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.     

径向偏振光束经三角形点阵列板衍射的偏振奇点

根据夫琅禾费衍射积分公式推导出径向偏振光束通过三角形点阵列板(TMP)衍射后的传输表达式,通过理论计算模拟并进行实验验证,得到衍射场光强分布图,衍射场光强成周期性排列。并用复Stokes场来分析径向偏振光束衍射场偏振奇点的变化规律。结果表明,衍射场的光强分布随着三角形点阵列板N值的改变而改变,偏振奇点的数目随着三角形点阵列板的N值的增大而增多。另外,光束参数(波长和光束束宽)、传输距离和三角形点阵列板的参数(三角板边长)均对偏振奇点的位置产生影响。改变三角形点阵列板的N值,伴随有偏振奇点的产生和湮灭现象。     

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。     

连铸板坯三角区裂纹成因分析及改进措施

通过对铸坯的低倍检验和三角区裂纹部位的电镜扫描,分析了连铸板坯三角区裂纹的类型和成因;提出了从钢水成分、二冷配水、驱动压力、轻压下、支撑段等方面的改进措施。其结果表明,钢中硫和铸坯的支撑段是影响三角区裂纹的主要因素。具体措施的实施,使三角区裂纹的发生率和级别大幅度下降。     

三角网格牙颌模型上的牙弓线半自动探测方法

介绍了一种在三维三角网格牙颌模型上半自动探测牙弓线的方法。首先使用三维激光测量系统对牙颌石膏模型进行测量获得数字模型;其次在网格模型上交互拾取4个参照点以确定咬合平面;接着使用顶点法矢和咬合平面作为参考,筛选出初始特征点并拟合出初始牙弓线;以初始牙弓线作为新的参考依据筛选出最终特征点,拟合最终参照点得到最终牙弓线。使用多副牙颌模型对该方法进行了测试,验证了该方法稳定、有效。     

面向曲面品质分析的高光曲率线法

高光线法在曲面品质分析领域广泛应用。针对高光线法信息单调的缺陷,首先探讨了三角网格曲面上高光线的生成原理,优化了曲面特征点法矢量算法,给出了更为高效的高光点追踪算法,引入NURBS插值曲线反算法用于计算高光线。由此提出了一种曲面品质分析新方法一高光曲率线法。最后通过高光曲率线法的分析实例,验证文中所提出新算法在曲面品质分析上的优越性。     

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。     

上三角矩阵代数上的保不变子空间格映射

设Tn是数域F上的n×n阶上三角矩阵代数,其中F是实数域R或复数域C.利用矩阵的可加性,证明了Tn上的每一个保不变子空间格的可加映射Φ为:Φ(A)=αA+φ(A)I ((A)A∈Tn),其中α是非零常数,φ∶Tn←F是可加映射,I∈Tn是单位算子.