长光程共振式二氧化碳气体光声传感器研究

二氧化碳(CO₂)是植物光合作用的原材料,也是一种温室气体,其过量地排放会影响动植物的生态环境。在碳达峰、碳中和的背景下,研制高灵敏度的CO₂检测装置具有重要意义。为了监测大气环境中CO₂含量的变化,设计了一种长光程共振式CO₂气体光声传感器,并以此搭建了光声检测装置。以中心波长为2 004 nm的分布式反馈激光器(DFB)作为激发光源,激光射入由漫反射材料制成的球型吸收腔,在腔内多次反射以增加气体的吸收路径。吸收腔外部被两个高热传导率的铝制半球包裹,降低由池体吸收光能后产生的热噪声。吸收腔上耦合一根声学管,当其工作在一阶纵向共振模态时,光声信号被放大,在管子末端达到极大值。为了进一步增大光声信号,通过饱和加湿样品的方式来加快CO₂气体的弛豫速率,加湿后的样品产生的光声信号是干燥样品的2.1倍左右。使用一系列浓度的湿润CO₂样品标定光声检测装置,结果表明,光声信号与浓度之间呈现良好的线性关系。在此基础上,通过对标准气体的检测实验,验证了装置的准确性与稳定性...     

A functional-group-based approach to modeling real-fuel combustion chemistry – III: Application to biodiesels

Real biodiesel fuels are mixtures comprising many high molecular weight components, making it a challenge to predict their combustion chemistry with detailed kinetic models. Our group previously proposed a functional-group approach (FGMech) to model the combustion chemistry of real gasoline and jet fuels; this approach has now been extended to model real biodiesel combustion and mixtures with petroleum fuels. As in our previous work, a decoupling philosophy is adopted for construction of the model. A lumped reaction mechanism describes the (oxidative) pyrolysis of fuels, while a detailed base chemistry model represents the oxidation of key pyrolysis intermediates. However, due to the presence of the ester group, several oxygenated species are identified as additional primary products and incorporated into the lumped reaction steps. In addition to the lumped reactions initiated by unimolecular decomposition and H-atom abstraction reactions, a lumped H-atom addition-elimination reaction is also incorporated as a new reaction class to account for the presence of double bonds. Stoichiometric parameters are obtained based on a multiple linear regression (MLR) model, which establishes relationships between the fuel's functional group distributions and the stoichiometric parameters of the lumped reactions. Global rate constants are developed from consistent rate rules obtained from pure fuels. New pyrolysis experimental data for methyl pentanoate/methyl nonanoate and methyl heptanoate/n-heptane mixtures (50%/50% in mol) are obtained in a jet-stirred reactor at atmospheric pressure. In general, kinetic models developed using the FGMech approach can reasonably reproduce all the validation targets obtained in this work, as well as those in the literature, confirming that functional-group-modeling is a promising approach to simulate combustion behavior of diesel/biodiesel surrogate fuels and real biodiesels.     

An analysis of the in-cylinder soots generated from the main- and post-injection combustion in diesel engines

In modern diesel engines, the exhaust soot primarily comes from the main-injection combustion and post-injection combustion. Therefore, to reduce the diesel soot emissions, it is essential to better understand the soots generated from the main-injection combustion (main-soot) and from the post-injection combustion (post-soot). This work focused on the properties of the main-soot and post-soot during the combustion process, including the primary particle size, nanostructure and soot mass. The in-cylinder soot samples were obtained using a self-developed total cylinder sampling system, and the primary particle size and nanostructure were determined using high-resolution transmission electron microscopy. The isolation of the post-soot was achieved by adding dimethyl ether to the intake gas instead of the real main-injection to create a simulated main-injection combustion environment for post-soot formation. Combustion analysis and numerical simulation results showed that the simulated combustion environment for post-soot formation generated by the DME combustion was very similar to that generated by the real main-injection combustion. During the combustion process, although the main-soot and post-soot exhibit similar variations in the primary particle size, the maximum primary particle size of the post-soot is smaller than that of the main-soot (23.38 nm for the main-soot and 20.51 nm for the post-soot). The main-soot and post-soot show almost the same trends in the nanostructure, as characterized by the fringe length, separation and tortuosity, throughout the combustion process. The introduction of the post-injection accelerates the reduction of the primary particle size of the main-soot and the increase in the structural order of the main-soot. Because a large number of the main-soot particles are oxidized during the post-injection combustion, the post-soot accounts for a considerable proportion in the engine-out soot (i.e., 42%).     

