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.     

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.     

A lattice Boltzmann model for solidification of water droplet on cold flat plate

Since the solidification of water droplet is the initial and essential process in the whole process of frosting, a model is developed by the lattice Boltzmann method (LBM) that applies the velocity and temperature distribution functions to investigate the solidification process of water droplet on cold flat plate. The thermal transport and liquid–solid phase transition in the present model are both based on the pseudo-potential model combined with the enthalpy formation. By this LB model, the solidification process is simulated in form of temperature and solid phase variations in water droplet on cold flat plate, and the shape of solid phase in freezing can also be predicted. In addition, we apply the present LB model to preliminarily study the frost formation process. Numerical results agree well with our experimental data.     

Effect of Temperature Change on Interfacial Behavior of an Acoustically Levitated Droplet

Under the microgravity environment, new and high quality materials with a homogeneous crystal structure are expected to be manufactured by undercooling solidification, since the material manufacturing under the microgravity environment is more static than that under the normal gravity. However, the temperature change on the interface of the material in space can affect on the material processing. The purpose of the present study is to investigate effect of the temperature change of interface on the large levitated droplet interface. A water droplet levitated by the acoustic standing wave is heated by YAG laser. In order to heat the water droplet by the laser heating, rhodamine 6G is solved in it to achieve high absorbance of the laser. The droplet diameter is from 4 to 5.5 mm. The deformation of the droplet interface is observed by high speed video camera. The temperature of droplet is measured by the radiation thermometer. It is noticed that the larger droplet under the higher sound pressure tends to oscillate remarkably by the laser heating.     

Effect of Laser Heating on Nonlinear Surface Deformation of Acoustically Levitated Droplet

Under the microgravity environment, new and high quality materials with a homogeneous crystal structure are expected to be manufactured by undercooling solidification, since the material manufacturing under the microgravity environment has no effect of gravity. However, the temperature change on the interface of the material in space is expected to affect on the material processing due to the changing of physical property corresponding temperature. The purpose of the present study is to investigate effect of the laser heating on surface deformation of large levitated droplet. A water droplet levitated in the acoustic standing wave is heated by YAG laser. In order to increase the water droplet temperature, rhodamine 6G is solved in it to achieve high absorbance of the laser. Droplet from 2.5 to 5.5 mm in diameter were levitated and heated. The deformation of the droplet interface has been observed by high speed video camera. We used the radiation thermometer for the measurement of the temperature of droplet surface. It is noticed that the heated droplet deforms with its resonance frequencies. The experimental result of shape oscillation makes it possible to simulate the oscillation.     

Solidification of droplets

The response of a crystallite to the thermal fields developing during the solidification of a droplet and the mode of growth on a S?L interface are discussed. Stresses are relieved by causing the crystallite to deform plastically. The dislocations and/or twins that arise alter the morphology of the S?L interface and can enhance its growth rate. The analysis concentrates on crystallites growing in the bulk or at the surface of a droplet. For the hexagonal metals Cd, Mg and Zn, when the temperature varies only through the thickness of a thin floating raft, the growth helixes arise mainly as a result of the relief of strain energy due to the action of surface tension forces. Their calculated height is lower than the height measured on spherical particles of Cd.     

Parameters controlling solidification of molten wax droplets falling on a solid surface

An experimental study was done to identify parameters that determine the shape of splats formed by droplets of paraffin wax impacting and freezing on a polished aluminum surface. Impact velocity was varied from 0.5 to 2.7 m/s and surface temperature from 23 to 73 °C. Droplet impact was photographed, and the splat diameter and liquid-solid contact angle measured from photographs. A simple energy conservation model was used to predict the maximum extent of droplet spread and the rate of droplet solidification. The extent of droplet solidification was found to be too small to affect droplet impact dynamics. Photographs showed liquid recoiling in the droplet center following impact on a cold surface (23 °C); the height of recoil diminished if either substrate temperature or impact velocity was increased. Droplet recoil was attributed to surface tension pulling back the periphery of the splat. Reducing the surface temperature increased surface tension, promoting recoil. At sufficiently large impact velocities droplets fragmented. A model based on the Rayleigh-Taylor instability was used to predict the number of satellite droplets that broke loose after impact.     

血管真空冷冻干燥过程的传热传质研究

本文采用无接触式称重法监测猪主动脉冷冻干燥过程,实时获取脱水速率,针对升华过程,通过采用准稳态传热模型结合脱水速率的变化来进行计算,获得升华温度、升华界面位移及传热量的变化规律.冻干参数分别设定为:预冻结温度-40 ℃、一次干燥-20℃、二次干燥10 ℃:、冻干箱压力10 Pa.结果显示,样品的平均含水量为74.24％...     

