度分布较为复杂,影响因素众多。     

塑料微齿轮微流道注塑模充填研究

使用圆形微流道进行塑料微齿轮注射成型充填研究,利用模流分析软件Moldflow进行数值模拟试验,通过改变注射速度参数(微流道深度尺寸和进口注塑压力),探讨熔融塑料在微流道内的充填状况,并结合实际注射成型实验,了解工艺参数改变对微齿轮充填的影响.为避免塑料在微结构件内产生充填不良等问题,提高成型质量,降低生产成本提供了理...     

磁场作用下液态包层中的流动传热和插件安全分析

国际热核聚变实验堆(ITER)的双冷锂铅(DCLL)包层涉及"磁-热-流-固"多物理耦合场问题。流道插件(FCI)作为DCLL包层的关键部件,具有隔绝热量传递和降低磁流体压降的作用。本文基于电势方法,采用PISO算法和相容守恒格式求解了包含Lorentz力的不可压Navier-Stokes方程;应用有限元方法计算了FCI在流场、热场和磁场耦合作用下的热应力和变形。采用顺序耦合法,分析了磁-热-流固耦合场中速度、温度、压力的分布形式,研究了流道插件结构内的热应力和热变形,通过MHD压降系数和热效率系数揭示了FCI结构对于包层内MHD效应和传热性能的影响规律。     

一种螺旋流道辊的数值模拟及结构优化

对已有的螺旋流道辊模型进行合理简化,运用流场仿真软件Fluent对流道辊的温度场进行了仿真分析,得到了流道辊的温度场,从而得到了流道辊内流体速率、流体温度场、辊筒表面温度场等。通过分析模拟仿真结果,得到了辊筒外表面温度不均匀的原因,并在此基础上,以提高辊筒外表面的温度分布均匀性为目标,对流道结构进行多次改进,找出一种最佳方案,明显改善了辊筒端部的温度分布均匀性,使辊面的有效利用长度延长了约50 mm。     

杆支撑换热器壳程的单元流道模型及流场分布

提出了一种杆支撑换热器壳程的代表性“单元流道”模型,使结构复杂的管壳式换热器数值模拟更加简便而高效,数值分析得到了杆支撑换热器壳程中心区域的单元流道流场、压力场和温度场分布细节信息,为杆支撑换热器的结构完善和研究强化传热机理奠定了理论基础。     

快速动态模温控制于高深宽比微结构注塑成型

本研究利用电磁感应加热技术结合冷却水以达到快速模具温度控制并应用于微结构注塑成型制程。实验结果成功利用快速模具温度控制技术将模具温度从60℃提高至140℃仅需3秒,同时利用CAE模拟技术以掌握高深宽比之微流道模具温度变化情形,并获得实验验证。仿真结果显示,电磁波能深入微流道底部加热,与模具表面的温差在2℃之内。搭配PMMA塑料成型深600μm、宽30～50μm(深宽比>12)之微结构,并成功地改善微结构转写性达到96%。     

球形转角形状对于改善微结构塑件填充不平衡的实验研究

在流道分级部位设置直角转角、球形转角改善微注塑件充填不平衡.设计带有直径分别为200μm与300μm微圆柱的孔的1模8腔注塑模具,以高密度聚乙烯(PE-HD)材料进行实验.结果表明,随着注射速率和模具温度的逐渐增加,球形转角对改善填充不平衡优于直角转角;随着微圆柱孔特征尺寸的增大,直角转角与球形转角对填充不平衡的改善均...     

