In-situ investigation and modeling of electrochemical reactions with simultaneous oxygen and hydrogen microbubble evolutions in water electrolysis

The development of water electrolyzer is challenging as we approach theoretical limits arising from electrochemical reactions and micro-scale bubble dynamics. In this research, two-phase flow and bubble dynamics are in-situ studied in a special designed single-channel electrolyzer. The devices fabricated by a 3D printer provide a whole vision of the electrochemical reaction within the channel. In-situ observations of channel-scale hydrogen and oxygen micro-bubbles dynamics are conducted, and the whole process of hydrogen evolution reactions (HERs) and oxygen evolution reactions (OERs) are simultaneously studied. The results indicate that all bubbles generate at the interface between the proton exchange membrane and the electrode wire, and the operating conditions have a great impact on the micro bubble evolution process. The bubble detachment diameter is inversely proportional to the flow velocity, but is in direct proportion to the current density. Finally, a mathematic model has been developed, and shows a good agreement with experimental data. Those results could help to better understand the bubble evolution mechanism, in order to further understand the electrochemical reaction.     

滚动压印装置研制与压印填充模拟

紫外滚动压印因其可以在室温下、普通大气环境中高效大面积地进行微结构制作而备受瞩目.利用自行研制的紫外滚动压印装置,实现了微结构的快速复制.实验中发现当压印速度达到某一特定值时,会不可避免地产生气泡缺陷.采用流体软件Fluent对滚动压印充填过程进行分析,来研究气泡缺陷的产生机理.结果表明,压印速度过高导致空气被堵在图形空腔内,最终形成气泡缺陷.同时,低黏度的胶液和小的接触角更利于模型填充.此研究对于获得优化的滚动压印工艺具有一定的意义.     

Effects of bubble coalescence dynamics on heat flux distributions under bubbles

Bubble coalescence events and single bubble nucleation, growth, and departure were observed with a charge‐coupled device (CCD) camera synchronized with a high‐speed data acquisition system measuring the heat fluxes at different positions underneath the bubble. Four different kinds of coalescence events took place with characteristic heat flux signatures for each type of event. The heat transfer measurements under four typical heaters representing different positions under the bubble showed very different heat flux characteristics during each ebullition cycle. The results show that transient conduction due to the movement of the contact line during bubble interactions, sliding, stretching, and oscillating is the main mechanism resulting in the high heat transfer rates during coalescence. Fast evaporation of the liquid layer trapped between the two bubbles and higher heat fluxes near the contact line as the microlayer evaporated also generate high heat flux spikes but have less influence due to their short duration and small influence area. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1735–1745, 2013     

Bubble breakup with permanent obstruction in an asymmetric microfluidic T‐junction

Bubble breakup with permanent obstruction in an asymmetric microfluidic T‐junction is investigated experimentally. The breakup process of bubbles can be divided into three stages: squeezing, transition, and pinch‐off stages. In the squeezing stage, the thinning of the bubble neck is mainly controlled by the velocity of the fluid flowing into the T‐junction, and the increase of the liquid viscosity can promote this process. In the transition stage, the minimum width of bubble neck decreases linearly with time. In the pinch‐off stage, the effect of the velocity of the fluid flowing into the T‐junction on the thinning of the bubble neck becomes weaker, and the increase of the liquid viscosity would delay this process. The evolution of the minimum width of the bubble neck with the remaining time before the breakup can be scaled by a power–law relationship. The bubble length has little influence on the whole breakup process of bubbles. © 2014 American Institute of Chemical Engineers AIChE J, 61: 1081–1091, 2015     

Mechanism and analytical models for the gas distribution on the SiC foam monolithic tray

