强化管内沸腾换热实验研究

主要研究在低过热度下微槽对流动沸腾换热特性的影响,分别以单工质甲醇和甲醇与甲苯的混合物为工质对不同流量情况下光管、直槽管和螺旋槽管的流动沸腾换热特性进行了实验研究。研究结果表明：对单工质甲醇来说,螺旋槽管可以明显起到强化传热作用,而且流量越低,强化传热效果越明显。对混合工质来说,当流量较低时,螺旋槽管强化传热效果不明显,而在流量较高时,强化传热效果比较明显。无论是单工质还是混合工质,直槽管在实验所能达到的壁面温度条件下不能起到明显的强化传热效果。还给出了螺旋槽管强化传热的定性解释。     

Modeling and performance analysis of an absorption chiller with a microchannel membrane‐based absorber using LiBr‐H2O,LiCl‐H2O,and LiNO3‐NH3

In order to develop compact absorption refrigeration cycles driven by low heat sources, the simulated performance of a microchannel absorber of 5‐cm length and 9.5 cm³ in volume provided with a porous membrane is presented for 3 different solution‐refrigerant pairs: LiBr‐H₂O, LiCl‐H₂O, and LiNO₃‐NH₃. The high absorption rates calculated for the 3 solutions lead to large cooling effect to absorber volume ratios: 625 kW/m³ for the LiNO₃‐NH₃, 552 kW/m³ for the LiBr‐H₂O, and 318 kW/m³ for the LiCl‐H₂O solutions given the studied geometry. The performance of a complete absorption system is also analyzed varying the solution concentration, condensation temperature, and desorption temperature. The LiNO₃‐NH₃ and the LiBr‐H₂O solutions provide the largest cooling effects. The LiNO₃‐NH₃ can work at a lower temperature of the heating source, in comparison with the one needed in a LiBr‐H₂O system. The lowest cooling effect and coefficient of performance are found for the LiCl‐H₂O solution, but this mixture allows the use of lower temperature heating sources (below 70°C). These results can be used for the selection of the most suitable solution for a given cooling duty, depending on the available heat source and condensation temperature.     

Effect of aspect ratio on saturated boiling flow in microchannels with nonuniform heat flux

Microchannel two‐phase flow is an effective cooling method used in microelectronics, in which the heat flux density is unevenly distributed usually. The paper is focused on numerical study the effect of aspect ratio on the flow boiling of microchannels with nonuniform heat flux. The heat source is a three‐dimensional (3D) integrated circuit. 3D microchannel model and volume of fluid method are coupled in numerical simulation. The results show that the aspect ratio has no relationship with the two‐phase pressure drop of the microchannel. It has a certain influence on the distribution of bubble shape. In terms of the heat transfer coefficient, the aspect ratio has a certain influence on a section of the inlet. Due to the nucleate boiling, the convective heat transfer in the remaining areas is the dominant factor and the average heat transfer coefficient is mainly determined by the heat flux at the bottom of the channel.     

基于一种微通道散热器的散热实验研究

微通道散热是近年来发展起来的一种新型散热措施.讨论了一种蜂窝状微通道散热器,并设计了基于该微通道散热器的散热系统.系统实验表明,当17cm2的微通道散热器在系统内流体流量为13.3ml/s时,可带走发热源约233.63W热能,同时微通道散热器工质温度能维持在52℃左右.实验还发现,要充分发挥微通道散热器的优势,散热系统中与环境交换热量的小型换热器的散热能力也非常重要.     

微通道板表面羟基化工艺研究北大核心

研究了3种微通道板基底羟基化的方法,测量了羟基化处理后微通道板基底表面水接触角及通道端面的形貌变化,分析了各种方法中微通道板基底的亲水性和腐蚀情况。实验结果表明:氨水双氧水溶液对基体表面的亲水性能提升不大,NaOH溶液对基体有腐蚀作用,经食人鱼溶液处理的基体表面亲水性明显提高且无腐蚀作用。研究了微通道板在食人鱼溶液中的浸泡时间和浸泡温度对表面亲水性的影响。结果表明:随着浸泡温度的增加,微通道板表面水接触角先减小后增大,当温度为80℃时达到极小值,浸泡时间对微通道板表面的亲水性影响不大。最终确定了微通道板表面羟基化工艺:浸泡温度为80℃,静置时间为20~60 min。     

基于光线追迹的方形微通道X光器件模拟

为研究方形微通道X光器件的成像特性,基于光线追迹法建立了一种方形微通道X光器件的数学模型。利用数值模拟的方法对比了方形微通道X光器件和传统多毛细管X光器件的传输效率,研究了方形微通道X光器件的潜在光源位置分辨能力、光斑形貌等成像特性。证实了方形微通道X光器件在深空探测和空间X射线成像领域的潜在应用价值。     

双T型微通道内气液液三相分散规律

研究了双T型微通道内气液液三相微分散过程,获得了水包油包气复乳型三相流和含有气泡的液液平行流两种主要流型,探讨了流体的进料顺序、三相表观流速和微通道内受限空间对于三相流型的影响,建立了流型分布图表。制备了平均体积在22～54 nl的单分散微气泡和平均体积在60～81 nl的油包气复合分散流体,根据气相在Rayleigh-Plateau效应下的破碎机制建立了基于气液相比的分散尺寸数学模型。     

DNA molecule stretching through thermo-electrophoresis and thermal convection in a heated converging-diverging microchannel

A novel DNA molecule stretching technique is developed and tested herein. Through a heated converging-diverging microchannel, thermal convection and thermophoresis induced by regional heating are shown to significantly elongate single DNA molecules; they are visualized via a confocal laser scanning microscopy. In addition, electrophoretic stretching is also implemented to examine the hybrid effect on the conformation and dynamics of single DNA molecules. The physical properties of the DNA molecules are secured via experimental measurements.     

Microdroplet coalescences at microchannel junctions with different collision angles

Microdroplet coalescence mechanism is very important for the miniaturization of multiphase chemical processes with microstructured devices. Using three working systems with different physical properties, this article presents an experimental study on the fluid dynamics of microdroplet coalescence at different microchannel junctions. The critical capillary number to distinguish coalescence or noncoalescence of microdroplet is investigated and its variations with droplet size, collision angle, and physical properties are analyzed with two important parameters – the film drainage time and droplet contact time. Experimental results indicate that microdroplet coalescence can be enhanced by reducing the droplet collision angle. The differences of microdroplet coalescences in confined microchannels and free‐flowing spaces are provided with the analysis of critical capillary number. A model equation is proposed to predict the critical capillary numbers in this study, which may provide valuable information for the design and development of new microstructured chemical device. © 2012 American Institute of Chemical Engineers AIChE J, 59: 643–649, 2013     

三维菱形结构微通道内气液传质与强化

研究了具有三维交错菱形结构的微通道对离子液体1-丁基-3-甲基咪唑四氟硼酸盐([Bmim][BF₄])水溶液吸收CO₂过程的传质增强作用。实验主要聚焦于弹状流和破碎弹状流。考察了弹状流型下气液流量、离子液体浓度对体积传质系数k_La、增强因子E、CO₂吸收率X及压力降ΔP的影响。结果表明,较之于直通道,三维菱形通道可以显著提高体积传质系数和CO₂吸收率,其增强因子可达2.1,压力降仅增加 0.9 kPa。提出了一个新的体积传质系数k_La预测式,预测效果良好。采用VOF法模拟了微通道内气液两相流动过程,获得了连续相的速度矢量场。三维菱形通道能诱导涡流,强化传质过程。