Modeling the impact of a molten metal droplet on a solid surface using variable interfacial thermal contact resistance

An analytical model of the true area of contact between molten metal and a rough, solid surface has been used to calculate thermal contact resistance and to predict how it changes with surface roughness, substrate thermal properties and contact pressure. This analytical model was incorporated into a three-dimensional, time-dependent numerical model of free-surface flows and heat transfer. It was used to simulate impact, spreading and solidification of molten metal droplets on a solid surface while calculating contact resistance distributions at the liquid–solid interface. Simulations were done of the impact of 4 mm diameter molten aluminum alloy droplets and 50 μm diameter plasma sprayed nickel particles on steel plates. Predicted splat shapes were compared with photographs taken in experiments and simulated substrate temperature variation during droplet impact was compared with measurements.     

An experimental study of freezing of supercooled water droplet on solid surface

Results of experimental investigations of the freezing of immobile water droplet on an aluminum plate are presented. The process was studied with the aid of a high-speed photo camera. The freezing of supercooled water contained in the surface droplet proceeds in a few stages: (i) preliminary heating of water and nucleation of ice microcrystals, (ii) relatively fast formation of the ice–liquid system with a transition to the state of thermodynamic equilibrium near the freezing temperature, and (iii) slow process of complete freezing. The rate and duration of each stage and the time of delay between the moment of action upon the supercooled droplet and the onset of freezing are estimated. Processes of supercooled and nonsupercooled water solidification are compared.

     

低温环境下水滴在冷表面结冰过程的实验研究

本文研究了环境温度的变化和实验表面改变对水滴结冰过程的影响。在温度可调的低温环境测试箱中搭建了观测水滴结冰的实验装置。用气压控制器控制液滴发生装置内的压强并使其产生水滴,通过相机记录水滴在冷表面上的结冰过程。实验结果表明:实验板表面的导热系数对水滴的结冰时间影响较大,环境温度为-22. 0℃时,红铜表面的结冰时间为3. 8 s,不锈钢表面的结冰时间为5. 0 s,均小于玻璃表面上的结冰时间15. 2 s;水滴在不同实验表面上结冰时均会产生凸起,但仅在玻璃表面上会发生明显的凸起塌陷现象。     

Experimental Investigation on Flash Evaporation of Saltwater Droplets Released into Vacuum

In this paper, the flash evaporation process of saltwater droplets released into vacuum is experimentally investigated. During the experiment, a saltwater (NaCl) droplet was suspended on a thermocouple junction, which was used to measure the temperature evolution. The droplet surface temperature was captured by an infrared thermal imager, and the shape variation was recorded by a high speed camera. According to the experimental results, the component and solution concentration has great influence on the evaporation process. With a rise of salt concentration in water, the evaporation rate decreases. The shape of temperature transition curve also depends on the salt concentration in solution, no matter whether it is higher or lower than the eutectic point (22.4%). The effects of environmental pressure, initial droplet temperature and initial droplet diameter on the temperature transition of droplets were also summarized based on the experimental data.     

水滴在真空室内结晶过程的模拟

模拟真空室内水滴结晶过程是模拟真空雾化法制备冰浆系统的第一步。模拟了液滴在真空室内从较高的温度降温到零度,之后到一定的过冷度,然后发生相变为液-固两相,到最后为全部固相的整个过程。在该过程中热质传递迅速发生,模拟结果表明蒸发或者升华控制了整个过程。以扩散模型和以对流模型所得到的结果都与实验结果在一定程度上吻合,这是因为通过对流或者导热所传递的热量比相变潜热要小几个数量级。在整个结晶过程中液滴温度的变化表明:液固相变要占据较长的时间,是一个更为重要的过程。考虑重力、浮力和粘滞力的影响给出了液滴在结晶过程中的运动方程。从该模型计算出的液滴结晶过程中下降距离比仅考虑重力作用下给出的要准确。研究结果为更好的设计真空闪蒸室提供了理论依据和参考。     

Influence of gas adsorption on optical transition energies of single-walled carbon nanotubes

The photoluminescence (PL) spectra of suspended single-walled carbon nanotubes (SWNTs) were measured in an ethanol gas atmosphere. When the gas pressure was decreased, the PL peaks were initially blue-shifted to a small extent before a rapid blue-shift took place at a transition pressure that depended on the temperature and diameter of the SWNT being measured. This pressure dependence is due to the adsorption of ethanol molecules on the SWNT surface. The optical transition energies measured below the transition pressure are intrinsic to the SWNT.     