3D打印技术制备新型微流道细胞培养系统

本文通过结合3D打印技术与软刻蚀技术制备了一种新型的微流道细胞培养系统(MPCCS)。该微流道培养体系具有可预先设计的细胞培养腔,用于细胞组织块培养。通过降解实验及数值模拟表明,微流道培养体系可以实现以下要功能:①调控营养物质的流动,减轻施加在细胞和支架上的剪切应力;②在降解过程中,保持支架的原始形态;③在支架降解时提供机械支撑。通过对于微流道系统内部的数值模拟,进一步表明流体在微流道灌注细胞培养系统(MPCCS)内部具有均匀的速度分布,可以为细胞培养提供均匀的营养供给。     

不同基板对羟基磷灰石生长的影响

人体骨骼的主要成分是羟基磷灰石(HAP)。本文通过共混制备聚二甲基硅氧烷(PDMS)/HAP和PDMS/生物玻璃复合材料作为生长基板,模拟体液(SBF)做生长溶液,研究了不同基板上HAP的生长。研究发现HAP晶体在PDMS复合材料上比在生物玻璃上生长更快,尺寸更大。     

多孔壁面微通道换热性能的实验研究

微通道散热器作为一种高效散热器件,广泛应用于微电子、光电、汽车、航天国防、能源等领域。针对传统光滑微通道传热面积小、换热性能偏低、沸腾迟滞等问题,本文提出一种多孔壁面微通道结构,并采用激光直写方法实现微通道多孔壁面的高效、稳定生成。该多孔壁面微通道显著增大了换热面积、促进流体的扰动、提供大量稳定沸腾核心,从而强化单相与两相沸腾传热。通过搭建微通道换热性能测试系统,测试对比了多孔壁面微通道与光滑微通道的单相对流、两相沸腾传热性能。发现多孔壁面微通道的Nu数相对于光滑微通道提升了21%~31%。在两相沸腾换热过程中,其粗糙多孔结构促进了沸腾气泡成核,其核态沸腾起始温度相比于光滑微通道降低了35%。同时粗糙多孔结构可以保证沸腾过程中的液体持续供给,从而大幅提升了沸腾换热能力,避免了干涸现象的提前发生,其两相沸腾换热系数相对于未处理的光滑微通道最大提升了83%。此外,还开展了不同流量下多孔壁面微通道的沸腾传热性能测试,发现在质量流率为G=500kg/(m²·s)下的沸腾换热系数相对于G=200kg/(m²·s)情况下最大提升了30%。     

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.     

Experimental and numerical studies of injection molding with microfeatures

Injection molding with microstructures was investigated both experimentally and theoretically. A series of injection molding experiments with PP and PMMA was carried out in a long and a short rectangular mold containing microchannels with the thickness of either 50 or 100 μm and an aspect ratio of 5. The filling lengths in the microchannels were affected by injection speed, mold temperature, and channel location. A high injection speed or high mold temperature resulted in a longer filling length. The filling length in the microchannels decreased as the filling time in the main flow region increased. All filling lengths can be merged into a single curve vs. Fourier number based on the microchannel thickness. Comparison was also made between the experimental measurements and numerical simulation. The mold/melt heat transfer coefficient was found to be a critical factor in determining the filling lengths. The local heat transfer coefficient provided a much better agreement than a constant heat transfer coefficient. POLYM. ENG. SCI., 45:866–875, 2005. © 2005 Society of Plastics Engineers     

Micromixing using induced-charge electrokinetic flow

The mixing effect of induced-charge electrokinetic flow (ICEKF) in a rectangular microchannel with embedded conducting hurdles is investigated in this paper. A correction method is suggested to estimate numerically the induced zeta potential on the conducting surface. Two-dimensional pressure-linked Navier-Stokes equation is used to model the flow field in the channel. The numerical results show flow circulations generated from the induced non-uniform zeta potential distribution along the conducting hurdle surfaces. It is demonstrated that the local flow circulations provide effective means to enhance the flow mixing between different solutions. The mixing enhancement effect is experimentally validated using PDMS based microchannels with embedded platinum hurdles. The dependence of the degree of mixing enhancement on the hurdle geometries and hurdle numbers is also predicted. The mixing using ICEKF described in this paper can be used in various microfluidics and lab-on-a-chip applications.     

Effect of Electric Field/Current on Liquid Phase Sintering

The sintering behavior of alumina containing different amounts of calcium–aluminum–silicate glass as sintering aid was analyzed under AC electric fields between 0 and 150 V/cm. Liquid phase sintering was enhanced by the electric field, and “flash sintering” behavior depending on the current density and power dissipation within the specimen could be observed. Current flowed only through the liquid phase at high temperature and enhanced the densification process by two effects: Joule heating and athermal response of the viscous liquid under the electric field. Joule heating increased the temperature within the specimen, whereas the applied electric field reduced the viscosity of the liquid phase promoting a more effective matter transport.     

