Film boiling of discrete droplets of mixtures of coal and water on a horizontal brass surface

The effect of coal on the evaporation of water from mixtures of coal and water was studied in a Leidenfrost-type experiment. Pulverized bituminuous coal was mixed with water in volume concentrations of 14% and 26%, with corresponding total droplet volumes of 0.052 cm³ and 0.055 cm³, respectively. Evaporation times were recorded by videotape over a range of plate temperatures from just above the Leidenfrost point of water to 670°K. The test surface used in all the experiments was brass, and the pressure was kept constant at atmospheric.Three stages in the evaporation lifetime of the coal/water droplets were observed: (1) evaporation of levitated sessile shaped drops; (2) a regime in wResults showed that evaporation times of water were lower in the presence of coal than for a pure water droplet containing the same volume of water as     

Explosive boiling of liquid droplets at their superheat limits

The maximum temperature limit at which liquid boils explosively is called the superheat limit of liquids. Using the droplet explosion technique, the superheat limit of several hydrocarbons and their mixtures was measured. The dynamic behavior of the bubble formed from the fully evaporated droplet at its superheat limit was investigated by measuring the far-field pressure signal from the bubble and by visualizing its behavior. The evaporation process of a droplet and the evolution of the bubble from the evaporated droplet were also studied theoretically by using an appropriate bubble nucleation and dynamics model. It was found that the superheat limit of liquids could be accurately predicted when correct vapor-liquid equilibrium data for the liquids were provided. The calculated far-field pressure signals, which indicate thermal damping, are in good agreement with observed one until the bubble disintegrates.     

Screening action and growth kinetics of electrodeposited mercury droplets

Electrolytically growing new phase centres exert a certain screening action in their near vicinity manifested in the formation of spatial zones around them where the nucleation process is practically arrested. This phenomenon, resembling similar effects in non-electrical crystallization processes, is due to the redistribution of the ohmic drop within the solution during the electrodeposition process. A triple-pulse technique is developed to visualize the screening action of single mercury droplets deposited on to platinum electrodes from Hg₂(NO₃)₂ solution and to measure the screening zone radii as a function of the growth parameters. Detailed measurements pertaining to the growth kinetics of mercury are presented showing that the electrolytic growth under the experimental conditions employed is governed by the ohmic resistance of the solution. A quantitative treatment of the experimental data is outlined based on an expression for the screening zone radius derived. The significance of the results obtained for the kinetics of the overall electrodeposition process prior to the formation of continuous layers is discussed.     

微柱表面液滴定壁温沸腾实验研究

利用高速摄像技术对去离子水液滴撞击微柱结构表面后的蒸发及核化过程进行观测。实验测得不同壁面温度下液滴蒸干时间,获得液滴沸腾曲线;发现相对光滑表面,微柱表面在50、60、70、80、120℃强化相变换热,120℃时强化比例最大,达到35.71%;壁温为90、100、110℃时,微柱表面无强化作用。从液滴直径和厚度的变化可知微柱表面液滴蒸发分为两个阶段：第一阶段,液滴直径不变,厚度变化;第二阶段,液滴厚度接近微柱高度,直径减小。随壁温升高,第一阶段时长显著缩短。液滴内部核化点密度和气泡平均直径随壁面温度的升高均有明显增大的趋势。需指出的是,液滴冲击对微柱表面液滴内部核化点分布有重要影响,受微柱结构及滴落冲击作用液滴内部成核气泡沿液滴半径呈辐射状分布。     

Enhanced boiling of mixtures

The degradation in the mixture pool boiling heat transfer coefficient is demonstrated to be much different for an enhanced boiling tube than for an ordinary smooth tube operating under identical conditions, usually being substantially less pronounced. This phenomenon is explored in the present study for the High Flux tube with its thin, porous metallic enhancement layer. The reduced degradation in the enhanced boiling heat transfer coefficient is shown to be the result of a liquid Prandtl number augmentation of the liquid-phase convection process inside the porous matrix, which partially counterbalances the negative effect of mass diffusion on the thin film evaporation process in enhanced boiling. It is thus concluded that the existing mixture boiling correlations for conventional smooth tubes, which only include the mass transfer effect on evaporation, will not be able to correlate the mixture effect for enhanced surfaces.     

Interdroplet heterogeneous nucleation of supercooled liquid droplets by solid droplets in oil-in-water emulsions

The crystallization of oil droplets inn-hexadecane oil-inwater emulsions was monitored by pulsed nuclear magnetic resonance (NMR). The emulsions initially contained an equal mixture of solid droplets and supercooled liquid droplets at either 6 or 8°C. The degree of crystallization in the droplets was determined by measuring the NMR signal 30 μs after a 90^o radio frequency pulse was applied to the sample. The signal from the solid droplets decayed rapidly after the radio frequency pulse, allowing the measured signal to be related to the fraction of liquid droplets. No crystallization was observed in a sample that contained only supercooled liquid droplets, but crystallization was observed when solid droplets of the same material were present. This indicated that crystallization was induced in the liquid droplets by the solid droplets and was most likely caused by interactions between solid and liquid droplets-that is, by interdroplet heterogeneous nucleation. The rate of induced crystallization decreased as the viscosity of the continuous phase was increased and the size of the droplets was increased, but was independent of droplet concentration (20–40%).     

