氧化铝有机纳米流体的流动传热基础特性

以氧化铝为纳米粒子、丙二醇和水为基础液体制备了氧化铝有机纳米流体,分别测量了它的沸点、热导率、比热容和黏度。以1％～5％（体积分数）的氧化铝纳米流体作为冷介质,测试了在车用机油冷却器中的传热系数和流动阻力。试验结果表明,纳米粒子能够显著强化基础液体在机油冷却器中的换热能力,粒子体积分数和流体温度是影响纳米流体热物性的重要因素。氧化铝纳米流体的沸点高于120℃,比热容随体积分数增加而降低,热导率、黏度和在机油冷却器中的传热系数均随粒子体积分数的增加而提高。在试验Ⅱ中,5％（体积分数）纳米流体的平均传热系数比基础液体提高了124.56％,而流动阻力增幅较小。     

用石蜡-CO_2超临界流体快速膨胀在流化床中进行细颗粒包覆

对超临界流体快速膨胀技术在流化床中进行细颗粒的表面包覆进行了研究 ,以实现细颗粒中关键成分的有效控制释放 .实验研究了含有包覆剂———石蜡的超临界二氧化碳流体通过微细喷嘴快速膨胀到装有细颗粒的流化床中 ,膨胀射流中所产生的微核在细颗粒表面均匀沉积 ,形成细颗粒表面薄层包覆 .结果分析表明 ,超临界流体快速膨胀前的温度是包覆过程的关键参数 ,通过控制操作过程参数可以获得良好的包覆结果     

Boiling local heat transfer enhancement in minichannels using nanofluids

This paper reports an experimental study on nanofluid convective boiling heat transfer in parallel rectangular minichannels of 800 μm hydraulic diameter. Experiments are conducted with pure water and silver nanoparticles suspended in water base fluid. Two small volume fractions of silver nanoparticles suspended in water are tested: 0.000237% and 0.000475%. The experimental results show that the local heat transfer coefficient, local heat flux, and local wall temperature are affected by silver nanoparticle concentration in water base fluid. In addition, different correlations established for boiling flow heat transfer in minichannels or macrochannels are evaluated. It is found that the correlation of Kandlikar and Balasubramanian is the closest to the water boiling heat transfer results. The boiling local heat transfer enhancement by adding silver nanoparticles in base fluid is not uniform along the channel flow. Better performances and highest effect of nanoparticle concentration on the heat transfer are obtained at the minichannels entrance.     

A new method to deagglomeration of PbTiO3 ultrafine powders

An innovative method for deagglomeration of ultrafine powders was developed in this paper. The method was based on electrohydraulic forces produced by electric breakdown discharge between two electrodes submerged in liquid. By controlling the process parameters (voltage, electrode separation and reaction time), PbTiO₃ ultrafine powders with spherical, tabular and aciculate shape were successfully deagglomerated. Two ways based on centrifugal sedimentation and laser-light scattering were used to determine the particle size distribution. Scanning electron microscope (SEM) images showed that the agglomeration of PT powders was greatly reduced.     

机械球磨时间对液相烧结Ba2Ti3Nb4O18陶瓷材料结构和性能的影响

Ba2Ti3Nb4O18是BaO-TiO2-Nb2O5体系中一种新型的介质材料,具有优良的微波介电性能.为满足低温共烧陶瓷技术(low temperature cofired ceramics,LTCC)对微波介质陶瓷材料的低温烧结要求,实现在900℃与银电极共烧,添加了质量分数为5%的ZnO-B2O3玻璃作助融剂,并研究了机械球磨时间对粉料粒径、陶瓷样品的烧结密度、显微结构和介电性能的影响.机械球磨6h的粉体粒径适中(约90nm),用该粉料制备的陶瓷样品可以在900℃致密烧结(大于理论密度的95%).且高频介电性能为(1MHz下测试):介电常数εt≈36,介电损耗tanδ≈2× 10-4,电容温度系数αc≈2.5×10-6/℃.同时微波介电性能良好:εt=33.3,品质因数和频率的乘积Qf=14274GHz.可与银电极共烧结作为LTCC介质瓷料.     