宽矩形硅微通道中流动冷凝的流型

对水力直径90.6 μm、宽深比9.668的矩形硅微通道中的流动冷凝过程进行了可视化研究。研究发现,宽矩形硅微通道中的冷凝,沿程主要有珠状-环状复合流、喷射流和弹状-泡状流等流型。在珠状-环状复合流区,冷凝液膜可覆盖通道竖直侧壁,而在通道长边上,仍然为珠状凝结。喷射流位置随着入口蒸气Reynolds数的增大而延后,通道截面形状对流动冷凝不稳定性也存在很大影响。喷射流之后为弹状-泡状流,弹状气泡沿程逐渐缩短,并在表面张力的作用下收缩成圆球形气泡。冷凝通道的平均传热系数将随着入口蒸气Reynolds数的增大而增大。     

Effect of microchannel structure on droplet size during crossflow microchannel emulsification

A monodispersed oil-in-water emulsion was continuously produced using a crossflow-type silicon microchannel plate in which a liquid flow path for the continuous phase was made, and on each side of the wall of the path, an array of regular-sized slits (microchannels) was precisely fabricated on a micrometer scale by photolithography. A flat glass plate was tightly attached to the microchannel plate to cover the top of the microchannels. Regular-sized oil (triolein) droplets were generated by pressing the oil through the microchannels into a continuous phase of 0.3 wt% aqueous sodium lauryl sulfate. The average size of the oil droplets was regulated within a range of 11.3 to 28.2 μm by changing the microchannel structural features such as the shape of the cross section and outlet, the equivalent diameter, and the length of the terrace, which is a flat area fabricated at the outlet of the microchannels. In every case, the droplet size distribution was narrow, and the geometric standard deviation was 1.03 or less.     

Bubble lengths in the gas–liquid Taylor flow in microchannels

Experimental results of measurements of the bubble and slug lengths in Taylor (slug) flow are presented. The experiments were carried out using 3 different straight microchannels (microreactor with square cross-section made of polydimethyloxosilane (PDMS); microreactor with circular cross-section made of glass; microreactor with rectangular cross-section made of polyethylene terephthalate modified by glycol (PETg)) and 4 different liquids (water, ethanol propanol and heptane). The results have been compared with the available literature correlations. It is concluded, that the values obtained from the correlation proposed by Laborie et al. [Laborie, S., Cabassud, C., Durant-Bourlier, L., Laine, J.M., 1999. Characterization of gas–liquid two-phase flow inside capillaries. Chem Eng Sci 54, 5723–5735] do not agree with the results of measurements, while the agreement of these results with the predictions obtained using the correlation proposed by Qian and Lawal [Qian, D., Lawal, A., 2006. Numerical study on gas and liquid slugs for Taylor flow in a T-junction microchannel. Chem Eng Sci 61, 7609–7625] is good. New, corrected values of the pre-exponential constant and the exponents in the Qian and Lawal [Qian, D., Lawal, A., 2006. Numerical study on gas and liquid slugs for Taylor flow in a T-junction microchannel. Chem Eng Sci 61, 7609–7625] correlation are proposed.     

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.     

Oil–water two‐phase flow in microchannels: Flow patterns and pressure drop measurements

This paper investigates oil–water two‐phase flows in microchannels of 793 and 667 µm hydraulic diameters made of quartz and glass, respectively. By injecting one fluid at a constant flow rate and the second at variable flow rate, different flow patterns were identified and mapped and the corresponding two‐phase pressure drops were measured. Measurements of the pressure drops were interpreted using the homogeneous and Lockhart–Martinelli models developed for two‐phase flows in pipes. The results show similarity to both liquid–liquid flow in pipes and to gas–liquid flow in microchannels. We find a strong dependence of pressure drop on flow rates, microchannel material, and the first fluid injected into the microchannel.