Subcooled boiling oscillations in natural circulation boiling loop at low pressure

Flow instabilities in a natural circulation boiling loop at a low pressure are reported. The oscillations at boiling incipience are primarily chaotic and bifurcate to quasiperiodic ones depending on inlet subcooling ΔT_sub and heater power Q. They also strongly depend on water volume Φ in the loop. We have presented power spectrums, attractor reconstructions, and Hurst exponents for the analysis of the experimental data. The analysis shows that the primary oscillations are very similar to geysering instability. Chaotic oscillations occur at low ΔT_sub or high Q, whereas quasiperiodic oscillations occur at high ΔT_sub or low Q. Our experiments also suggest that wall superheat exceeding a critical value triggers the instability. © 2013 American Institute of Chemical Engineers AIChE J, 60: 375–386, 2014     

Coagulative stability of miniemulsion droplets

Miniemulsion droplets were stabilized with a mixed emulsifier system consisting of an anionic surfactant such as sodium dodecyl sulfate and a costabilizer, which is typically a low-water-soluble, long-chain alkane or alcohol such as hexadecane or hexadecanol. The surfactant, anionic in nature, provided an electrostatic barrier against coagulation, and the costabilizer retarded molecular diffusion of the monomer or Ostwald ripening. The coagulation mechanism operative for the stabilization of the miniemulsion droplets is theoretically considered here. The understanding provides insight into the role of the surfactant. The fate of the miniemulsion droplets during the course of polymerization is also discussed. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Combustion of coal-water slurry droplets

The combustion of single droplets of coal-water slurries was studied experimentally by determining the instantaneous size and mass of the droplets and theoretically modelled for the char agglomerate by allowing for the presence of the ash layer as well as the residual volatiles. Comparison of the theoretical results with and without finite-rate kinetics substantiates the hypothesis that the combustion is basically diffusion-limited.     

纳米流体沸腾核化的热力学探讨

基于经典的核化理论,将纳米流体看成是一个均匀的体系,不考虑颗粒的团聚及其在壁面处的富集和沉积,仅考虑体相纳米颗粒的影响。将纳米颗粒类比为大分子溶质,基液类比为溶剂,则纳米流体便可看作是浓度极稀的溶液,借用溶液热力学的相关理论,并假设气液界面处的颗粒是单层嵌入的方式且排布均匀,分别建立了纳米流体的均相和非均相核化模型,分析探讨颗粒对核化的影响。结果表明,纳米颗粒在界面处的吸附可减小核化的平衡胚泡半径,降低沸腾核化势垒,有利于核化的发生。     

蒸发焓及沸点估算的基团拓扑空间方法

基于传统基团划分方法提出了描述有机化合物分子拓扑结构的新方法,用基团模指数表征基团在分子中的位置,建立了有机化合物物性估算的基团拓扑空间方法.提出了估算正常沸点下蒸发焓ΔH_vb和沸点T_b的表达式,并分别用503种和669种有机化合物拟合出了相应的基团参数.蒸发焓ΔH_vb和沸点T_b估算的平均相对偏差分别为0.962%和1.40%.与广泛应用的其他方法进行比较,结果表明本方法具有更高的准确度和可靠性.     

宽馏分重整汽油及直馏汽油芳烃快速分析

建立了对宽馏分的重整原料油及生成油芳烃的色谱分析方法,采用OV—275为固定液的色谱柱,试验表明该方法简捷,准确度和重现性良好,适用于汽油质量监控快速分析。     

Flow boiling and pool boiling critical heat flux in water and ethylene glycol mixtures

Flow boiling and saturated pool trailing were studied in water, and in ethylene glycol and aqueous antifreeze solutions. Boiling took place from an electrically heated cast iron surface at pressures close to atmospheric. In the flow boiling tests, velocities and subcoolings were varied up to 3 m/s and 36°C respectively, and effects of changes in flow passage geometry were examined. In 50 weight percent antifreeze mixtures the pool boiling critical heat flux increased relative to pure ethylene glycol in aqueous solution. Both critical heat flux and heat transfer coefficients decreased as percent water in the mixtures tested was reduced.     

Interactions between fine combustion droplets

Interactions between fine combustion droplets were directly observed using microscopic flow visualization and high speed photography. The observations revealed “attracting-revolving-repulsing” interactions between the droplets. Force analyses showed that the traditionally considered interparticle forces, including drag, gravitation, the Coulomb force and the van der Waals force, cannot explain these interactions. However, the induced dipoles on the droplets due to the non-uniform distribution of surface charges on the fine droplets have important influence on such interactions. Therefore, the inter-dipole forces must be taken into account in the interaction force analysis as well as the Coulomb force between the net charges. The inter-dipole force includes components in the radial and azimuthal directions and is inversely proportional to r⁴. This force causes the particles to revolve and repel each other at small distance. The combined effects of the inter-dipole force and the traditionally considered forces give a complete explanation for the particle interactions.     

Spectral properties of diesel fuel droplets

Absorption spectra of several types of diesel fuel are studied experimentally. Index of refraction of these fuels is calculated using subtractive Kramers-Krönig analysis. The ageing process of fuels is simulated by prolonged boiling. Radiative properties of diesel fuel droplets are calculated using the Mie theory and a simplified approach, based on approximations of absorption and scattering efficiency factors. It is pointed out that the accuracy of the simplified approach is sufficient for practical applications in the visible and infrared ranges, for various types of diesel fuel, and for droplet radii in the range from 5 to 50 μm. The monodisperse approximation is shown to be applicable for the analysis of infrared radiative properties of realistic polydisperse diesel fuel sprays.