Enhancement of thermal conductivity of ethylene glycol based silver nanofluids

Nanofluid is a kind of new engineering material consisting of solid particles with size typically of 1-100 nm suspended in base fluids. Nanofluids offer excellent scope of enhancing thermal conductivity of common heat transfer fluids. In the present study, nanofluids are synthesized using silver nitrate (precursor), ethylene glycol (reducing agent), and poly(acrylamide-co-acrylicacid) (dispersion stabilizer). The different concentrations of silver nanofluid (1000-10,000 ppm) were synthesized. The silver particles present in colloidal phase have been characterized by EDX, XRD, UV-visible spectroscopy, Zeta potential and transmission electron microscopy (TEM). The stability as well as thermal conductivity of these nanofluids was determined with a transient hot-wire apparatus, as a lapse of time after preparation. Typically, 10,000 ppm silver nanofluid exhibited rapid increase in the particle size with the passage of time. Thermal conductivity of silver nanofluids increased to 10, 16, and 18% as the amount of silver particles in nanofluid were 1000, 5000, and 10,000 ppm, respectively. After 30 days of preparation, the thermal conductivity of 1000 and 5000 ppm silver nanofluids decreased slightly from 10% and 15% to 9% and 14%, respectively. In addition, the thermal conductivity of 10,000 ppm nanofluid was decreased from 18% to 14% after 30 days. It is very interesting to report that the silver particles were aggregated in early stage of preparation (up to 15 days), which leads to the increase in the size of silver particles. However, no significant change was observed after 15 days which indicates the stability of silver nanofluids.     

Kinetic Investigation of Reaction Between Metallic Silver and Nitric Acid Solutions

In this study, the reaction kinetics between metallic silver and nitric acid solutions was investigated by taking into consideration the parameters of temperature, solid‐to‐liquid ratio, stirring speed, nitric acid concentration, particle size and addition of sodium nitrite. It was determined that the dissolution rate of the process increased with decreasing particle size and solid‐to‐liquid ratio, and increasing nitric acid concentration, reaction temperature and the amount of the sodium nitrite in the solution. In addition, it was observed that the stirring speed had more effect on the dissolution rate at low stirring speeds than at high stirring speeds. In the present study, the examination of shrinking core models of fluid‐solid systems showed that the dissolution of metallic silver in the nitric acid solutions was controlled by the reaction on the surface. A semiempirical model, which represented well the process, was developed by statistical methods. The activation energy of the process was found to be 57.66 kJ mol^–1.     

Wet dielectric separation

We have studied the possibility of using electrical phenomena in order to selectively separate minerals with various densities and dielectric constants in non-homogeneous electric fields. This technique is based on levitation forces acting on a solid particle immersed in dielectric fluid.When the gradient of the electric field is directed downwards, the levitation force in the dielectric fluid (pure liquid or electrical colloids) will be in an upwards direction to float sinking minerals by counterbalancing their density mismatch with the liquid medium.The separation process can be made by using two possible modes: either by direct application of electrical forces or by change in the density and dielectric constant of a liquid medium in which the mineral particles are placed. We have investigated several electrode configurations in order to produce electrical fields which are strongly non-homogeneous. Further investigations on the dielectric properties of liquids have been performed.     

Surface tension of Nanofluid-type fuels containing suspended nanomaterials

ABSTRACT: The surface tension of ethanol and n-decane based nanofluid fuels containing suspended aluminum (Al), aluminum oxide (Al2O3), and boron (B) nanoparticles as well as dispersible multi-wall carbon nanotubes (MWCNTs) were measured using the pendant drop method by solving the Young-Laplace equation. The effects of nanoparticle concentration, size and the presence of a dispersing agent (surfactant) on surface tension were determined. The results show that surface tension increases both with particle concentration (above a critical concentration) and particle size for all cases. This is because the Van der Waals force between particles at the liquid/gas interface increases surface free energy and thus increases surface tension. At low particle concentrations, however, addition of particles has little influence on surface tension because of the large distance between particles. An exception is when a surfactant was used or when (MWCNTs) was involved. For such cases, the surface tension decreases compared to the pure base fluid. The hypothesis is the polymer groups attached to (MWCNTs) and the surfactant layer between a particle and the surround fluid increases the electrostatic force between particles and thus reduce surface energy and surface tension.     

