常规空调热泵系统的R32替代研究述评

R32由于具有ODP为零、GWP值小于R410A等优越的环境性能,逐渐被认识到可以用于替代R410A。这里对R410A和R32的循环性能、润滑油的选用、可燃性、充注量、换热器性能等相关问题进行了综述。大量研究表明,与R410A系统相比,R32系统的压缩机耗功略高,制冷量较高,同时排气温度也高出很多,具有极大的替代R410A的潜力。然而,相关研究也指出要使其能够在大范围推广使用,除了要有效解决排气温度过高的问题之外,还需要解决其微可燃性在设计应用中所受到的限制。通过解读国内外法规政策发现,国内外对R32的充注量限制有放宽的趋势,这对R32在常规空调热泵系统中替代R410A和R22的研究工作将是一个巨大的推动。     

FDI Experience Location and Subsidiary Mortality

Purpose: This study examines the relationship between an MNE’s experience with foreign direct investments (FDI) and subsidiary mortality when considering learning limitations that an MNE are susceptible to under the influence of national cultural differences. Methods: We apply event history analysis on a sample of subsidiaries of Korean MNEs during 1990–2006. Results: We find that subsidiary mortality has a negative relationship with FDI experience from countries culturally similar to both the MNE’s home country and the focal host country; an inverse U-shaped relationship with both experience from countries culturally similar to the home country but different from the focal host country, and experience from countries culturally different from the home country but similar to the focal host country. Experience from countries culturally dissimilar to both the focal host country and the home country has a negative effect when a subsidiary is located in countries dissimilar to the home country, and an insignificant effect when a subsidiary is located in countries similar to the home country. Conclusions: Our findings show that prior experience has different effects depending on where it is accumulated and that MNEs are susceptible to limitations both when drawing inferences from their FDI experiences and when applying these experiences toward subsequent FDIs.     

Scalable Video Multicast Over Multi-Antenna OFDM Systems

We propose a framework for efficient scalable video multicast over downlink orthogonal frequency-division multiplexing (OFDM) systems with multiple transmit antennas. In conventional video multicast systems, the achievable transmission rate is determined by the user with the worst channel condition, and the system saturates the capacity when the number of users increases. To accommodate the heterogeneous channel conditions and device capabilities of various users, scalable video coding (SVC) encodes video streams into base and enhancement layers. We exploit the advances in multi-antenna OFDM and the layered nature of SVC, and propose a framework for scalable video multicast which guarantees the base layer quality for all users while making best use of limited resource for the enhancement layer of users with good channel conditions. We show that the resource allocation that includes the transmit precoding, subcarrier allocation, and bit and power allocation is a very difficult optimization problem. A low-complexity suboptimal algorithm is proposed which is suitable for practical implementations. Simulation results are provided to show the effectiveness of the proposed solution.     

基于包含谐波模型的加权最小二乘估计算法的电压暂降检测方法

针对动态电压恢复器需要以较快的速度检测出电网电压暂降,提出一种基于对电网电压建立谐波模型的加权最小二乘估计算法的电压暂降检测方法.通过重置协方差,在电网电压的谐波分量较高时该算法也能准确快速地判断出电压暂降.由于建立了谐波模型,稳态误差不会包含大量谐波分量,因此重置协方差的阈值可以设置得较小,使得该算法能够较快地检测出深度较小的电压暂降.通过设置不同的实验条件,验证了该算法具有良好的检测性能.     

750kV海西开关站母线电量不平衡原因及对策

文章就电力系统中变电站母线电量不平衡特例进行分析,并对出现问题提出相应的解决措施,为计量人员解决此类问题提供一些参考。     

基于单片机的印刷品色彩检测系统

鉴于CIEXYZ颜色空间日益广泛应用于彩色印刷复制过程控制和印刷品质量评价,本文设计一种在线印品色彩质量检测系统,该系统可实现测量并显示色彩的CIEXYZ的功能,且仪器工作稳定、性能良好。     

铁掺杂二氧化钛光催化剂的性能研究

采用溶胶-凝胶法制备了铁掺杂二氧化钛粉体(Fe-TiO2),采用XRD、UV-Vis、PL、纳米粒度分析仪等方法对其进行表征和分析。结果表明,Fe掺杂降低了TiO2的相转变温度,拓展了TiO2对可见光的吸收范围,有效地抑制了光生电子-空穴对的复合。光催化降解亚甲基蓝(MB)的实验表明,Fe-TiO2粉体在普通日光灯下对亚甲基蓝的降解率达90.36%,明显优于德国Degussa公司生产的P25纯TiO2光催化剂对亚甲基蓝的降解率43.58%。     

多孔材料[ZnNa（H3btc）（H2O）2]·（H2O）2的合成、晶体结构及吸附性能研究

合成了一种多孔金属有机骨架材料[ZnNa(H3btc)(H2O)2].(H2O)2,分析晶体结构,并采用体积法气体吸附实验,考察了该配合物的吸附气体性能。实验测试结果表是:该配合物为双核,中心Zn2+与三羧基和二个水构成双四面体配位模式),中心Na+与四羧基和二个水构成正八面体配位模式;该配合物对CO2饱和吸附量较大,在p=20bar,t=20℃条件下达到14.25mmol/g,且随着温度升高,可缩短饱和吸附时间,但吸附性能有所下降。     

刀具断屑槽对切削过程影响的数值模拟

本文通过有限元法模拟切削过程,对硬质合金刀片断屑槽在切削过程中的影响做了研究。结果表明采用平滑的槽型曲面有利于降低主切削力;断屑槽能够降低轴向力和径向力,进而可以帮助提高加工精度和工件的表面质量。切削过程中,有断屑槽的刀片表面等温线分布集中,而没有断屑槽的刀片表面有多个最高温度区域且温度更高,不利于刀片的使用寿命。另外,有断屑槽的刀片所产生切屑的有效应力、有效应变更大,更适合于加工塑性好的材料。