脉冲MIG焊熔滴过渡最佳方式的控制

基于一个脉冲过渡一个熔滴且熔滴体积不变的思想,提出了在峰值电流和基值电流不变的条件下采用送丝速度预置和峰值弧压调节峰值时间与基值时间的控制方式。试验结果表明:采用这种控制方式,能够在不同送丝速度或不同干伸长下实现一个脉冲过渡一个熔滴且熔滴体积基本不变。     

Real‐Time Monitoring of Colloidal Crystallization in Electrostatically‐Levitated Drops

Colloidal crystallization is analogous to the crystallization in bulk atomic systems in various aspects, which has been explored as a model system. However, a real‐time probing of the phenomenon still remains challenging. Here, a levitation system for a study of colloidal crystallization is demonstrated. Colloidal particles in a levitated droplet are gradually concentrated by isotropic evaporation of water from the surface of the droplet, resulting in crystallization. The structural change of the colloidal array during crystallization is investigated by simultaneously measuring the volume and reflectance spectra of the droplet. The crystal nucleates from the surface of the droplet at which the volume fraction exceeds the threshold and then the growth proceeds. The crystal growth behavior depends on the initial concentrations of colloidal particles and salts which determine the overall direction of crystal growth and interparticle spacing, respectively. The results show that a levitating bulk droplet has a great potential as a tool for in situ investigation of colloidal crystallization.     

Theoretical Investigation on Rapid Evaporation of a Saline Droplet During Depressurization

This paper reports a theoretical investigation on rapid evaporation of a saline droplet during depressurization. A mathematical model was developed to simulate the droplet temperature variation by considering the ambient pressure change, the heat transfers due to evaporation and convection at the droplet surface, accompanying the heat and mass transfer inside the droplet. The component diffusion and the temperature gradient inside the droplet were mainly discussed by comparing the numerical droplet temperature with the experimental data. The result shows that, the variation of internal concentration is small, while the temperature gradient within the droplet is significant during the evaporation process. In addition, the influencing factors of the droplet temperature variation were analyzed, such as: the final ambient pressure, theinitial salt concentration and the initial droplet temperature. The present model calculations help to understand the thermodynamic process of rapid evaporation of a saline droplet during depressurization.     

Influence of fluorinated self-assembled monolayer on wetting dynamics during evaporation of refrigerant–oil mixture on metal surface

Reducing wettability of a metal surface is a promising method for enhancing boiling heat transfer of refrigerant–oil mixture on the metal. As fluorinated self-assembled monolayer (F-SAM) coating is effective for wettability reduction, its influence on wetting dynamics including meniscus shape, contact angle, contact line velocity and rising liquid height during evaporation of refrigerant–oil mixture on metal surface were experimentally investigated. The refrigerant–oil mixture was prepared by R141b and NM56, the oil mass fraction ranged from 0 to 10 wt%, and the surface roughness ranged from 0.028 to 1.166 µm. The results show that during evaporation of refrigerant–oil mixture, the presence of F-SAM changes the evaporation mode to be constant contact line velocity followed by both constant contact angle and contact line velocity, while decreases the rising liquid height. The results suggest that larger surface roughness and higher oil mass fraction are preferred when using F-SAM to reduce surface wettability.     

脱硫废水烟气喷射蒸发流动特性实验研究

确保喷雾液滴在接触烟道壁面前完全蒸发,是保障电站脱硫废水在锅炉尾部烟道内蒸发处理安全运行的关键。喷雾液滴的破碎、聚并等动力学行为,以及液滴群的粒径分布和速度等因素的影响机制,是喷雾蒸发的主要特性。设计搭建了热态风洞实验台,利用激光粒度分析仪和粒子图像测速仪（particle image velocimeter,PIV）,在不同的引射空气压力、喷嘴水流量,以及风速、加热空气温度等条件下,对喷雾液滴群的粒径变化和速度变化进行了测量和分析。实验结果表明：以大液滴形态离开喷嘴的射流在引射气流的携带作用下,因破碎而形成小液滴,而后液滴间聚并效果会显现出来。液滴初始粒径仅与引射气体压力和水流量有关;风速的提高一定程度上会促进液滴间的聚并。提高高压气体压力、温度、风速以及减小水流量均有助于提高液滴群速度,其中提高风速对液滴群的增速效果最为明显。研究结果为喷雾的数值模拟及工程应用改进方向提供了参考。     

Numerical simulation of gas-droplets mixing and spray evaporation in rotary spray desulfurization tower

