Size effects in physicochemical transformations in aerosol systems with nanoparticles

The influence of the size effects on the physicochemical processes occurring in aerosol systems with nanoscale particles is studied theoretically. In particular, such processes as homogeneous nucleation, coalescence of nanoparticles, trapping of impurity atoms by nanoparticles are considered taking into account the size effects.     

Thermoelastic damping of graphene nanobeams by considering the size effects of nanostructure and heat conduction

Thermoelastic damping of nanobeams by considering the size effects of nanostructure and heat conduction is studied herein. The size effect of nanostructure is investigated based on Euler–Bernoulli beam assumptions in the framework of nonlocal strain gradient elasticity, and the size dependence of heat conduction is taken into account by incorporating phase-lagging and nonlocal effects. Closed-form solutions of thermoelastic damping and quality factor characterized by thermoelastic coupling are derived. Graphene nanoribbon is chosen as a nanobeam. The effects of relaxation time, aspect ratio, elastic modulus, thermal expansion, and thermal conductivity on quality factor of graphene nanobeams are discussed in detail.     

Geometrical effects on ionic diffusion in carbon-carbon symmetric supercapacitors

There is a great need to understand the structural dependence of supercapacitors for the development of advanced electrode materials that can be used to improve the electrochemical performance and structural integrity. In this work, we investigate the effects of the size and pore width/size of activated carbon microspheres (ACMSs) with diameter in a range of 2.7 ± 0.54 to 9.45 ± 1.81 μm and average pore width in a range of 1.60 to 2.0 nm on the electrochemical impedance characteristics of carbon-carbon symmetrical supercapacitors. Both the size and pore width/size of the ACMSs have negligible effects on contact resistance. The activation energy for the migration/diffusion of electrolyte ions in the supercapacitors made from the ACMSs of nearly same porous structure increases nonlinearly with the increase of the average size of the ACMSs, which is likely associated with overscreening effect. The activation energy for the migration/diffusion of electrolyte ions in the supercapacitors made from the ACMSs of nearly same size increases first and then decreases as the pore width/size increases.     

尺寸效应和表面效应对纳米颗粒比热容的影响

从弹性介质假设出发，考虑内部和表面原子对比热容的贡献不同，建立起纳米颗粒比热容的理论模型。分析了尺寸效应、温度和表面原子振动软化对纳米颗粒比热容的影响，提出比热容与尺寸和温度之间的关系；以氧化铜颗粒为计算对象，具体计算考虑表面和尺寸效应的比热容，结果与实验数据符合较好。     

Numerical study of methane and air combustion inside a small tube with an axial temperature gradient at the wall

A numerical study on CH₄ and air premixed combustion inside a small tube with a temperature gradient at the wall was undertaken to investigate the effects of inlet velocity, equivalence ratio and combustor size on combustion characteristics. The simulation results show that the inlet velocity has a significant influence on the reaction zone, and the flame front shifts downstream as the inlet velocity increases. The results also show that, the inlet velocity has no obvious effects on the flame temperature. The highest flame temperature is obtained if the equivalence ratio is set to 1. It is disclosed that the combustor size strongly influences the combustion characteristics. The smaller the combustor size is, the more difficult it is to maintain the steady combustion. The smallest combustor size that the stable flame can be sustained is determined mainly by the wall temperature of the micro-combustor under the given conditions. The higher the wall temperature is, the smaller the smallest combustor size. Therefore increasing wall temperature is an effective way to realize flame stabilization for a given combustor size.     

住宅室内发热状况调查

根据北京市98户居民住宅室内发热量的问卷调查数据,采用统计分析的方法,研究现有住宅的室内发热状况,并分析住户建筑面积、建筑户型、家庭人员数、家庭结构、收入等级等各个因素与住宅室内发热状况之间的相互关系。研究结果表明住户家庭人员数和收入等级是影响住宅室内发热状况的关键因素,而住宅室内发热量主要在250W～450W的范围内变化。     

基于CFD正交实验的引射雾化喷嘴仿真研究

气水比对喷雾冷却换热有重要影响,为研究不同结构和工况对引射雾化喷嘴喷雾气水比的影响,对实验中用到的环流引射雾化喷嘴,采用流体力学仿真软件FLUENT建立了环流气体辅助雾化喷嘴模型,对该喷嘴的流场进行了数值模拟。分析模拟气体压力、喷嘴的出水口伸出长度、气体环缝大小等参数对引射喷雾量的影响。结果表明:气体环缝大小对气水比影响最显著;进水管出口端的负压值随伸出长度先增加再减小,水孔伸出长度存在一最佳位置L=0.10mm;工作气体消耗量主要受气体环缝大小影响,其次是工作压力大小,水口伸出长度影响最小。     

蜂窝汽封流场及泄漏特性的数值模拟研究

建立了蜂窝汽封三维流场数值模型,系统地分析了蜂窝孔径尺寸、蜂窝高度、入口压力等结构与运行参数对汽封流场及泄漏特性的影响.计算结果与分析表明,在运行参数不变的情况下,降低蜂窝孔径大小或增加蜂窝高度都有利于降低汽封泄漏量,提高密封效果.而在结构参数不变的情况下,汽封泄漏量随入口压力增大而增加,而随旋转速度增大出现减小的趋势.     

