液滴撞击低温超疏水表面运动状态的数值模拟研究

本文针对液滴撞击固体表面后出现的反弹和黏附两种运动状态,通过引入基于连续温度函数的能量方程,建立了用于计算液滴运动、传热和相变过程的数值模型,并与实验结果对比,验证了数值模型的准确性。进而模拟了具有不同韦伯数We和奥内佐格数Oh的液滴撞击低温超疏水表面的运动、传热和相变过程。结果表明：液滴撞击后的运动状态主要取决于奥内佐格数Oh,临界范围是0.022-0.026,而基本不受韦伯数We的影响。当液滴撞击后的运动状态为反弹时,韦伯数We越大的液滴,最大铺展直径越大。而当韦伯数相同时,随着奥内佐格数Oh的增大,液滴与表面的接触时间越大。     

悬垂水滴与悬浮气泡表面气体水合物形成特性对比

基于悬垂水滴和悬浮气泡表面形成气体水合物的可视化耐高压实验装置,分析探讨了反应压力、温度、水质等因素对水滴和气泡表面气体水合物成核和生长规律的影响.对已有的关于研究单个静止悬垂水滴和悬浮气泡表面气体水合物生长特性的实验现象及结果进行了对比分析,得出结论:温度和压力是影响表面水合物结晶与生长的重要因素;温度的降低或压力的升高均使水合反应速度加快.研究为发展喷雾法和鼓泡法这两种强化制备水合物的方式提供了有效的实验支撑.     

Research on droplet transfer in oscillating arc narrow gap GMA welding

In this paper, the droplet transfer in oscillating arc narrow gap gas metal arc （ GMA ） welding was studied. According to the experimental results, the oscillating arc has effect on the droplet transfer mode. The droplet transfer frequency in narrow gap groove is higher than that in bead-on-plate welding. Because of the change of arc location in narrow gap groove, the droplet transfer in oscillating arc narrow gap changes regularly. The droplet transfer frequency near groove sidewall is higher than that at the middle of narrow gap groove.     

DYNA-FOGR MAXI-PROTM 2P 2743型超低量喷雾机雾化性能的实验研究

针对一种先进的新型喷药机具DYNA-FOGR MAX I-PROTM 2P 2743型超低量喷雾机,应用雾滴谱显微镜测定方法在实验室测量了该喷雾机的雾滴粒谱及喷雾量与粒谱的关系。试验表明该机喷射雾滴直径NMD为25μm,VMD为50μm,适用合于大型仓库、车站、码头、垃圾场等大面积区域的灭虫、灭菌防治作业。     

MECHANISMS OF DRYING OF SKIN-FORMING MATERIALS.III. DROPLETS OF NATURAL PRODUCTS.

ABSTRACT

Drying studies were carried out on single droplets suspended from a rotating nozzle in a horizontal wind tunnel. Droplets were of solution of a third type of skin-forming material, namely the natural products skim milk or fructose. Droplet weight and temperature were measured during the drying process. Dried crusts were recovered and examined by Scanning Electron Microscopy.

With this type of material a skin first forms at any drying condition on the surface of the droplet at a cerlain stage of drying. A crust subsequently forms beneath the skin. The final dried panicle retains the structure and properties of the original powder, unless the material is dried at very high temperatures. e.g. >300 °C. which result in burning.

An increase in air temperature resulted in the formation of a smoother skin offering a higher resistance to vapour diffusion. At temperatures over 100 °C droplets exhibited puffing or ballooning phenomena; i.e. they inflated and deflated many times before forming the final dried particles.     

Evaporation of Pure Droplets in the Convective Regime Under High Mass Flux

An analytical expression analogous to the D² law was introduced, based on a mass transfer consideration, to describe the evaporation of pure droplets in the convective regime based on the Ranz-Marshall-type correlation for the Sh number. Comparison was made to single droplet drying experiments of pure water. It was found that the effect of mass transfer coefficient depression is significant under the high mass flux condition from the droplet. An expression was obtained to approximate the total time required for complete evaporation of a pure droplet. This expression can be used as a quick computation tool for more fundamental studies on droplet evaporation under controlled ambient conditions.     

Optimization Of The Operation Of A Drying Heat Pump Using Superheated Steam

ABSTRACT

A numerical solution of a model which describes the optimal operation of a novel concept of heat pump for drying applications based on the theory of minimum energy cycles is presented. The thermodynamic cycle of the drying heat pump involves vapor condensation in a Laval nozzle, removal of the liquid phase in a separator as well as compression of the working medium, which is superheated vapor.     

Effect of viscoelasticity on drop dynamics in 5:1:5 contraction/expansion microchannel flow

Recently, we reported how viscoelasticity affects drop dynamics in a microchannel flow using the finite element-front tracking method (FE-FTM). In this work, we investigate drop dynamics for a wider range of parameters: viscosity ratio between droplet and medium (χ), capillary number (Ca), droplet size, and fluid elasticity. The Oldroyd-B model is adopted as the constitutive equation for the viscoelastic fluid. We observe that the drop deformation in a microfluidic channel is dependent on Ca, which is more pronounced for smaller χ values. The present work shows that viscoelasticity plays an important role in drop dynamics with increasing χ values for Newtonian droplet in viscoelastic medium, which can be attributed to high normal stress developed in narrow film thickness between droplet and channel for higher χ values. We also study circulation problem inside droplets, which is important in practice, such as in droplet reactor application. The present work shows that circulation intensity is enhanced with decreasing χ values. We find that the relevance of viscoelastic effects on internal circulation is dependent on χ values, and the circulation intensity is distinctively decreased with increasing elasticity for high χ values for Newtonian droplet in viscoelastic medium. We expect that the present work be helpful not only in controlling droplets but also to improve our physical insight on drop dynamics in microchannel flows.     

Pattern analysis of a single droplet impinging onto a heated plate

The time scale for measuring the heat transfer phenomena, such as the Leidenfrost phenomenon, is much longer than the hydrodynamic transient period of a droplet impinging onto a heated surface. However, the Leidenfrost temperature has been long since taken as the criterion to classify characteristic regimes for not only heat transfer but also hydrodynamic impact patterns of liquid‐solid‐interface problems in the literature. The impingement phenomena of a single droplet onto a heated plate for We <200 were experimentally classified as five different characteristic patterns in this paper. They are [I] completely wet, [II] wet film boiling, [III] transition, [IV] dry rebounding, and [V] satellite dry rebounding impact patterns. The former two patterns belong to wet impact, while the latter two are dry impact. This work reveals that the hydrodynamic impact parameter does influence the classification of characteristic impact patterns, especially for the dry impact patterns. For high impact Weber numbers, the dry impact happens to the plate surface temperature much lower than the commonly believed Leidenfrost temperature due to the squeeze film effect. The evolutions that impact changes from one to another characteristic patterns are also discussed. This paper offers a systematic and reliable database for future numerical simulations. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(8): 579–594, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20089     

Director Field in a Bipolar Configured Nematic Droplet

Polymer dispersed liquid crystal(PDLC) have found its application in flexible displays and switchable windows. Such applications require dimension of the droplets to be several micro meters[1]. Recently, it is found that droplets in PDLC may gather together in some districts by special manufacture process, to form switchable period holographic gratings. Such an application requires dimension of the droplets less than 200 nm[2]. In spite of dimension of the droplets, whether in micrometers …