KF稳定性判据的进一步分析

对ＫＦ稳定性判据做了进一步分析，得到一个新的界面稳定性判据：Ｇ≥ｍＧｃ－ｋΓＶ２／Ｄ２Ｖ／Ｄ，发现绝对稳定性不只出现在凝固速度极高的条件下，极高的温度梯度也会导致绝对稳定性．这个结果对快速凝固和激光表面处理技术具有指导意义．     

A model of a liquid droplet impinging on a high-temperature solid surfaces

The paper deals with a model of a liquid droplet vertically impinging on a heated solid surface. The model uses the following assumptions. The value of the wall temperature is taken to be such that the droplet-wall interaction would proceed via gas-vapor interlayer (T 400 °C). The droplet liquid is incompressible and nonviscous. The droplet surface is taken to be free, with its deformation caused by the effect of external pressure distributed over the droplet surface. The pressure is made up by two components, of which the first one is the surface tension pressure due to the curvature of the droplet surface; the second component is the pressure of vapor between the droplet and wall, which is determined by analyzing the process of vapor escape from the vapor interlayer. The motion of liquid within the droplet is taken to be potential and axisymmetric. The equations of droplet motion are solved relative to the potential of the vector field of velocity. The suggested model is used to perform numerical calculations of the droplet collision process, and the obtained results are compared with the data of other authors.Translated from Teplofizika Vysokikh Temperatur, Vol. 42, No. 6, 2004, pp. 921–927.Original Russian Text Copyright © 2004 by A. V. Gulikov, I. I. Berlin, and A. V. Karpyshev.     

Droplet cooling in atomization sprays

Transport between droplets/particles and a gas phase plays an important role in numerous material processing operations. These include rapid solidification operations such as gas atomization and spray forming, as well as chemical systems such as flash furnaces. Chemical reaction rates and solidification are dependent on the rate of gas-particle or gas-droplet transport mechanisms. These gas-based processes are difficult to analyze due to their complexity which include particle and droplet distribution and the flow in a gas field having variations in temperature and velocity both in the jet cross-section and in the axial distance away from the jet source. Thus to study and properly identify the important variables in transport, these gas and droplet variations must be eliminated or controlled. This is done in this work using models based on a single fluid atomization system. Using a heat transport model (referred to as thermal model) validated using single fluid atomization of molten droplets and a microsegregation model, the effect of process variables on heat losses from droplets was examined. In this work, the effect of type of gas, droplet size, gas temperature, gas-droplet relative velocity on the heat transport from AA6061 droplets was examined. It is shown that for a given gas type, the most critical process variable is the gas temperature particularly as affected by two-way thermal coupling and the droplet size. The results are generalized and applied to explain the difference in droplet cooling rate from different atomization processes.     

In-situ observation of solid-liquid interface transition during directional solidification of Al-Zn alloy via X-ray imaging

The morphological instability of solid/liquid(S/L) interface during solidification will result in different patterns of microstructure. In this study, two dimension(2 D) and three dimension(3 D) in-situ observation of solid/liquid interfacial morphology transition in Al-Zn alloy during directional solidification were performed via X-ray imaging. Under a condition of increasing temperature gradient(G), the interface transition from dendritic pattern to cellular pattern, and then to planar growth with perturbation was captured. The effect of solidification parameter(the ratio of temperature gradient and growth velocity(v), G/v) on morphological instabilities was investigated and the experimental results were compared to classical "constitutional supercooling" theory. The results indicate that 2 D and 3 D evolution process of S/L interface morphology under the same thermal condition are different. It seems that the S/L interface in 2 D observation is easier to achieve planar growth than that in 3 D, implying higher S/L interface stability in 2 D thin plate samples. This can be explained as the restricted liquid flow under 2 D solidification which is beneficial to S/L interface stability. The in-situ observation in present study can provide coherent dataset for microstructural formation investigation and related model validation during solidification.     

Freezing a water droplet on an aligned Si nanorod array substrate

When a water droplet is dried on a vertically aligned Si nanorod array surface, the nanorods are bundled together. To understand how bundles are formed, a water droplet is frozen rapidly on a Si nanorod array surface observed under a cryo-SEM (scanning electron microscope). The nanorods in the precursor film form similar bundles as those dried in air. But the nanorods under the apparent frozen water droplet are only slightly deformed. We propose that the bundling of nanorods is caused by non-uniform water-nanorod interaction, which could happen either during the water spreading or drying process. Therefore, controlling the liquid-nanostructure interaction could minimize the bundling. In addition, the rapid freezing process does not preserve the water inside the nanochannels, and almost all the water forms ice on top of the nanorod surface, either as a planar interface or as particles, depending on the locations. The separated ice-nanorod interface will have potential applications in chemical separation and crystal growth.     

接触角和质量分数对液滴冻结过程的影响

本文通过FLUENT软件的凝固/熔化模型,模拟了接触角及质量分数对纯水和氯化钠溶液在冷表面冻结过程的影响,选择铜片为亲水表面,纳米膜表面为疏水表面,对液滴在不同表面特性条件下的冻结过程进行实验研究。结果表明：液滴在冷表面的冻结特性与接触角、质量分数有关。当溶液质量分数一定时,接触角越小,液滴冻结速度越快,完全冻结时间越短;在冻结过程的初始时刻,接触角越小,液滴底部温度越低;当冻结时刻相同、液滴高度一致时,液滴表面的温度和液相分数均比液滴内部低;接触角相同时,溶液质量分数与液滴的开始冻结温度成反比,与完全冻结时间成正比。对比实验结果与模拟可知,不同质量分数的氯化钠液滴在接触角为60°和100°时,冻结时间的变化趋势一致,但实验值大于模拟值。