Effect of the Entrained Air Void on Strength and Interfacial Transition Zone of Air-Entrained Mortar

In order to facilitate the development and application of air entraining agents (AEA) in the high performance concrete, entrained air void structure parameters (air void size range from 10 to 1 600 mu m) of 28 d sifted mortar were measured by image analysis method. The relationship between the air void size distribution and strength of mortar was studied by methods of grey connection analysis and multiple linear regression analysis. The multiple linear regression equation was established with a correlation coefficient of 0.966. The weight of the affection of hierarchical porosity on the compressive strength ratio was also obtained. In addition, the effect of air voids on the paste-aggregate interfacial transition zone (ITZ) was analyzed by microhardness. The results show that the correlation between different pore size range and the compressive strength is negative. The effect of air void size distribution on 28 days compressive strength is different: under the condition of similar total porosity, with the increase of the porosity of the air void size, ranging from 10 to 200 mu m, and the decrease of the porosity, ranging from 200 to 1 600 mu m, the average air void diameter and mean free spacing are decreased; as well as the width of ITZ. On the contrary, the microhardness of the ITZ is increased while the compressive strength loss is decreased.

Transition Periods Between Sea Ice Concentration and Sea Surface Air Temperature in the Arctic Revealed by an Abnormal Running Correlation

This study used the synthetic running correlation coefficient calculation method to calculate the running correlation coefficients between the daily sea ice concentration(SIC) and sea surface air temperature(SSAT) in the Beaufort-Chukchi-East Siberian-Laptev Sea(BCEL Sea), Kara Sea and southern Chukchi Sea, with an aim to understand and measure the seasonally occurring changes in the Arctic climate system. The similarities and differences among these three regions were also discussed. There are periods in spring and autumn when the changes in SIC and SSAT are not synchronized, which is a result of the seasonally occurring variation in the climate system. These periods are referred to as transition periods. Spring transition periods can be found in all three regions, and the start and end dates of these periods have advancing trends. The multiyear average duration of the spring transition periods in the BCEL Sea, Kara Sea and southern Chukchi Sea is 74 days, 57 days and 34 days, respectively. In autumn, transition periods exist in only the southern Chukchi Sea, with a multiyear average duration of only 16 days. Moreover, in the Kara Sea, positive correlation events can be found in some years, which are caused by weather time scale processes.

Flat-plate hypersonic boundary-layer ?ow instability and transition prediction considering air dissociation

The effects of air dissociation on ?at-plate hypersonic boundary-layer ?ow instability and transition prediction are studied. The air dissociation reactions are assumed to be in the chemical equilibrium. Based on the ?at-plate boundary layer, the ?ow stability is analyzed for the Mach numbers from 8 to 15. The results reveal that the consideration of air dissociation leads to a decrease in the unstable region of the ?rst-mode wave and an increase in the maximum growth rate of the second mode. High frequencies appear earlier in the third mode than in the perfect gas model, and the unstable region moves to a lower frequency region. When the Mach number increases, the second-mode wave dominates the transition process, and the third-mode wave has little effect on the transition. Moreover, when the Mach number increases from 8 to 12, the N-factor envelope becomes higher, and the transition is promoted. However, when the Mach number exceeds 12, the N-factor envelope becomes lower, and the transition is delayed. The N-factor envelope decreases gradually with the increase in the altitude or Mach number.

Localisation and phase transition of acoustic waves in a soft medium containing air bubbles

We study via numerical experiments the localisation property of an acoustic wave in a viscoelastic soft medium containing randomly-distributed air bubbles. The behaviours of the oscillation phases of bubbles are particularly investigated in various cases for distinguishing efficiently the acoustic localisation from the effects of acoustic absorption caused by the viscosity of medium. The numerical results reveal the phenomenon of ＇phase transition＇ characterized by an unusual collective oscillation of bubbles, which is an effective criterion to unambiguously identify the acoustic localisation in the presence of viscosity. Within the localisation region, the phenomenon of phase transition persists, and a remarkable decrease in the fluctuation of the oscillation phases of bubbles is observed. The localisation phenomenon will be impaired by the enhancement of the viscosity factors, and the extent to which the acoustic wave is localised may be determined by appropriately analyzing the values of the oscillation phases or the amount of reduction of the phase fluctuation. The results are particularly significant for the practical experiments in an attempt to observe the acoustic localisation in such a medium, which is in general subjected to the interference of the great ambiguity resulting from the effect of acoustic absorption.

