The ablation characteristics of laser-assisted pulsed plasma thruster with metal propellant

In this study,a laser-assisted pulsed plasma thruster (LA-PPT) with a novel configuration is proposed as an electric propulsion thruster which separates laser ablation and electromagnetic acceleration.Owing to the unique structure of the thruster,metals can also be used as propellants,and a higher specific impulse is expected.The ablation quality,morphology,and plume distribution of various metals (aluminium alloy,red copper,and carbon steel) with different laser energies were studied experimentally.The ablation morphology and plume distribution of red copper were more uniform,as compared to those of other metals,and the ablation quality was higher,indicating its greater suitability for LA-PPT.The plume generated by nanosecond laser ablation of aluminium alloy expanded faster,which indicated that the response time of the thruster with aluminium alloy as the propellant was shorter.In addition,when the background pressure was 0.005 Pa,an obvious plume splitting phenomenon was observed in the ablation plume of the pulsed laser irradiating aluminium alloy,which may significantly reduce the utilisation rate of the propellant.     

覆岩结构类型与开采覆岩损害特征分析

煤层开采打破了上覆岩层原有的平衡,必然引起岩层的移动、破断,导致应力场和裂隙场的改变,工作面出现矿压显现。上覆岩层的岩性和组合结构对工作面开采覆岩损害特征分布影响显著,一般岩性越坚硬,上覆岩层应力集中程度越高,采动裂隙越发育,矿压显现也越明显;坚硬岩层尤其是关键层对覆岩的破断运动起控制作用,覆岩破断发展到一定层位后软弱岩层往往是抑制覆岩破断进一步向上发展的关键。当工作面上覆岩层同时存在远近场关键层时,近场关键层的周期性破断引起裂隙带范围内岩层的运动和工作面周期来压;而远场关键层的周期性破断导致地表裂缝或台阶下沉,工作面压力急剧上升,有时甚至导致煤矿动压灾害,尤其是远近场关键层同步破断时动力现象更明显。通过地面钻孔使远场关键层提前破断是防止此类动力灾害的关键。     

Performance assessment of gas crossover phenomenon and water transport mechanism in high pressure PEM electrolyzer

To improve proton exchange membrane (PEM) electrolyzes’ performance the voltage loss through them should be avoided. In this work, it is intended to analyze losses including of diffusion loss, ohmic loss due to electrode, bipolar plate (BP), and membrane resistances, and gas crossover associated with the water transferring mechanisms. All of the losses are associated with water transferring mechanisms, which is created due to electro-osmoic drag, pressure differential between the anode and cathode sides, and diffusion. Furthermore, the effect of membrane thickness, cathode pressure, and operating temperature on the hydrogen crossover is examined. In addition, the contribution of ohmic loss due to electrode bipolar plate (BP), and membrane resistances is studied and, the contribution of different losses on the cell performance is discussed. Results show that raising cathode pressure from 1 to 40 bar lead to the increment of anodic hydrogen content from 1.038% to 21% at the specific current density of 10,000 A/m2. Enhancing the thickness of membrane has considerable impact on decrementing anodic hydrogen content, but the mass transfer loss rises from 0.022 to 0.027 V with enhancing membrane thickness from 50 to 300 μm, respectively. Furthermore, the contribution of voltage losses, assigned to each of losses are equal to 85%, 3%, and 12% for activation, diffusion and ohmic losses, respectively. It is found that, from the reported contribution for ohmic loss, the contribution of electrode BP, and membrane resistances are 31% and 69%, respectively.     

Motion trajectory prediction model of hydrogen leak and diffusion in a stable thermally-stratified environment

The motion trajectory of hydrogen leakage is an essential safe issue for the application of hydrogen energy. A dimensionless fast-running motion trajectory prediction model is proposed to predict the dispersion characteristics of the buoyant jet of hydrogen leakage for the accident. The impact of different leakage angles, leakage velocity and thermal stratification of ambient air on hydrogen leakage behavior was analyzed. The new developed model was verified by experimental results in literatures. Leakage hydrogen can flow upwards freely in a uniform environment. However, it shows an oscillating trajectory at a certain height in a thermally stratified environment, which is so called “locking phenomenon”. The trajectory of hydrogen leakage is upward and hydrogen gathers at the top of the space to form stratification in a uniform environment, while the hydrogen leakage shows an oscillating trajectory at a certain height in a thermal stratification environment. With the increase of Froude number Fr, it shows that the stable height and maximum height of the leakage airflow have a trend of rising first and then falling in a thermally stratified environment. The findings are expected to give guidance in real-world situations, for example, a larger Fr value and a larger temperature gradient can lead to a decrease in the stable height in the thermally stratified environment. It is found that the fitting of the stable height with different temperature gradients satisfies the power function relationship. This work is expected to be helpful for reducing hydrogen leakage accumulation and explosion risk.     

低流量下等高差自然循环系统倒流现象实验研究

自然循环系统由于具有固有安全性而被广泛应用于核电站中。等高差自然循环系统是一种特殊的系统，其加热段与水箱之间是通过上下水平段连接。在低流量条件下，这类自然循环系统会发生水箱冷水倒流至上水平段的现象，目前对该现象的研究并不常见。对等高度差自然循环系统的上水平段倒流特性进行了可视化实验研究。实验结果表明，上水平段倒流长度随流量的增加而减小，随着加热段出口流体与倒流流体温差的增大而增加。研究发现，上水平段倒流现象主要受加热段出口流体惯性力和与倒流流体形成的浮升力共同影响，并且倒流现象的发生可以通过Richardson数来判定。     

北票矿区地质动力区划

地质动力区划法用以揭示地壳的区域岩体断块结构以及评估其力学状态，逐级使用地形图并结合其它方法查清活断裂，由各级活断裂所分割的Ⅳ级断块结构，展出了矿区现今构造运动的结果，以此为构造格架，确定主应力方向和构造应力区，进行区域构造应力场数值模。     

液压传动爬行现象的机理分析与控制

本文从机理上对液压传动中爬行现象进行了分析,提出了控制液压爬行的措施。     

采用异步机矢量控制的矿井提升机开机特性的分析及改进

分析了矢量控制的原理和数学模型，结合矿井提升机的工作特性，通过Matlab仿真，指出造成提升机下坠的根本原因及影响因素，并提出了相应的解决方案，即采用复合控制策略，在原有系统基础上增加给定前馈和扰动前馈环节.计算机仿真结果表明：此方案可大大减轻乃至消除提升机下坠现象.即使称重有误差，产生的下坠现象也非常轻微.     

载人航天器舱内通风空调特性和数值模拟

由于载人航天器这一密闭狭小空间及其微重力这一特殊环境，舱内通风空调问题和热舒适环境与地面的HVAC问题不同，微重力效应特别是自然对流大为减弱影响着通风换热效果。本文首先分析了舱内通风空调问题的特殊性，通过无量纲分析和计算微重力的流体力学效应，然后利用FLUENT软件对两种集中通风方式进行数值模拟。模拟结果表明：微重力下几乎不存在“冷风下坠”或者“热羽”现象；集中斜进风在一定的进风角度和Re数下出现分岔解现象；与常重力相同通风条件下，微重力下自然对流的减弱使得舱内温度降低，换热减少，因此满足常重力热舒适要求的通风条件不一定满足微重力下热舒适性的要求。