Influence of T-semi attached rib on turbulent flow and heat transfer parameters of a silver-water nanofluid with different volume fractions in a three-dimensional trapezoidal microchannel

This study aimed at exploring influence of T-semi attached rib on the turbulent flow and heat transfer parameters of a silver-water nanofluid with different volume fractions in a three-dimensional trapezoidal microchannel. For this purpose, convection heat transfer of the silver-water nanofluid in a ribbed microchannel was numerically studied under a constant heat flux on upper and lower walls as well as isolated side walls. Calculations were done for a range of Reynolds numbers between 10,000 and 16,000, and in four different sorts of serrations with proportion of rib width to hole of serration width (R/W). The results of this research are presented as the coefficient of friction, Nusselt number, heat transfer coefficient and thermal efficiency, four different R/W microchannels. The results of numerical modeling showed that the fluid's convection heat transfer coefficient is increased as the Reynolds number and volume fraction of solid nanoparticle are increased. For R/W=0.5, it was also maximum for all the volume fractions of nanoparticle and different Reynolds numbers in comparison to other similar R/W situations. That's while friction coefficient, pressure drop and pumping power is maximum for serration with R/W=0 compared to other serration ratios which lead to decreased fluid-heat transfer performance.     

低温冷冻靶微管燃料充注流动冷凝特性

低温冷冻靶是惯性约束核聚变装置的关键部件之一。冷冻靶靶丸位于黑腔内部,需要利用微管完成燃料充注。针对燃料在微管内发生气液相变后流入靶丸这一关键过程,研究了微通道流体与常规流体流动传热的差异。基于微通道特性对流体流动传热方程进行修正,同时建立了气液相变模型,对微充气管内燃料充注过程进行了数值计算分析。得到重力和表面张力的影响,在微通道中,重力作用可以忽略,表面张力起重要作用。得到了微尺度效应包括速度滑移和温度跳跃对流动传热过程的影响。对多种充气管结构进行比较分析,为选型提供指导。通过选择不同进口条件和出口条件,对充注量控制和充注条件选择提供了指导方案,实际充注时需要同时提高进口温度和压力,保证连续可控充注。     

微针肋槽道内去离子水换热特性

设计了槽肋比为1:2和2:1的矩形大长宽比微针肋散热器,并实验研究了去离子水在其内的流动换热性能。结果表明：当进口温度为40 ℃、微针肋槽道在雷诺数小于650、最高壁面温度低于77 ℃时,单位面积散热量可达21.32 W/cm²。当雷诺数一定时,同一个槽道壁面温度沿着流动方向不断增加、同一个位置壁面温度随着加热功率的增加而增大,局部努谢尔数沿着流动方向先减小后逐渐增加并趋于定值。当针肋流动换热长度较长时,其入口效应可以忽略,槽道平均努谢尔数随着雷诺数的增大而增大,与加热功率无关;为了更好地表达微针肋槽道内的换热特性,考虑了槽肋比、流动雷诺数等影响,拟合了去离子水在微针肋槽道内的对流换热关系式。     

Flexible particle flow‐focusing in microchannel driven by droplet‐directed induced‐charge electroosmosis

We report herein a novel microfluidic particle concentrator that utilizes constriction microchannels to enhance the flow‐focusing performance of induced‐charge electroosmosis (ICEO), where viscous hemi‐spherical oil droplets are embedded within the mainchannel to form deformable converging‐diverging constriction structures. The constriction region between symmetric oil droplets partially coated on the electrode strips can improve the focusing performance by inducing a granular wake flow area at the diverging channel, which makes almost all of the scattered sample particles trapped within a narrow stream on the floating electrode. Another asymmetric droplet pair arranged near the outlets can further direct the trajectory of focused particle stream to one specified outlet port depending on the symmetry breaking in the shape of opposing phase interfaces. By fully exploiting rectification properties of induced‐charge electrokinetic phenomena at immiscible water/oil interfaces of tunable geometry, the expected function of continuous and switchable flow‐focusing is demonstrated by preconcentrating both inorganic silica particles and biological yeast cells. Physical mechanisms responsible for particle focusing and locus deflection in the droplet‐assisted concentrentor are analyzed in detail, and simulation results are in good accordance with experimental observations. Our work provides new routes to construct flexible electrokinetic framework for preprocessing on‐chip biological samples before performing subsequent analysis.     

