Droplet Size Distribution and Sphericity Measurements of Low‐Density Sprays Through Image Analysis

An image analysis technique has been developed in order to determine the drop size distributions of sprays produced by low‐velocity plain cylindrical jets. The particle sizing method is based on incoherent backlight images. Each drop is analyzed individually in the image. The two‐dimensional image resulting from the projection of the three‐dimensional object shape (the drop) on a screen (the video sensor surface) is modeled. The model, based on the point spread function formulation, has been developed to derive a relation between contrast and relative width of individual drops. This relation is used to extend the domain of validity of drop size in terms of size range, out of focus and image resolution. The shape parameter is determined for each drop image through morphological analysis. Spherical and non‐spherical droplets are then sorted on the basis of this parameter. Non‐spherical drops are regarded as non‐fully atomized liquid bulks or coalesced drops. Finally, the droplet size distribution of true spherical droplets is established for a low‐velocity plain cylindrical liquid jet.     

Equilibrium shape and stability of a charged drop in an air flow under an electric field

The pressure balance on the surface of a charged liquid drop moving along a uniform electrostatic field is analyzed. The liquid is assumed to be nonviscous and incompressible. In the approximation linear in deformation amplitude, the equilibrium shape of the drop as a function of the charge, field strength, and velocity of travel can be both a prolate and an oblate spheroid. Critical conditions for the surface instability of such a drop are obtained analytically in the form of a relationship between the charge, field strength, and velocity of travel. An instability criterion is found by extrapolating to large Reynolds numbers. This makes it possible to fit the earlier model of a corona-initiated lightning in the vicinity of large charged water drops or hailstones to the charges of the drops, field strengths, and velocities of travel (relative to the medium) typical of thunderclouds.     

Singular jets and bubbles in drop impact

We show that when water droplets gently impact on a hydrophobic surface, the droplet shoots out a violent jet, the velocity of which can be up to 40 times the drop impact speed. As a function of the impact velocity, two different hydrodynamic singularities are found that correspond to the collapse of the air cavity formed by the deformation of the drop at impact. It is the collapse that subsequently leads to the jet formation. We show that the divergence of the jet velocity can be understood using simple scaling arguments. In addition, we find that very large air bubbles can remain trapped in the drops. The surprising occurrence of the bubbles for low-speed impact is connected with the nature of the singularities, and can have important consequences for drop deposition, e.g., in ink-jet printing.     

水平梯度表面能材料表面上的滴状凝结换热系数

本文在均质表面的单个球缺形液滴换热模型和液滴通用尺度分布规律的基础上,结合梯度表面能材料表面的液滴分布和凝结换热特性,得到了一维水平梯度表面能材料表面上的滴状凝结换热计算式。在此基础上,研究了壁面过冷度、接触角梯度、工质物性等参数对梯度表面能材料表面滴状凝结换热性能的影响。结果表明:随着过冷度的增加和凝结工质汽化潜热的增大和表面张力的减小和接触角梯度的增大,平均表面凝结换热系数会增大。     

Heat-transfer coefficient of a fine heated wire in a gas flow

Heat exchange between a linear heat source and a gas flow is studied theoretically. The coefficient of source-to-gas heat transfer is found as a function of the gas velocity in the limit of zero temperature drop. It is shown that the heat-transfer coefficient depends linearly on the gas flow velocity for Peclet numbers 0.1–1.0.     

Off-centre impact on a horizontal fibre

Fibre filters that consist of a network of randomly oriented thin fibres are commonly used to recover the liquid phase from an aerosol. During the filtration process, drops of the aerosol impact the solid fibres of the filter. If the impact velocity is smaller than a threshold velocity V_c, the drop is entirely captured by the fibre whereas if the velocity is larger than V_c, only a small portion of fluid remains trapped on the solid fibre. While this threshold velocity has been determined when the impact is centred – i.e. when the trajectory of the drop and the axis of the fibre do intersect – fewer studies are related to off-centre impact. Here, we focus on the influence of the relative position of the drop and the fibre on the impact. For velocity larger than V_c, we vary the eccentricity and measure the ability of the fibre to retain the liquid phase. Quite surprisingly, we show that off-centre impact situation enhances the ability of the fibre to capture a portion of the drop.     

Thermal expansion of nanostructured PbS films and anharmonicity of atomic vibrations

The dependences of the coherent scattering region size and thermal expansion coefficient α of a PbS nanofilm on the annealing temperature in the range of 293–473 K and on the duration of annealing at a constant temperature of 423 K have been measured. It has been found that the thermal expansion coefficient α of the PbS nanofilm is almost twice as much as the coefficient α of coarse-grained lead sulfide. It has been shown that the large difference in the coefficients α is associated with the small size of particles in the film, which leads to an increase in the anharmonicity of atomic vibrations. The contribution from the small size of particles to the thermal expansion coefficient of the PbS nanofilm has been evaluated theoretically.     

