高功率二极管泵浦固体激光器研究的进展

对高功率二极管泵浦固体激光器若干新的进展。相关技术和潜在应用作了评述和分析。     

Diffusioosmotic flow in rectangular microchannels

The diffusioosmosis of an electrolyte solution inside a uniformly charged rectangular channel at steady locally developed conditions is the subject of this study. Utilizing a finite element based numerical procedure, we try to estimate the errors incurred by modeling the actual rectangular geometry of typical microchannels as a slit. We demonstrate that the flow pattern and direction are generally dependent upon the width‐to‐height ratio of the channel. Such a finding, besides showing the ineffectiveness of the slit geometry in representing a rectangular channel of small aspect ratio, informs us of another mechanism of controlling the diffusioosmotic flow. Inspections of the mean velocity reveal that, although it drastically grows by increasing the aspect ratio at smaller values of this parameter, no significant change is observed when the aspect ratio is 5 or higher. The same trend is observed when EDL is shrunk and is considered as a basis for the introduction of a slip‐like velocity, similar to the concept of the Helmholtz–Smoluchowski electroosmotic velocity, which will be of high practical importance when dealing with a micronsized channel. Because of its significance, an expression is presented for this slip velocity utilizing the curve fitting of the results, assuming a typical Peclet number.     

Electroosmotic flow mixing in zigzag microchannels

In this study we performed numerical and experimental investigations into the mixing of EOFs in zigzag microchannels with two different corner geometries, namely sharp corners and flat corners. In the zigzag microchannel with sharp corners, the flow travels more rapidly near the inner wall of the corner than near the outer wall as a result of the higher electric potential drop. The resulting velocity gradient induces a racetrack effect, which enhances diffusion within the fluid and hence improves the mixing performance. The simulation results reveal that the mixing index is approximately 88.83%. However, the sharp-corner geometry causes residual liquid or bubbles to become trapped in the channel at the point where the flow is almost stationary, when the channel is in the process of cleaning. Accordingly, a zigzag microchannel with flat-corner geometry is developed. The flat-corner geometry forms a convergent-divergent type nozzle which not only enhances the mixing performance in the channel, but also prevents the accumulation of residual liquid or bubbles. Scaling analysis reveals that this corner geometry leads to an effective increase in the mixing length. The experimental results reveal that the mixing index is increased to 94.30% in the flat-corner zigzag channel. Hence, the results demonstrate that the mixing index of the flat-corner zigzag channel is better than that of the conventional sharp-corner microchannel. Finally, the results of Taguchi analysis indicate that the attainable mixing index is determined primarily by the number of corners in the microchannel and by the flow passing height at each corner.     

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。     

Numerical simulation of immiscible liquid-liquid flow in microchannels using lattice Boltzmann method

Immiscible kerosene-water two-phase flows in microchannels connected by a T-junction were numerically studied by a Lattice Boltzmann (LB) method based on field mediators.The two-phase flow lattice Boltzmann model was first validated and improved by several test cases of a still droplet.The five distinct flow regimes of the kerosene-water system,previously identified in the experiments from Zhao et al.,were reproduced.The quantitative and qualitative agreement between the simulations and the experimental dat...     

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。     

Halo-based black point detection method of microchannel plate北大核心CSCD

根据微通道板黑点检测原理与黑点光晕特征设计了一种检测方法,该方法利用圆提取技术实现荧光屏图像的半径和圆心提取,通过高斯拉普拉斯算子实现目标黑点粗检测。基于黑点在不同电压下外围呈现光晕的特征,利用对比度受限的直方图均衡化并结合阈值分割的光晕检测法实现黑点光晕的提取。测试结果表明,该方法可以将荧光屏划痕、灰尘与微通道板黑点有效区分,从而实现黑点的自动提取。     

一种透射式软X光带通方法研究

基于微通道板(MCP)X射线光学, 提出了一种透射式软X光带通方法. 通过三种通道结构的MCP-X光传输特性比较, 给出了带通设计方法. 利用北京同步辐射装置开展了方孔MCP和滤片标定. 结果表明, MCP透射谱具有宽能带选择范围和高效率的特点, 并且在1 keV以下可以在多个多能点实现100 eV带宽的带通设计. 关键词： X射线光学 微通道板 软X光带通