Effects of raw material particle size on the briquetting process of rice straw

Biomass feedstocks need to be milled or chopped into particles before briquetting, and the particle size has great effects on the energy consumption and product quality. In this study, the effects of the particle size on the rice straw briquetting process were investigated. The raw materials were milled or chopped into four different sized test materials. Experiments were carried out with an electronic universal testing machine and a self-designed single pellet unit on the basis of a simplex-centroid design. Several parameters, including briquetting time, energy consumption, maximum extrusion force, product compressive strength, and product density, were tested and recorded. The experimental data were processed by the methods of regression analysis and variance analysis. Finally, effects of raw material particle size on the briquetting process, energy consumption, maximum extrusion force, product compressive strength, and product density were obtained. Results showed that, compared with simple sized materials, mixed materials achieved lower energy consumption, higher product compressive strength, and higher product density.     

乙醇/柴油混合燃料喷雾粒度分布特性研究

采用激光粒度分析仪对乙醇/柴油混合燃料稳态自由喷雾粒度分布进行了试验研究,并就不同喷油压力、喷孔直径对乙醇、柴油及乙醇/柴油混合燃料稳态自由喷雾粒度分布的影响进行了对比研究。试验结果表明:各种燃料喷雾索特平均直径(SMD)的空间分布沿喷雾轴线均呈逐渐减小的趋势,其中柴油喷雾的SMD在20～40μm范围内变化,乙醇喷雾较柴油喷雾具有更小的SMD,且其空间分布较为均匀;随着乙醇含量的增加,乙醇/柴油混合燃料喷雾的SMD不断减小,其SMD大小和空间分布均匀性介于柴油喷雾和乙醇喷雾之间。     

Thermal performance investigation of a single medium temperature phase change microcapsule used for wind power absorption and heat storage system

Phase change microcapsules have a wide application in the heat storage system. The medium temperature heat storage systems such as medium temperature solar thermal plants, waste heat recovery systems and wind power absorption systems. In order to analyse the effects of configuration parameters and materials on phase change heat transfer process in a single medium temperature microcapsule, an enthalpy-transforming model was applied to trace the location of the solid-liquid interface and obtain the liquid fraction at different time in the melting process. Based on this model, the effects of particle size, the effects of wall thickness, the effects of wall materials and different medium temperature phase change materials were analyzed. The numerical results show that the larger particle size has a longer melting time, the melting time of 50 μm particle size and 250 μm particle size is 0.036 s and 2.48 s, respectively. In addition, the melting time of microcapsules with different wall thicknesses from the 1μm to 9μm is the same i.e., 0.14 s. Therefore, the wall thickness has little effect on the melting time of microcapsules. Besides, the microcapsule with the erythritol as inner material and the polystyrene as wall material has the longest melting time. Furthermore, the thermal conductivity of the wall materials is the main factor affecting the melting time. Moreover, the product of latent heat and density of phase change material is the main factor of the melting time.     

SIZE EFFECTS ON THE THERMAL CONDUCTIVITY OF THIN METALLIC WIRES: MICROSCALE IMPLICATIONS

This study examines the influence of radial thickness on the thermal conductivity of thin metallic wires. While size effects on the electrical conductivity of thin wires have been discussed in the literature, research into size effects on thermal conductivity still requires investigation. At such small length scales, the assumption that the reduced electrical conductivity can be simply related to the reduced thermal conductivity through a Wiedemann-Franz relation is subject to question. This study uses the Boltzmann transport equation for electrons to determine the thermal conductivity of a thin wire directly. Electrons are treated as the primary heat carriers in a thin wire with a thermal gradient along the axis. A single-crystal, defect-free, metallic thin wire is considered in the derivation. An expression is presented which accounts for the radial size effects on axial thermal conductivity. The derived thermal conductivity is compared to expressions for the reduced electrical conductivity, and the applicability of the Wiedemann-Franz relation is discussed.     