基于Gama-Theta转捩模型的气冷涡轮气热耦合研究

基于非结构化混合网格,采用有限体积法求解了N-S方程,对流项格式为AUSM+,梯度的求解方法为加权最小二乘法,求解器采用LUSGS隐式求解方法,并进行了预处理和网格重排序。与导热方程耦合,搭建了气热耦合平台,采用直接耦合方法,交界面互相传递温度,并且采用能保证通量守恒的面积加权类的插值方式实现数据传递,将γ-Reθt两方程转捩模型应用在流场程序中。通过MARKII叶片4311和5411工况的实验结果进行了验证,结果表明采用转捩模型对压力分布的影响不大,与实验值吻合较好,通过间歇因子的分布可以看出,γ-Reθt模型成功地预测了从层流到湍流的转捩过程,在层流区采用γ-Reθt模型计算的涡粘系数与真实流动情况更加吻合,由于涡粘系数对传热的影响很大,从而有效地提高了边界层的层流区和转捩区的传热计算的精度,得到的温度和换热系数与实验值吻合更好,但是该模型是和SST模型耦合在一起的,由于SST模型的局限性,所以对激波和边界层干扰区域的模拟产生了7%左右的温度误差。

Preparation and properties of transition metal nitrides caged in N-doped hollow porous carbon sphere for oxygen reduction reaction

A series of transition metal nitrides(MxNy,M=Fe,Co,Ni)nanoparticle(NP)composites caged in N-doped hollow porous carbon sphere(NHPCS)were prepared by impregnation and heat treatment methods.These composites combine the high catalytic activity of nitrides and the high-efficiency mass transfer characteristics of NHPCS.The oxygen reduction reaction results indicate that Fe2N/NHPCS has the synergistic catalytic performance of higher onset potential(0.96 V),higher electron transfer number(~4)and higher limited current density(1.4 times as high as that of commercial Pt/C).In addition,this material is implemented as the air catalyst for zinc−air battery that exhibits considerable specific capacity(795.1 mA·h/g)comparable to that of Pt/C,higher durability and maximum power density(173.1 mW/cm2).

地区空气污染的“压力”与企业绿色转型的“动力”——基于城市PM2.5和公司并购的实证发现

地区空气污染会影响域内各类市场主体行为,包括微观企业决策。碳达峰、碳中和的提出要求各地加强空气污染治理,推动企业绿色转型,而绿色并购是企业绿色转型的重要方式。利用2013—2018年我国A股上市的污染企业发起的并购为样本,并手动搜集整理确定绿色并购样本,实证研究地区空气污染对域内企业绿色转型的影响。研究发现,地区空气污染加剧会提升域内污染企业绿色并购的意愿。在此基础上,通过并购前后企业信息披露质量和社会责任承担变化,研究发现地区空气污染加剧下的企业绿色转型并非源于自主绿色转型发展的内生“动力”,更多是外部“压力”下的“工具主义”行为。从影响路径来看,地区空气污染引致的压力会通过行业竞争和融资约束这两个途径影响污染企业绿色转型的“动力”。最后,异质性检验发现在市场化程度高和非国有企业样本,地区空气污染加剧对企业绿色并购的促进作用更为显著。

轨道车辆空气弹簧刚度试验统型设计与研究

为实现轨道车辆空气弹簧静态和动态刚度试验模块化、标准化统型设计目标,对试验统型设计思路进行了分析;设计了一种新型的空气弹簧径向轴向组合密封模块化试验装置,并与传统密封装置进行了对比;重点利用新型装置探究了不同加载速度对刚度的影响,以及受迫和自由这两种不同振动方式对动刚度、阻尼、共振频率、共振倍率的影响。结果表明:统型试验装置能够满足试验要求,且装置数量减少了50%,存放空间缩减了50%,设计效率提高了60%,加载速度对静刚度的影响并不明显,受迫和自由这两种不同振动方式所测试的动态性能数据基本一致。

隧道防火门分流对端部临界送风风速的影响

在单洞双线轨道交通隧道发生火灾时,人员从火源上游防火门疏散,防火门的分流作用将影响端部送风风速。采用理论计算与数值模拟相结合的方法探究其影响关系,首先通过理论分析推导出了端部临界送风风速公式;随后,利用PyroSim软件建立了全尺寸模型,并针对不同数量防火门、火源热释放速率、隧道高度的分流情况进行了研究。结果表明:在相同火源热释放速率下,防火门的分流流速随着防火门开启数量的增多而减少;相同位置防火门的分流流速随火源热释放速率的增大呈指数增长的关系;防火门总分流量随防火门开启数量与热释放速率的0.3次方之积呈线性增长的关系,而受隧道高度的影响较小;结合理论推导,建立了端部临界送风风速的数学模型。研究结果为隧道火灾情况下的人员疏散和通风设计提供了科学依据。