热稳定单原子催化剂的理性构筑:从原子级结构到实际应用

80%以上的工业生产过程涉及催化,如化工生产、能源转换、制药和废物处理等等.催化剂的使用显著提高了生产效率,降低了生产成本,为国民经济、地球环境和人类文明的可持续发展做出了很大贡献.为了满足日益增长的生产需求和最大的经济效益,开发高效、稳定、低成本的新型催化剂已成为当务之急.金属中心负载在载体上的负载型金属催化剂因其较好的催化活性和相对较低的金属用量而受到广泛关注.研究发现,负载型结构可增强传热和传质并增加活性金属中心的分散度,从而影响催化性能.此外,负载金属的颗粒尺寸对催化剂的性能有很大影响.迄今为止,科学家们一直在通过减小金属颗粒尺寸和提高原子利用效率来提高催化剂的活性.原子级尺寸的颗粒通常表现出与大尺寸颗粒显着不同的物理和化学性质,而当活性位点的尺寸缩小到单个原子时,单原子催化剂的概念应运而生.对于单原子催化剂,金属原子中心通过配位被载体中的缺陷锚定,从而调整金属原子的电子云分布.这种配位调整使得单原子催化剂拥有与传统催化剂不同的性能.作为催化领域的新前沿,单原子催化剂已经在许多催化反应中表现出前所未有的活性和选择性.然而,许多报道的单原子催化剂在高温环境或长期催化应用中容易受到奥斯特瓦尔德熟化过程的影响,从而导致催化剂烧结和失活.而烧结的原因在于金属原子和载体之间较弱的相互作用.失活催化剂的再生和回收将大大增加工业生产的时间和经济成本.因此,开发具有优异热稳定性的单原子催化剂以满足工业需求是十分必要的.本综述首先总结了近年来关于热稳定型单原子催化剂合成方法的基础研究,并从原子尺度上分析了这些方法所构建的金属中心的结构形态和配位环境.此外,结合近些年的研究中新的表征技术与理论计算手段解释了热稳定性的来源.重点讨论了热稳定单原子催化剂的实际催化应用.分析了热稳定单原子催化剂在热催化应用中的独特作用机理、并尝试为确定催化过程中真正的活性中心以及通过原子级调控手段进行高活性热稳定单原子催化剂的合成提供理论指导.最后总结了热稳定单原子催化剂发展的主要问题,并简要分析了单原子催化领域的研究挑战和发展前景.     

CoMn@NC催化5-羟甲基糠醛常压氧化为2, 5-呋喃二甲酸二甲酯

2, 5-呋喃二甲酸二甲酯(DMFDCA)这一生物质衍生的增值化学品是石油基聚合物单体对苯二甲酸(TPA)的理想替代品。本研究采用一步共热解法合成了两种廉价金属修饰的氮掺杂多孔碳催化剂CoMn@NC,并将其用于5-羟甲基糠醛(HMF)在温和条件下的需氧氧化。由Co₃Mn₂@NC-800催化HMF在50 ℃和常压氧气的条件下反应12 h后,得到产率为85%的DMFDCA。多孔催化剂的高比表面积提高了传质效率。Co纳米粒子(NPs)和呈原子级分散的Mn与掺杂在碳中的氮配位形成M―N_x。富含吡啶氮的碳基体中的缺电子金属位点有利于HMF和氧的活化。氧形成的超氧自由基阴离子的存在确保了半缩醛中间体和5-(羟基甲基)-2-糠酸甲酯(HMMF)的羟甲基的脱氢氧化,从而高选择性得到DMFDCA。该催化剂性能稳定,可适用于各种取代芳醇。该催化体系具有用于生产聚合物单体羧基酯的应用潜力。     

PVDF应力测量技术及在混凝土冲击实验中的应用

介绍了使用PVDF(polyvinylidene fluoride,聚偏二氟乙烯)应力计的应力直接测量技术。在SHPB装置上进行自制PVDF应力计动态压电系数的标定实验,分析了应力集中、横向泊松效应、摩擦效应对PVDF应力计信号及动态压电系数的影响。用PVDF应力计进行了混凝土的冲击压缩实验。利用混凝土前后端面PVDF应力计信号分析了实验过程中混凝土试样的应力均匀性。与应变计直测应变技术相结合得到了混凝土的动态应力-应变关系。     

Numerical study of buoyancy- and thermocapillary-driven flows in a cavity

Thermocapillary- and buoyancy-driven convection in open cavities with differentially heated endwalls is investigated by numerical solutions of the two-dimensional Navier-Stokes equations coupled with the energy equation. We studied the thermocapillary and buoyancy convection in the cavities, filled with low-Prandtl-number fluids, with two aspect-ratiosA=1 and 4, Grashof number up to 10⁵ and Reynolds number ⋎Re⋎≤10⁴. Our results show that thermocapillary can have a quite significant effect on the stability of a primarily buoyancy-driven flow, as well as on the flow structures and dynamic behavior for both additive effect (i.e., positiveRe) and opposing effect (i.e., negativeRe).     

气相爆轰波在90矩形弯管中传播时胞格结构的演化

利用烟迹技术实验获得气相爆轰波在90矩形弯管中传播时留下的胞格图案，详细分析了胞格结构在矩形弯管中的演化规律。同时分析了90矩形弯管的曲率半径和预混气体初始压力对胞格结构演化的影响。实验所采用的预混气体初始压力为33.3、26.7、24.0、16.0、10.7 kPa，90矩形弯管的曲率半径分别为100、50 mm。研究爆轰波胞格结构在弯管中的演化过程有助于深入了解气相爆轰波绕射机理和石油天然气等管道设计。     

Numerical investigation of transient laminar natural convection of air in a tall cavity

Transient laminar natural convection of air in a tall cavity has been studied numerically. The Navier-Stokes and Energy equations were solved by the accurate projection method (PmIII), in which the derived Poisson equation for pressure potential was solved by the approximate factorization one method (AF1). The aspect ratio of the tall cavity is 16, and the Prandtl number of air filled in the tall cavity is 0.71. To obtain the numerical results of heat transfer by natural convection of air in the tall cavity, the second order schemes for the space and time discretizations were utilized. The availability of the numerical algorithm was also assessed by considering the natural convection of air in a square cavity which is differentially heated from side walls. It was found that the overall Nusselt numbers for the Rayleigh numbers covering the range from 1000 to 100000 reveal a good agreement with measured data. When Ra takes the value in the range from 100000 to 600000, the overall Nusselt number have a relative deviation less than 18% from the experimental data. For the suddenly heating mode, the multicellular flow pattern occurs when Rayleigh number belongs to the range of Ra from 7000 to 20000. or greater than 115000. At the critical number of cats' eye instability, the cell distance is just twice of the cavity width. This is rather similar to the observed result for Bénard problem. When Ra is over 115000, a further increase of heat flux across the tall cavity causes serious cell-breaking. There are 8 cells when Ra = 600000.