Experimental Investigations of the Binary Interaction of Polydisperse Sprays

Experimental investigations of the interaction of two polydisperse semi-hollow cone sprays are presented. The process, although of considerable significance for the chemical industry and applications like flue gas cleaning, has not been well-covered in the existing literature. This may be due to difficulties in getting general results from experiments involving particular geometries, like conical sprays, with fixed spray angle and geometrical arrangement of the nozzles. The present work develops a representation of the effects of the spray interaction on the spray drops in the resulting two-phase flow. The measurement technique used is phase-Doppler anemometry (PDA), which provides information about the size and two velocity components of the drops at each measurement position in the sprays. A factorial design of the experiments allows the influence of the intersection angle and the liquid flowrate of the sprays on an integral mean drop size in a spray cross section to be quantified. For varying values of these parameters, the downstream evolution of the interacting sprays is quantified in terms of the smoothness of profiles of the number-mean drop size. The collisional interaction of the spray drops is identified as the reason for the observed increase of the mean drop size caused by the spray interaction.     

The wetting angle of very small and large drops

On the basis of Born-Green-Yvon integral equations for the density distribution functions, an approximate integral equation is established for the profile of the surface of the drop. Numerical solutions and analytical solutions for limiting cases are obtained for this profile. Equations relating the angle at the leading edge and in its vicinity to parameters characterizing the interaction forces between the molecules of the liquid and between those of the liquid and solid are derived for large and for very small drops on a horizontal solid surface. One concludes that there is a rapid spatial variation of shape near the leading edge, that for large drops the measured macroscopic wetting angle is reached at a distance of about 20 to 40 Å from the leading edge, and that for very small drops the wetting angle is weakly size dependent. A condition for drop stability is established, which if not satisfied, the liquid will spread over the surface of the solid.     

Viscosity and electric properties of water aerosols

The flow of water mist in a narrow duct has been studied experimentally. The profile of the velocity of drops has been measured, and the viscosity of the mist has been calculated using the Navier–Stokes equation. It has been found that at low gradients of the rate of shear the viscosity of the mist can exceed that of clean air by tens and even hundreds of times. The electric charge of the drops has been measured. It has been found that the viscosity of the mist differs from that of clean air at gradients of the rate of shear that are less than the frequency of the establishment of electric equilibrium between the drops. A comparative analysis of the viscosities of the mist and a drop cluster has been carried out, and the dependence of the viscosity of the water aerosol on the radius and the charge of the drops has been predicted. The possible role of aerosols that contain submicron drops in the known “clear air turbulence” problem has been shown.     

Nucleation and growth on a superhydrophobic grooved surface

The growth dynamics of water drops condensed on a superhydrophobic geometrically patterned surface were studied. Drop size evolution at early and intermediate times is self-similar. Drop growth laws do not differ for a flat surface because of a reduction of both drop and substrate dimensionality. A striking observation is the instantaneous drying of the top surface of grooves at a point in time due to coalescence of the drops with a completely filled channel. At late times, only a few large drops grow connected to the channels, in a mixed Wenzel-penetration regime.     

倾斜均质表面上等径水滴聚合过程的可视化实验

对倾斜均匀表面上等直径水滴的聚合过程及特性进行了可视化实验研究,获得了水滴直径和表面倾角等参数对液滴聚合过程中液滴液桥半径、接触角和接触线变化特性的影响,分析了水滴聚合对其运动的影响.实验结果表明:表面倾角越大,下滑的临界半径越小;液滴的直径越大,液滴聚合后越容易下滑;液滴聚合可以加快液滴的运动,使下滑临界半径减小.     

The effect of volatility on the thermophoretic motion of a high-viscosity sphere in a binary gaseous mixture with regard for thermodiffusion and Stefan effects

The effect of the evaporation coefficient of a large aerosol particle on the velocity of its thermophoretic motion in a binary gaseous mixture is studied. The thermodiffusion of the mixture components and Stefan phenomena are taken into consideration. The results of this study are more general than those obtained previously. The conventional theories of thermophoresis, as applied to a volatile high-viscosity liquid drop, are extended for the cases of weak and moderately intense diffusive evaporation.     

气溶胶抛撒过程中首次破碎液滴的尺寸分布

对气溶胶爆炸抛撒过程中,首次破碎液滴的尺寸分布进行理论和数值研究.基于热力学第二定律的最大熵增理论,建立首次破碎过程满足的方程组约束条件,给出破碎后液滴尺寸分布的确定方法,并对Air-blast喷管实验和Samirant相关实验进行数值模拟预测,与实验数据符合较好.     