空地一体遥感技术在轨道交通安全保护区巡查中的应用

随着城市轨道交通建设的不断发展,轨道交通沿线安全保护区巡查工作的范围越来越大,内容越来越多,区域内的情况越来越复杂。针对存在的问题,本文构建了一套空地一体的遥感综合监测体系,在巡查工作中引入航空摄影测量、低空无人机监测、车载全景扫描、定点实时监控等多种遥感监测技术及DSM差分计算、深度学习智能识别等多种数据处理技术,并以上海为例开展应用实践。结果表明,本文方法具有可行性,对建立全覆盖、高频次、智能化的巡查模式有参考借鉴价值。

FLIGHT CLASSIFICATION MODEL BASED ON TRANSITIVE CLOSURE ALGORITHM AND APPLICATION TO FLIGHT SEQUENCING PROBLEM

A new arrival and departure flight classification method based on the transitive closure algorithm （TCA） is proposed. Firstly, the fuzzy set theory and the transitive closure algorithm are introduced. Then four different factors are selected to establish the flight classification model and a method is given to calculate the delay cost for each class. Finally, the proposed method is implemented in the sequencing problems of flights in a terminal area, and results are compared with that of the traditional classification method（TCM）. Results show that the new classification model is effective in reducing the expenses of flight delays, thus optimizing the sequences of arrival and departure flights, and improving the efficiency of air traffic control.

Research progress of transition metal compounds as bifunctional catalysts for zinc-air batteries

Zinc-air batteries(ZABs)are widely studied because of their high theoretical energy density,high battery voltage,environmental protection,and low price.However,the slow kinetics of oxygen reduction reaction(ORR)and oxygen evolution reaction(OER)on the air electrode limits the further application of ZABs,so that how to develop a cheap,efficient,and stable catalyst with bifunctional catalytic activity is the key to solving the development of ZABs.Transition metal compounds are widely used as cathode materials for ZABs due to their low cost,high electrocatalytic activity,and stable structure.This review summarizes the research progress of transition metal compounds as bifunctional catalysts for ZABs.The development history,operation principle,and mechanism of ORR and OER reactions are introduced first.The application and development of transition metal compounds as bifunctional catalysts for ZABs in recent years are systematically introduced,including transition metal oxides(TMOs),transition metal nitrides(TMNs),transition metal sulfides(TMSs),transition metal carbides(TMCs),transition metal phosphates(TMPs),and others.In addition,the shortcomings of transition metal compounds as bifunctional catalysts for ZABs were summarized and reasonable design strategies and improvement measures were put forward,aiming at providing a reference for the design and construction of high-performance ZABs cathode materials.Finally,the challenges and future in this field are discussed and prospected.

高压气动减压阀内部湿空气凝结特性

针对高压气动减压阀内部结露结冰的现象,建立高压湿空气凝结流动数学模型,开展高压气动减压阀内部结露机理研究。以湿空气和液滴为流动介质,采用湿蒸汽凝结理论和多组分运输模型,通过C语言和Fluent-UDF编程开发了高压超音速湿空气凝结流动数值模型,搭建实验平台进行数值模型验证。利用该数值模型对高压气动减压阀内部湿空气凝结流动特性进行仿真,并分析了高压超音速流动过程中湿空气的流动凝结行为。结果表明:高压压缩气体流经节流阀口为绝热等熵过程,产生焦耳-汤姆逊正效应,气体体积迅速膨胀,压力和温度急剧下降,流速达到超音速流动;阀口附近下游区域大量成核,液滴快速增长,形成主要凝结区。高压湿空气凝结流动模型可为高压减压阀结构优化及进一步结冰机理研究提供参考。

大型叠层笼养肉鸡舍气流组织模拟与优化

针对大型叠层笼养肉鸡舍过渡性通风模式下温度分布不均的问题,通过现场实测建立三维数学模型,采用计算流体动力学方法对试验鸡舍进行气流组织模拟分析,提出改进方案并进行测试验证。结果显示:鸡舍内部气流分布不均,大部分气流从鸡舍上方空间流出舍外,未与肉鸡充分换热;鸡舍长度方向上温差大,前后最大温差可达4℃,给整体调控带来难度。在天花板下安装导流板可以明显增加肉鸡生活区气流速度,肉鸡生活区域平均风速提高了29%~61%,有效提高了室外空气利用率和鸡舍换热效率,使前后温差控制在2℃以内。研究表明天花板下导流板的合理使用可以有效改善鸡舍气流组织,使鸡舍内温度保持良好的均匀性。

中国家庭能源转型的环境—健康—低碳效益及经济成本

家庭能源绿色低碳转型是一个关系到中国数亿居民福祉且待妥善解决的问题。基于中国健康与养老追踪调查数据研究发现,使用天然气或电力替代散煤等固体燃料能显著降低暴露人群慢性阻塞性肺疾病、心脏病和中风等疾病的患病概率。在此基础上,以陕西省家庭能源转型为例,利用患病概率估计值测算居民健康改善带来的环境效益,同时考虑温室气体减少的低碳协同效益以及转型的经济成本,全面评估三种家庭能源清洁转型方式的净效益。结果表明,基于陕西省的电力结构和能源价格等现状,三种转型方式的净效益均为负,不同转型方式的负效益程度具有明显差别。为了加快家庭部门实现清洁能源转型,短期内需要政府通过提供补贴等方式引导支持居民采用高效的用能设备,同时加快天然气市场完善和电力部门清洁低碳化转型。