Effects of the particle size and temperature on the efficiency of nanofluids using molecular dynamic simulation

ABSTRACT

Nanofluids are conventional heat transfer fluids with suspended nanoparticles to enhance their thermal conductivity. However, enhancement of thermal conductivity is coupled with increased viscosity. This study investigates the efficiency of nanofluids (ratio of thermal conductivity and viscosity enhancement) with the effects of particle size and temperature using molecular dynamic (MD) simulation. The efficiency of nanofluids is improved by increasing particle size and temperature. The thermal conductivity enhancement increases with increasing particle size, but is independent of temperature; the viscosity enhancement decreases with increasing particle size and temperature. Particle size variation is therefore shown to be more effective than temperature control.     

粒度级配、添加剂种类对宁东煤成浆性影响的实验研究

总结了宁东煤的特点,认为期内水质量分数高,难以制备高质量分数的水煤浆。通过实验,研究了添加剂种类、煤粉级配对低变质宁东煤成浆性的影响。结果表明,采用双峰级配可配制高性能的水煤浆。     

Effects of particle sizes on performances of the horizontally buried-pipe steam generator using waste heat in a bioethanol steam reforming hydrogen production system

The discrete element geometric model of the horizontally buried-pipe steam generator was set up. The effects of particle size (20 mm–80 mm) on performances of the horizontally buried-pipe steam generator using waste heat in a bioethanol steam reforming hydrogen production system was studied. When the particle size increases, the particle layer flatness decreases, the particle layer flow ununiformity increases. The volatility of the particle residence time distribution increases with the particle size increases, and the standard deviation of the particle residence time increases. When the particle size increases, the voidage of the particle system increases. So the particle thermal resistance in the steam generator increases with the particle size increases, the steam production of the generator decreases, and the system hydrogen production of decreases.     

大型水利工程与生态平衡

兴修大型水利工程将会打破原有的生态平衡 ,但对生态平衡的影响效应直接取决于人类对保护生态平衡的重视程度及一个国家现阶段的经济发展水平。大型水利工程的修建 ,对生态平衡的良性循环有着巨大的副作用 ,但对不良生态平衡 ,却可以使之朝着有利于人类健康的方面发展。因此在兴建大型水利工程的前前后后 ,要将环保意识贯穿于工程规划设计及施工组织管理的每一环节     

F级联合循环冷端系统优化方案研究

以某F级燃机联合循环为例,研究凝汽器端差、凝汽器循环水温升、冷却塔逼近度、凝汽器面积、冷却塔面积、冷却倍率等因素对经济性的影响。     

Latest developments on the viscosity of nanofluids

The past decade has seen the rapid development of nanofluids science in many aspects. Number of research is conducted that is mostly focused on the thermal conductivity of these fluids. However, nanofluid viscosity also deserves the same attention as thermal conductivity. In this paper, different characteristics of viscosity of nanofluids including nanofluid preparation methods, temperature, particle size and shape, and volume fraction effects are thoroughly compiled and reviewed. Furthermore, a precise review on theoretical models/correlations of conventional models related to nanofluid viscosity is presented. The existing experimental results about the nanofluids viscosity show clearly that viscosity augmented accordingly with an increase of volume concentration and decreased with the temperature rise. However, there are some contradictory results on the effects of temperature on viscosity. Moreover, it is shown that particle size has some noteworthy effects over viscosity of nanofluids.     

Effects of grain size and dislocation density on the susceptibility to high-pressure hydrogen environment embrittlement of high-strength low-alloy steels

The tensile properties of several high-strength low-alloy steels in a 45 MPa hydrogen atmosphere at ambient temperature were examined with respect to the effects of grain size and dislocation density on hydrogen environment embrittlement. Grain size was measured using an optical microscope and dislocation density was determined by X-ray diffractometry. Both grain refinement and a reduction in dislocation density are effective in reducing the susceptibility to embrittlement. The steel that has high dislocation density or large grain size inclines to show a smooth intergranular fracture surface. Given only the grain size and dislocation density, a simple approximation of the embrittlement property of high-strength steel could be obtained. This method could be useful in selecting candidate materials in advance of the mechanical tests in high-pressure hydrogen gas.     

Diameter-controlled carbon nanotubes and hydrogen production

Simultaneous production of hydrogen and carbon nanomaterials over Ni-loaded ZSM-5 catalysts via catalytic decomposition of methane was investigated. The effects of nickel particle size and reaction temperature on the hydrogen production, catalyst deactivation and the morphologies of the carbon nanotubes were investigated. Two catalyst were prepared: Ni/ZSM-5(300) – predominant size of the Ni particles 30–60 nm and nNi/ZSM-5(300) predominant size of the Ni particles 10–20 nm.