The effect of the evaporation coefficient on the thermophoresis of a volatile single-component drop in a binary gas mixture

A theory of the uniform thermophoretic motion of a liquid volatile spherical drop in a binary gas mixture is developed based on hydrodynamic analysis. One of the components undergoes the phase transition on the surface. The solution of the problem makes it possible to estimate the effect of the evaporation rate on the rate and direction of thermophoresis, as well as on the distributions of the velocity, temperature, and concentration of the volatile component. The thermal diffusion of the gas mixture, together with Stefan and capillary phenomena, is taken into account. The velocity of thermophoretic transport is expressed through the evaporation coefficient of the drop by the formula that generalizes the known results of the conventional theories for the cases of weak and moderately intense diffusive evaporation of a liquid drop.     

Measurement of the continuous Lehmann rotation of cholesteric droplets subjected to a temperature gradient

In 1900, Otto Lehmann observed the continuous rotation of cholesteric drops when subjected to a temperature gradient. This thermomechanical phenomenon was predicted 68 years later by Leslie from symmetry arguments but was never reobserved to our knowledge. In this Letter, we present an experiment allowing quantitative analysis of the Lehmann effect at the cholesteric-isotropic transition temperature. More precisely, we measure the angular velocity of cholesteric drops as a function of their size and the temperature gradient and we show that applying an electric field can stop the drop rotation. From these observations and a theoretical model we estimate the Lehmann coefficient nu.     

Radius of electron-hole drops in silicon

The radius of electron-hole drops at nucleation threshold is investigated as a function of temperature in Si using a small shift of their main luminescence line due to their surface energy. For T > 10 K, experiment and theory are consistent if we take into account a sticking coefficient (～ 4%) at the drop surface. For T ? 10 K our data indicate that the drop size is certainly small, but the measured radii are not likely. This suggests that other effects should be considered, such as the drop curvature energy for example.     

Influence of heterogeneous chemical reactions on the slip velocity of an inhomogeneous multicomponent gaseous mixture

In constructing a theory of thermal diffusiophoresis of volatile aerosol particles it is necessary to have boundary conditions for the tangential velocity component which allow for the presence of heterogeneous chemical reactions. Conditions of this sort have been obtained by a number of authors [R. N. Gupta et al., Technical Papers, AIAA 22nd Aerospace Science Meeting, AIAA 19th Thermophysics Conference, New York (1985), pp. 465–490; D. V. Kul’ginov, Tech. Phys. 63, 940 (1993); A. V. Bogdanov et al., Preprint No. 1051, Fiz-Tekh. Inst. Akad. Nauk. SSSR, Leningrad (1986)]. The results of Gupta et al. are in the form of analytical expressions, but their computations actually used Maxwell’s method, which is of low accuracy. Kul’ginov et al. and Bogdanov et al. used the method of matched asymptotic expansions, which did not permit them to get simple analytical expressions. In the present paper the slip velocity is calculated by the Loyalka method. Analytical expressions are obtained for the slip coefficients, and the results of numerical calculations are presented. It is shown that in the presence of concentration gradients of the components of the gaseous mixture along the surface of an aerosol particle, the slip velocity can acquire new terms due to the change in the sticking coefficient along the catalytic surface. Expressions in final form are given for these terms. Zh. Tekh. Fiz. 67, 29–33 (May 1997)     

基于格子Boltzmann方法的自驱动Janus颗粒扩散泳力

Janus颗粒的自驱动力研究对于纳微米尺度驱动力课题具有重要意义,本文针对Pt-SiO_2型Janus颗粒,基于格子Boltzmann模型及动量交换法提出了计算其扩散泳力的方法,通过与实验数据对比修正验证了模型准确性,并通过分析证明了此类Janus颗粒的扩散泳力与速度场无关,进一步模拟比较了不同形状颗粒的自驱运动.分析发现,对于体积相等形状不同的Janus颗粒,扩散泳力主要由轴线投影面积决定,此外反应面积也会对扩散泳力产生影响.     

Effect of CO2 shielding gas on metal droplet formationin arc welding

We have developed a unified arc electrode model that enables us to make predictions of the time development of molten drops from the welding wire in gas metal arc welding. The wire is taken as the positive electrode, and the effects of surface tension, magnetic pinch forces, and convection within the drop are taken into account to predict drop detachment for any given arc current. For pure argon, we have previously predicted the sharp transition that is observed experimentally at about 300 A between globular transfer at low current, when drop diameters are larger than the wire diameter, and spray transfer, for currents above 300 A, when drop diameters are smaller than the wire diameter. In this paper, we predict that addition of 25% of CO₂ to the argon leads to an increase in the transition current to more than 325 A, also in agreement with published experimental results. For pure CO₂, we find a significantly different drop behavior due to the more constricted arc. Both small and large drops are produced, with many very small drops being produced successively between each large drop