Motion and evaporation of droplets significantly affect the semidry flue gas desulfurization efficiency and long-term operation. Both the flow field distribution and the heat and mass transfer in the spray towers are studied by numerical simulation, and the process of droplet motion and evaporation is analyzed in detail. Then, two indices, mixing variance and droplet mass-weighted life, are provided to quantify gas droplet mixing and the droplet group evaporation time. The simulation results show that the radial penetration distance of the droplets is longer with the diameter increase, and the appropriate swirl number improves the mixing between the flue gas and droplets. With the increase of droplet diameter and velocity, the droplet distribution in the tower is more widely, obtaining the optimum mixing variance. The droplet mass-weighted life is promoted linearly with the increase of average droplet diameter and the decrease of flue gas temperature. With flue gas temperature increase from 458 k to 488 K, the droplet mass-weighted life decreases linearly by 31%. In comparison, the initial droplet velocity and spray angle have a slight effect on the droplet mass-weighted life.     

Control of particle-deposition pattern in a sessile droplet by using radial electroosmotic flow

In this technical note, we report an experimental investigation of radial electroosmotic flow (EOF) as an effective means for controlling particle-deposition pattern inside an evaporating droplet, which has a potential application to biochemistry and analytical chemistry especially for sample preparation steps. Using the microelectrode, which consists of the circular electrode around the rim of droplet and the point electrode at the center of the droplet, we generate the radial electric field at the bottom of the electrolyte droplet. The electric field developed between the center electrode and the circular electrode causes a radial EOF in the vicinity of the bottom of the droplet. By changing the applied voltage, the strengths and directions of the radial EOF are controlled at one's own discretion, and thus, we can modify the solute distribution inside the droplet during evaporation. When the radial EOF compensates the natural outward flow at a suitable choice of electrical voltage, the particles are uniformly distributed at the entire droplet spot. Moreover, with strong radial EOF, all the particles are deposited at the center rather than at the rim. We also carry out a simple theoretical investigation of flow field inside the droplet with Smoluchowski slip velocity condition to show how the particles travel during evaporation.     

Role of vapor flow in the mechanism of levitation of a droplet-cluster dissipative structure

The rate of water evaporation has been experimentally determined under conditions that ensure the formation of a dissipative structure of the droplet cluster type [1, 2]. It is shown that, in the region of localization of a droplet cluster, the velocity of the vapor-air flow is sufficient to maintain the levitation of droplets over the liquid surface according to the Stokes mechanism.     

Airborne cell analysis

A miniaturized analysis system for the study of living cells and biochemical reactions in microdroplets was developed. The technique utilizes an in-house-developed piezoelectric flow-through droplet dispenser for precise reagent supply and an ultrasonic levitator for contactless sample handling. A few-cell study was performed with living primary adipocytes. Droplets (500 nL) containing 3-15 individual cells were acoustically levitated. The addition of beta-adrenergic agonists into the levitated droplet using the droplet dispenser stimulated adipocyte lipolysis, leading to free fatty acid release and a consequent pH decrease of the surrounding buffer. The addition of insulin antagonized lipolysis and hence also the decrease in pH. The changes in pH, i.e., the cell response in the droplet, were followed using a pH-dependent fluorophore continuously monitored by fluorescence imaging detection. An image analysis computer program was employed to calculate the droplet intensities. To counteract droplet evaporation, found to affect the fluorescence intensities, a separate dispenser was used to continually add water, thus keeping the droplet volume constant.     

Features of the Diffusiophoresis of Aerosol Droplets with Allowance for the Influence of the Evaporation (Sticking) Coefficient and Internal Flows

A new theory of diffusiophoresis of large volatile spherical aerosol droplets, which is a further development of the previous investigations, has been formed. Account has been taken of the influence of the evaporation coefficient of the droplet liquid, the surfacetension coefficient variable along the droplet surface, and internal flows in the droplet on the diffusiophoresis velocity. The formulas obtained enable one to directly find the velocity of motion of single large aerosol droplets in a binary gas mixture inhomogeneous in concentration.     