Silicon carbide (SiC) foam material has been applied as monolithic tray for distillation column in our previous study. A systematic understanding of the gas distribution process on the foam tray should help to the design of commercial application. In this article, local gas holdup distribution and bubble size distribution are used to measure the gas distribution. The local gas holdup is tested by the conductive probe and the number of test point is counted in different local gas holdup. The bubbles are captured by the high‐speed camera to measure the bubble size. Bubble size is calculated as ellipsoidal bubble and counted with different pore sizes. Furthermore, a three‐stage process model is put forward to explain the uneven distribution of gas phase, and verified by the experimental values. The results show that the structure and the thickness of SiC foam is the decisive factor for the gas distribution performance. © 2015 American Institute of Chemical Engineers AIChE J, 61: 4509–4516, 2015     

DCM模式下交错并联磁集成双向DC/DC变换器的稳态性能分析

针对目前交错并联磁集成双向DC/DC变换器工作在DCM模态下的分析不全面这一研究现状,以两相电感进行耦合的两相交错并联磁集成Buck/Boost双向DC/DC变换器为例,在轻载时关断MOS管,利用MOS管的体二极管具有单向导电的特性,避免电感电流出现负值,从而降低变换器的能量损耗,提高变换器的效率。首先深入分析了运行在Buck状态下的双向DC/DC变换器的DCM模态的稳态性能。其次,分析了变换器的工作状况转换条件,求出各模态转换的临界负载电流。并通过引入的变比与其他参数之间的关系具体分析变换器在各种工作情况(CCM/DCM)下每相相电流的变化规律,同时找出变换器的总输出电流的变化规律。最后,通过仿真和实验的具体数据验证理论分析的可靠性。     

An enriched FEM technique for modeling hydraulically driven cohesive fracture propagation in impermeable media with frictional natural faults: Numerical and experimental investigations

In this paper, an enriched finite element technique is presented to simulate the mechanism of interaction between the hydraulic fracturing and frictional natural fault in impermeable media. The technique allows modeling the discontinuities independent of the finite element mesh by introducing additional DOFs. The coupled equilibrium and flow continuity equations are solved using a staggered Newton solution strategy, and an algorithm is proposed on the basis of fixed‐point iteration concept to impose the flow condition at the hydro‐fracture mouth. The cohesive crack model is employed to introduce the nonlinear fracturing process occurring ahead of the hydro‐fracture tip. Frictional contact is modeled along the natural fault using the penalty method within the framework of plasticity theory of friction. Moreover, an experimental investigation is carried out to perform the hydraulic fracturing experimental test in fractured media under plane strain condition. The results of several numerical and experimental simulations are presented to verify the accuracy and robustness of the proposed computational algorithm as well as to investigate the mechanisms of interaction between the hydraulically driven fracture and frictional natural fault. Copyright © 2015 John Wiley & Sons, Ltd.     

基于激光视觉原理测量玻璃中气泡的尺寸

为了实现玻璃中气泡尺寸的在线测量,基于激光视觉原理,采用线结构激光器、线阵CCD相机和运动控制系统在实验室搭建玻璃气泡的动态测量系统。投射到玻璃表面的线结构激光光斑经变焦镜头成像在相机的光敏阵列上,相机的横向扫描与运动平台的纵向扫描配合,利用Sapera CamExpert图像采集软件获得玻璃气泡的灰度图像;理论上讨论了图像的纵向和横向精度,给出了气泡图像失真的判断依据及工作像素时钟频率与玻璃板纵向运动速率之间的匹配关系;采集了不同纵向运动速率下气泡的灰度图像,利用Sapera Architect软件测量气泡图像的横向和纵向像素数,通过理论计算得出气泡的横向和纵向尺寸,并与直尺测量结果比较,发现纵向尺寸相对误差为0.18,横向尺寸相对误差为0.05。结果表明,在相机工作距离、焦距和像元尺寸确定的情况下在误差允许的范围内该系统可以用来测量气泡的横向尺寸。     

随机参差重频法抗雷达导引头距离波门前拖研究

文章分析了DRFM距离拖引干扰的工作原理,建立了干扰的信号模型。根据自动增益控制(AGC)变化对干扰进行了检测,在此基础上利用随机参差重频的方法对距离波门前拖进行抑制,在不同的干信比下分别进行了外场试验验证,通过对试验结果的分析表明了该方法能够有效的抑制干扰。