气冷冷凝相变脱白法系统计算研究

根据气冷冷凝相变脱白系统设计的需要,开发了排放气体温度、湿度的计算程序,在此基础上研究了冷空气温度、湿度和风量对脱白效果的影响,结果显示:气冷冷凝相变脱白法除雾存在临界风量线,这条线由冷空气温度、湿度和风量共同决定;冷凝相变脱白法比混风脱白法和混合冷凝脱白法效果好。

城市轨道交通车站空调冷冻水系统定温差变供水温度和大温差控制策略转换条件研究

[目的]为实现城市轨道交通车站空调冷冻水系统在定温差变供水温度和大温差两种工况下的整体能耗最低,需对这两种控制策略的转换条件进行研究。[方法]以郑州某城市轨道交通车站为例,基于TRNSYS(瞬时系统模拟程序)建立定温差变供水温度和大温差两种工况下的空调冷冻水系统模型,在满足空调冷冻水系统末端制冷量需求的前提下,分别研究冷冻水温差不变、供水温度为7~12℃,以及供水温度不变、供回水温差为5~10℃时对空调冷冻水系统能耗的影响。[结果及结论]两种控制策略的转换条件为:负荷率小于等于40%时,使用定温差变供水温度控制策略更节能;负荷率大于50%时,更适合采用大温差控制策略。该转换条件不仅能满足车站人员舒适度的要求,还实现了两种控制策略联合调控的目标,最终达到降低空调冷冻水系统整体能耗的目的。

载镍铁双金属微纳米纤维的构筑及其氧催化性能

开发高效的非贵金属氧还原/氧析出(oxygen reduction/evolution reaction,ORR/OER)双功能电催化剂对发展锌空气电池(zinc air batteries,ZABs)至关重要。文章以载有镍(Ni)、铁(Fe)金属的有机框架化合物为母体,利用静电纺丝技术制备了多级结构微纳米纤维,后经高温碳化,获得载有Ni、Fe双金属的微纳米碳纤维催化剂。探究了金属类型及负载方式对催化剂的ORR和OER催化性能的影响,结果表明:Ni和Fe之间的协同作用能够有效增强Ni基催化剂的OER催化活性,但Ni的引入会抑制Fe基催化剂的ORR催化活性。通过分步负载法使得Ni和Fe金属原子分离锚定时制备得到的纤维催化剂(Ni@PCF-Fe),具有较好的ORR/OER双功能催化性能:ORR半波电位为0.793 V(vs.RHE),OER在10 mA/cm^(2)电流密度时的电位为1.590 V。以Ni@PCF-Fe与氧化钌(RuO2)为正极催化剂组装的ZABs具有较好的电化学储能性能:开路电压为1.43 V,功率密度为108.48 mW/cm^(2)。

轨道交通车辆空气弹簧智能监测系统

针对轨道交通车辆空气弹簧在劣化过程中本体高度变化难以检测,压力测量精度不高,缺乏智能预警功能等问题,设计了轨道交通车辆空气弹簧智能监测系统。对这一监测系统的结构、硬件、软件、工作过程进行了介绍,并分析了技术创新点。这一监测系统应用智能传感技术、数据转储技术、智能预警技术,可以精确测量空气弹簧内部间隙变化、内压值变化,实现智能预警提示检修,进而能够提高检修效率,节约检修成本。

基于智能巡检机器人系统平台的列车空气泄漏超声波检测装置设计

民众日益增长的交通出行需求,直接推动了列车及其营运线路的开通与增长,也给列车日检工作带来了不断增大的压力。作为保障列车制动安全的重要环节,空气制动管路的密封性检测在列车日检作业中不可或缺。为满足实际需求,弥补当前智能巡检机器人巡检功能短缺,文章以双臂机器人为平台,基于泄漏小孔产生湍流、湍流产生超声波的原理,采用“一板两头”结构型式,设计开发了一套空气泄漏超声波检测装置。从信号采集、信号放大和滤波等预处理功能层面对装置结构和硬件配置展开设计,并基于FFT与Parseval定理设计了气体泄漏检测判断算法。通过模拟检测不同孔径(0.125 mm,0.200 mm,0.400 mm)的小孔在不同工况下的泄漏状况,验证了试验样机在0.3s采集时间内能满足接收与正确判断50°锥角、600mm范围内气体泄漏信号的功能需求。