多孔介质内颗粒流动特性的格子Boltzmann模拟

地下岩石孔隙中小颗粒的运移和沉积会使得储层渗透性能降低,影响石油开发。为了探究悬浮颗粒在多孔介质中的流动过程,采用格子Boltzmann方法对三维多孔介质内流体和颗粒的运动过程进行了数值模拟,采用有限体积颗粒法构建多孔介质中骨架颗粒和悬浮颗粒。通过Half-Way反弹格式实现流体与颗粒间的相互作用,考虑孔隙结构、入口流速、孔隙率和颗粒直径对颗粒流动特性的影响,探究颗粒的运移和沉积规律。结果表明,入口速度对不同孔隙结构下颗粒的运动作用显著。随着入口速度增大,颗粒与颗粒、孔隙壁面以及流体之间的动量和能量交换作用增强,缩短了颗粒的运移路径,颗粒沉积率逐渐变小,颗粒拟温度增大。孔隙率的下降强化了颗粒间的碰撞,孔隙率由0.581降低至0.400,使得颗粒拟温度提升至9倍。颗粒拟温度随颗粒直径的增加而增加。但随着孔隙率增加,颗粒轴向速度增加,颗粒最高轴向速度可达入口流速的11倍,而颗粒接触力降低。     

Al2O3纳米流体动态喷射真空制冰特性

配制溶质为Al₂O₃纳米粒子、黄原胶为分散剂,经超声波振荡后分散均匀稳定的Al₂O₃-H₂O纳米流体。利用真空动态喷射制冰实验装置,设定初始压力300 Pa,研究在有无固体吸附作用、不同纳米流体浓度、不同纳米颗粒粒径和不同供液流量下的真空闪蒸制冰特性。结果表明,吸附作用对真空制冰系统稳压效果显著且有助于捕捉实验中的水蒸气;当冷却介质温度低于吸附剂初始温度时,吸附剂有更好的吸附能力但冷却温度不宜过低;纳米流体浓度越大、纳米粒子粒径越小,更利于制取高含冰率的冰浆;对于同一浓度的纳米流体,合理控制供液流量可以获得不同含冰率的冰浆;在2%（质量）NaCl下,各粒径的纳米流体浓度不宜超过0.05%（质量）。     

Solid-liquid separation in suspension atomization

The aim of this paper is to define the conditions controlling the fragmentation process within the atomization of a suspension. Correlations for the droplet diameter of a suspension spray generated by a twin-fluid nozzle have been derived. Two separate regimes in suspension atomization have been identified with respect to the solid particle size. The atomized droplets from suspensions containing relatively fine solid particles are suspension droplets (containing liquid and solid particles). In this case a correlation for the drop size distribution in the spray of a twin-fluid nozzle has been deduced. Droplet size measurements in the suspension spray with varying solid particle sizes showed that when the suspended solid particle size exceeds a critical value, solid particles and liquid will be more and more separated. This effect is indicated by a bimodal size distribution in the suspension spray. It is shown that complete solid-liquid separation in the suspension spray may be achieved, where the pure liquid drops are significantly smaller than the separated solid particles. The critical process conditions where the solid-liquid separation process is found will be derived. Depending on the operating conditions of the atomizer, the resulting pure liquid droplet size is equal or less than the hydraulic diameter.     

Ag纳米流体浸没射流冲击换热特性

采用超声膜扩散法一步制备出水基Ag纳米流体作为实验工质,并对不同质量分数的Ag纳米流体在受限浸没阵列射流冲击针肋热沉中的流动和换热特性进行了实验研究.结果表明:采用超声膜扩散法制备的Ag纳米颗粒粒径分布均匀,平均粒径只有4.8 nm;表面活性剂对纳米流体的黏度影响较大;相同射流速度下,与基液(水+表面活性剂)相比,Ag...     

水力空化混合强化超临界流体辅助雾化制备罗红霉素超细微粒

分析了超临界流体辅助雾化（SAA）过程,发现饱和器内超临界二氧化碳与溶液的混合是SAA成功的关键因素之一,由此引入了水力空化混合器以强化饱和器内两相间的传质。在自行组建的引入水力空化混合器的超临界流体辅助雾化（SAA-HCM）装置上,以罗红霉素为模型药物,考察了混合器压力、沉淀器温度、溶剂、进料中CO₂与液体溶液流量比（R）和溶液浓度对微粒形态和粒径的影响。结果表明,水力空化混合器能有效地强化两相间的传质,SAA-HCM工艺可制备出罗红霉素超细微粒,大部分微粒形态呈球形,通过改变操作参数可制得粒径在1～3 μm的适于吸入式给药的气溶胶药物微粒和粒径小于1 μm的超细微粒。     

Preparation and characterization of carbon nanofluid by a plasma arc nanoparticles synthesis system

Heat dissipation from electrical appliances is a significant issue with contemporary electrical devices. One factor in the improvement of heat dissipation is the heat transfer performance of the working fluid. In this study, we used plasma arc technology to produce a nanofluid of carbon nanoparticles dispersed in distilled water. In a one-step synthesis, carbon was simultaneously heated and vaporized in the chamber, the carbon vapor and particles were then carried to a collector, where cooling furnished the desired carbon/water nanofluid. The particle size and shape were determined using the light-scattering size analyzer, SEM, and TEM. Crystal morphology was examined by XRD. Finally, the characterization include thermal conductivity, viscosity, density and electric conductivity were evaluated by suitable instruments under different temperatures. The thermal conductivity of carbon/water nanofluid increased by about 25% at 50°C compared to distilled water. The experimental results demonstrated excellent thermal conductivity and feasibility for manufacturing of carbon/water nanofluids.     