基于微通道构型的微流体流动控制研究

微通道不仅仅是作为流体流动的单元, 更是进行流体控制的工具,微通道自身特性和特征用在实现微流体的驱动、进样、混合、分离以及液滴的产生、控制等方面已经表现出了良好的效果.由于微通道中比表面积非常大, 表面效应极大影响流体流动,近年来多数研究集中在应用表面效应来实现微流体驱动与控制,而以利用微通道结构特征实现流体流动控制为目标的研究成果相对较少.为了提高对通道构型作用的认识,主要介绍了基于微通道构型的无可动部件的流体微阀和基于微通道构型微小液滴的产生及流动控制器两个方面的发展情况,表明微通道构型在微流体控制中同样可以发挥重要作用,甚至有望带来微流控技术的突破.      

Flow pattern based correlations of two-phase pressure drop in rectangular microchannels

Numerous pressure drop correlations for microchannels have been proposed; most of them can be classified as either a homogeneous flow model (HFM) or a separated flow model (SFM). However, the predictions of these correlations have not been compared directly because they were developed in experiments conducted under a range of conditions, including channel shape, the number of channels, channel material and the working fluid. In this study, single rectangular microchannels with different aspect ratios and hydraulic diameters were fabricated in a photosensitive glass. Adiabatic water-liquid and Nitrogen-gas two-phase flow experiments were conducted using liquid superficial velocities of 0.06–1.0 m/s, gas superficial velocities of 0.06–72 m/s and hydraulic diameters of 141, 143, 304, 322 and 490 μm. A pressure drop in microchannels was directly measured through embedded ports. The flow pattern was visualized using a high-speed camera and a long-distance microscope. A two-phase pressure drop in the microchannel was highly related to the flow pattern. Data were used to assess seven different HFM viscosity models and ten SFM correlations, and new correlations based on flow patterns were proposed for both HFMs and SFMs.     

基于解析法的X光分幅相机静态增益特性仿真

 采用解析法对X光分幅相机中微通道板单孔内电子运动、碰壁和二次发射一系列过程进行了建模与求解,依托商业Lorentz-2D软件,得到了二次电子初能量分布、电子碰壁前能量分布、渡越时间分布等仿真曲线。实验测量了分幅相机的静态灵敏度曲线,与仿真结果对比分析表明：光电子首次碰撞深度是影响相机增益的重要因素,给出了修正后的增益与电压关系表达式,定性分析了工作偏压等参数对首次碰撞深度的影响。     

微通道换热器大温差条件下流动换热研究

随着高效预冷器在航天航空领域发挥越来越重要的作用,紧凑高效换热器的研究成为了人们关注的热点。本文基于紧凑微通道换热器的几何特征,针对矩形截面平行流道换热器内超临界压力低温流体(氢和氦)在大温差条件下的流动换热现象进行数值模拟研究。通道截面边长小于1 mm,热流体氦和冷流体氢的进出口温差均大于600 K。通道内流体换热系数在顺流和逆流条件下有不同的变化趋势,并出现峰值。换热量随着通道宽度的增大而增大,流动压降随着通道宽度的增大而减小。冷热流体逆流时换热量大,压降较小,但对换热器材料要求较高。     

Imaging properties of a tetra wedge readout

The decoding principle of a tetra wedge anode, which is a development of the wedge and strip anode, is described. The influence of charge cloud size on decoding accuracy is studied using the Monte Carlo method. Simulation results show that the decoding error is large when the size of charge clouds collected by the anode is small. Thus, the charge clouds collected by the tetra wedge anode should reach a necessary size to ensure accurate decoding. Finally, using the ultraviolet photon counting imaging system, the linearity and the spatial resolution of the system are tested. Experimental results show that the system has a good linearity and the spatial resolution is better than 100 μm.