Experimental and Theoretical Investigation on Rapid Evaporation of Ethanol Droplets and Kerosene Droplets During Depressurization

This paper reports an experimental and theoretical study of rapid evaporation of ethanol droplets and kerosene droplets during depressurization. For experimental method, an ethanol droplet or a kerosene droplet was suspended on a thermocouple, which was also used to measure the droplet center temperature transition. And the droplet shape variation was recorded by a high speed camera. A theoretical analysis was developed based on the heat balance to estimate the droplet center temperature transition, and the evaporation model proposed by Abramzon and Sirignano was used to describe the droplet vaporization. According to the experimental data and theoretical analysis, both of the environmental pressure and the initial droplet diameter have a prominent influence on the droplet temperature transition. Comparing the evaporation processes of ethanol droplets and kerosene droplets with water droplets, the ethanol droplets have the fastest evaporation rate, followed by water, and the evaporation rates of kerosene droplets are the slowest. Also it was found that a bubble can easily emerge within kerosene droplet, and its lifetime is more than 1 s.     

Evaporation of additive droplets in a pulse MHD generator

Theoretical investigation into evaporation of additive droplets in the combustion chamber of a pulse MHD generator were undertaken. Flow in the chamber is considered as stationary and one-dimensional; mixing in a direction perpendicular to flow is believed to be ideal, and mixing is lacking in the flow direction. It is suggested that droplets are monodisperse, spherical, and motionless relative to the gas medium. The droplet evaporation can be taken as occurring in the diffusion mode. The specific heat c _p and heat conductivity coefficient are taken to be constant and independent of temperature and the concentration of components. The Lewis number is believed to be the unit value; and the Soret and Dufour effects, negligible. A formula for calculation of the droplet evaporation rate with allowance made for chemical reactions occurring in liquid and gas media is obtained.     

MEASUREMENT OF THE KINETICS OF SOLUTION DROPLETS IN A CONTINUOUSLY MIXED CHAMBER

ABSTRACT

An experimental system is designed for the measurement of the evaporation and growth kinetics of individual solution droplets. Electrostatically charged droplets, nominally tens of micrometers in diameter, are suspended in a hyperboloidal electrodynamic chamber by balancing the droplet ueight against a uniform electrostatic field. By controlling the mixing dynamics in the chamber with specific flow configurations, a continuously mixed chamber is achieved. Consequently the Instantaneous chamber relative humidity is predicted from an exponential law with o characteristic relaxation time which is given by the ratio of the chamber volume and the gas volumetric flow rate. The evaporation and growth kinetics of phosphoric acid droplets is measured for relative humidity changes between 30 and 80%. Comparison between experimental and theoretical instantaneous droplet masses reveals less than a 5% deviation.The present system is employed to analyze the effect of adsorbed hexadecanol surfactant molecules on the evaporation and growth kinetics of phosphoric acid droplets. It is found a critical coverage exists which will result in a dramatic reduction in the evaporation kinetics of solution droplets. ConcomiCantly, the condensation coefficient is reduced from unity to 4.0 x 10-5 as droplet kinetics occur in the presence of a complete monolayer of hexadecanol.     

Unrestrained early age shrinkage of concrete with polypropylene,PVA, and carbon fibers

Plastic shrinkage cracking due to restrained shrinkage is a primary problem that often occurs in concrete structures with a relatively large surface area, such as concrete walls, bridge decks, slabs, and overlays. These applications are susceptible to rapid changes in temperature and humidity resulting in high water evaporation and high potential for shrinkage cracking. Free shrinkage at early age is one of the most influential factors leading to tensile stresses in concrete structures. When the tensile stress is higher than the tensile strength, cracking occurs. The addition of micro-fibers in amounts as small as 0.1% by volume is an effective method to control plastic shrinkage cracking. However, the effect of fibers on the free shrinkage and water evaporation of concrete is not clearly understood. The main objective of the study described in this paper is to evaluate the influence of fibers on the free shrinkage and water evaporation of fiber reinforced cement composites during the first 24,h after mixing, with particular attention to fiber content, fiber bond, and fiber stiffness. Prismatic concrete specimens of 1000,mm in length and 100 × 60,mm in cross section were tested to measure their unrestrained shrinkage strain. Simultaneously, flat concrete specimens of dimensions 327 × 230 × 50,mm were tested to determine loss of water by evaporation. The tests were carried out under adverse environmental conditions known to encourage high shrinkage, namely high temperature, low relative humidity, and exposure to high volume and velocity of air flow. Three types of fibers, polypropylene, PVA, and carbon fibers were investigated at four different fiber volume fractions, namely: 0.1%, 0.2%, 0.3%, and 0.4%. Experimental results are presented and conclusions are drawn.     