Aggregate behaviour of liquid fluidised beds

New and long published experimental observations of the onset of aggregate (bubbling) behaviour in liquid fluidised beds are shown to be in agreement with the predictions of a recently published model of the fluidisation process. For a given liquid, the transition from particulate to aggregate fluidisation depends on both the density and size of the suspended particles: the lower the particle density the larger is the size necessary for a transition to occur: thus for fluidisation by ambient water, lead particles larger than 200 μm and soda glass particles larger than 2000 μm will exhibit aggregate behaviour; below a critical particle density no such transition is possible.     

Dielectric properties of silver-doped nanoporous silicate glasses in the temperature range between –50 and +250°C

The results of measuring dielectric parameters of nanoporous silicate glasses with a nanopore size of 3.5 and 25.7 nm, either doped with silver or not, in the temperature range between–50 and +250°C and the frequency range of 0.1–10⁶ Hz are reported. It is demonstrated that when silver nanoparticles are formed in glass pores, some silver remains in the form of subnanosized molecular clusters Ag _n and molecular complexes Ag _n –(OH) _m . The key dielectric properties of the relaxation centers are determined for different temperature ranges.     

Absolute Mass and Size Measurement of Monodisperse Particles Using a Modified Millikan's Method: Part I—Theoretical Framework of the Electro-Gravitational Aerosol Balance

A new method for accurate mass and size measurement of monodisperse particles is proposed. In this method, charged aerosol particles are introduced into parallel plate electrodes similar to the Millikan cell, and the number of particles left suspended after a certainty holding time has elapsed is measured. The particle survival rate as a function of the voltage applied to the electrodes is used to determine the particle mass. The particle size is deduced by using the particle density which is determined in a separate experiment. The expression of the particle survival function, which is defined as the survival rate as a function of the mass, for particles with and without Brownian diffusion is derived. The sensitivity of this method to the number average diameter, as well as other size distribution parameters, is analyzed on the basis of the survival function.     

纳米流体液滴在水平加热面上的变形行为特性

采用格子Boltzmann力矩模型模拟了纳米流体液滴在水平加热面上的变形过程,结果表明纳米流体液滴的变形经历了销联、解销阶段,并且由于纳米颗粒的强销联作用,在蒸发的后期可以明显看出环形沉积,这是在考虑布朗运动、粒子间相互作用等作用力后由于纳米颗粒的强销栓作用而表现出的纳米流体的独有性质。模拟结果与文献中的实验结果作了对比,表现出良好的一致性,表明格子Boltzmann方法可以成功模拟纳米流体液滴在水平加热表面蒸发过程中的变形特点。     

Experimental Study of Spray Drying and Atomization with a Two-Fluid Nozzle to Produce Inhalable Particles

A series of 50 experiments was performed to investigate particle size and yield from three laboratory spray dryers with two-fluid nozzles, using an aqueous solution of egg albumin as the feed. First principles theory suggests that the main parameters affecting final particle size should be atomization gas flowrate and velocity, feed solution flowrate, solution concentration, and inlet and outlet gas temperature. The experiments demonstrated that atomization gas velocity at the nozzle correlated better than volumetric gas flowrate to the final particle size obtained. Increasing atomization gas flow gave a decrease in particle size, levelling off at high nozzle gas velocities approaching sonic values. Liquid flowrate had little effect, and hybrid ratios such as the ALM (atomization gas to liquid flow ratio) did not correlate well to measured final particle size for this system. This seems likely to apply to aqueous systems with temperature limitations, where liquid flowrates will be relatively low compared to gas flowrates (both atomization and main gas flow). Particle size generally increased with solution concentration, and in some cases scaled exactly, assuming a constant initial droplet size and final particle density. All experiments were performed below the boiling point and therefore temperature had little effect. Particles of 2–3 microns were obtained at high atomization gas flows, indicating initial droplet sizes of 4–7 microns at most, while lower flowrates gave substantially larger particles and a wider size distribution.