Capillary Origami Inspired Fabrication of Complex 3D Hydrogel Constructs

Hydrogels have found broad applications in various engineering and biomedical fields, where the shape and size of hydrogels can profoundly influence their functions. Although numerous methods have been developed to tailor 3D hydrogel structures, it is still challenging to fabricate complex 3D hydrogel constructs. Inspired by the capillary origami phenomenon where surface tension of a droplet on an elastic membrane can induce spontaneous folding of the membrane into 3D structures along with droplet evaporation, a facile strategy is established for the fabrication of complex 3D hydrogel constructs with programmable shapes and sizes by crosslinking hydrogels during the folding process. A mathematical model is further proposed to predict the temporal structure evolution of the folded 3D hydrogel constructs. Using this model, precise control is achieved over the 3D shapes (e.g., pyramid, pentahedron, and cube) and sizes (ranging from hundreds of micrometers to millimeters) through tuning membrane shape, dimensionless parameter of the process (elastocapillary number C_e), and evaporation time. This work would be favorable to multiple areas, such as flexible electronics, tissue regeneration, and drug delivery.     

Mixing of vaporizing droplets on liquid substrates caused by festoon instability

A new configuration for a microfluid mixer, in which a mixed volume in the form of a volatile liquid droplet rests on a layer of water, is proposed. The mixing occurs owing to the injection of festoons emerging in a droplet under the influence of evaporation themocapillary convection from the periphery of the droplet to its central part. A linear relationship between the injection rate and the injection layer temperature is established, and it is shown that a droplet has time to repeatedly mix during its lifetime.     

Self-propelled drop jumping during defrosting and drainage characteristic of frost melt water from inclined superhydrophobic surface

Drainage behavior of frost melt water from bare, hydrophobic and superhydrophobic surface was experimentally investigated. The self-propelled droplet jumping during defrosting on superhydrophobic surface was captured and the self-drainage characteristic on inclined superhydrophobic surface was observed. In addition, the influence of surface inclined angle on drainage time and surface temperature after complete drainage was analyzed. It is found that the occurrence of spontaneous jumping of defrosted droplet was induced by drop coalescence, which can overcome the surface adhesion, leading to frost melt droplets suspending on superhydrophobic surface. Moreover, the frost melt water can be entirely released with no water droplets retention on superhydrophobic surface at larger than 30° inclined angle, which cannot happen on bare and hydrophobic surface. Furthermore, the surface can keep at low temperature after complete defrosting. The results show that the superhydrophobic surface has an excellent self-drainage capability, which provides possibility for improving defrost efficiency.     

Electrohydrodynamic generation and delivery of monodisperse picoliter droplets using a poly(dimethylsiloxane) microchip

We developed a drop-on-demand microdroplet generator for the discrete dispensing of biosamples into a bioanalytical unit. This disposable PDMS microfluidic device can generate monodisperse droplets of picoliter volume directly out of a plane sidewall of the microfluidic chip by an electrohydrodynamic mechanism. The droplet generation was accomplished without using either an inserted capillary or a monolithically built-in tip. The minimum droplet volume was approximately 4 pL, and the droplet generation was repeatable and stable for at least 30 min, with a typical variation of less than 2.0% of drop size. The Taylor cone, which is usually observed in electrospray, was suppressed by controlling the surface wetting property of the PDMS device as well as the surface tension of the sample liquids. A modification of the channel geometry right before the opening of the microchannel also enhanced the continuous droplet generation without applying any external pumping. A simple numerical simulation of the droplet generation verified the importance of controlling the surface wetting conditions for the droplet formation. Our microdroplet generator can be effectively applied to a direct interface of a microfluidic chip to a biosensing unit, such as AMS, MALDI-MS or protein microarray-type biochips.     

The effect of an axial temperature gradient on the steady motion of a large droplet in a tube

In this paper we analyse the steady motion of a large droplet of fluid with temperature-dependent surface tension in a cylindrical tube subject to a constant axial temperature gradient in the asymptotic limit of small Capillary and Marangoni numbers. The leading order problem obtained is a generalisation of the classical work of Bretherton (J. Fluid Mech. 10 (1961) 166–188) for an inviscid droplet with constant surface tension and leads to a generalised version of the well-known Landau-Levich equation for the shape of the free surface in the transition regions. Numerically calculated values for the leading order thickness of the thin film surrounding the droplet and fractional velocity correction are presented, and both are found to be monotonically increasing functions of the Marangoni number.A preliminary version of this work was presented by the author at The 1993 ASME Winter Annual Meeting held in New Orleans, Louisiana, USA from November 28th to December 3rd 1993.