水滴在-20℃表面结冰成尖现象研究

水滴在-20℃的平板上凝固后会形成尖顶圆锥型,该特殊形状的出现与水的反常膨胀有关。研究水的反膨胀现象,能解释和解决生活中很多相关领域的问题。通过修正理想的层状结冰模型为凹槽状结冰模型的方法,进一步解释了成尖现象的机制,并对影响液滴成尖的条件进行了半定量的探究。

表面水滴对特高压直流输电线路电晕特性的影响

雨、雾天气情况下导线表面形成的水滴会导致导线周围电场分布发生严重畸变,从而改变导线的电晕特性。我国第1条特高压直流输电工程——云广线处于多雨地区,导线电晕特性需考虑水滴因素的影响。为研究导线表面水滴对特高压输电线路电晕特性的影响程度,划分了导线表面水滴的等级,建立了导线电晕特性测试系统,测量了不同水滴等级下,电晕笼中导线的起晕电压、可听噪声水平等特征量,获得了导线表面水滴对导线电晕特性的影响规律,并据此对我国特高压直流输电线路的工程设计提出了建议。研究结果表明:导线的起晕电压随导线表面水滴等级的提高迅速下降,最低可降到原来的一半左右;起晕后,导线发生剧烈的舞动,导线表面的水滴在电场的作用下会被"蒸发"或甩掉;水滴会使导线起晕时的可听噪声值降低;在相同的电压作用下,导线的可听噪声水平随导线表面水滴等级的增加而提高。

表面水滴对交流导线电晕特性的影响

雨天条件下,输电线路导线表面形成的水滴会畸变导线周围电场,从而改变导线的电晕特性。笔者基于实验室小电晕笼,利用实验室研发的电晕损失测量系统,测量了不同水滴等级（水膜、一滴水、中等水滴、饱和水滴）下导线的电晕损失,获得了表面水滴对导线电晕起始特性的影响规律。同时,笔者针对自然界中雨水电导率的差异,研究了不同的水滴电导率对电晕放电特性的影响。结果表明：水滴的存在严重影响了导线在交流电压下的电晕损失特性,相同电压作用下,表面附着水滴的导线电晕损失比干净导线电晕损失大得多,且随着水滴等级的提高而增加;在30~680μS/cm电导率范围内,水滴电导率对交流导线电晕放电影响不明显;导线的起晕电压随导线表面水滴等级的提高依次下降,最低可降到原来的34.02%左右。

复合绝缘子污层弱憎水性条件下的表面水滴形态

研究固体污层法中复合绝缘子表面的弱憎水性污层对高压线设计优化具有重要意义。采用调节污层惰性成分中硅藻土和高岭土的质量比实现对迁移一定时间后污层表面憎水性的微调,得到经过细分的亲水性及弱憎水性污秽。本文观察了不同成分污层表面水滴的形态变化且针对具有不同成分污层的复合绝缘子,并进行人工污秽试验。利用傅里叶变换红外光谱分析手段实现对迁移前后弱憎水性污层中小分子扩散情况的检测。试验结果表明:复合绝缘子表面污层呈弱憎水性时,虽然传统的憎水性表征——喷水分级及静态接触角测量结果同完全亲水性时相似,但内部出现了明显的憎水性小分子迁移现象。这些小分子出现在污层内部的后果是对水滴渗入污层过程产生抑制作用,该抑制作用会对水滴在污层表面扩散过程中的形态产生影响。进一步地,这些影响还将在污闪试验中妨碍盐分的浸出,使闪络电压升高。

航空发动机结冰试验中水滴特性参数分析

通过建立水滴在低温低马赫数气流中的传热传质以及运动数学模型,和以Fluent软件解析雾化喷嘴周围气流的总压分布和速度分布的基础上,分析了水滴直径、水滴速度、水滴表面温度和液态水含量等特性参数,在不同的来流速度、来流温度和水滴初始温度条件下的变化。结果表明,在来流速度、水滴初始速度以及直径大小相同条件下,水滴与气流的温差,是影响发动机进口截面液态水含量的主要因素。对结冰试验过程中模拟结冰云雾气象条件的参数选取进行了建议。

二元冰真空制备技术分析与研究

简要介绍了二元冰真空制备的原理和设备,分析了国内外关于真空环境下冰晶形成过程的理论和实验研究,改进了描述水滴表面温度与时间关系的数学模型。

Transition of super-hydrophobic states of droplet on rough surface

Twelve samples with periodic array square pillars microstructure were prepared on the silicon wafer by plasma etching techniques, on which space b of the square pillars increased from 5 to 60 μm. In order to study the effect ofb on the wettability of the rough surface, the effects of apparent contact angle （CA） and sliding angle （a） of the droplet on the rough surface were measured with the contact angle meter. The results show that the experimental values of CA well agree with the classical wetting theory and a decreases with the increase of b. Two drop shapes exist on the samples＇ surface, corresponding to the Cassie state and the Wenzel state respectively. The contact state in which a drop would settle depends typically on the size of b. On the role of gravitation, the irreversible transition of a drop from Cassie state to Wenzel state should occur at a certain space of the square pillars. Since the transition has implications on the application of super-hydrophobic rough surfaces, theoretically, the prediction of wetting state transition on square pillar array micro-structured surfaces provides an intuitionistic guidance for the design of steady superhydrophobic surfaces.

Simulation of droplet topography on superhydrophobic surface

We established a novel droplet model (with-gravity model) to show the gravity effect of the droplet in the contact angle experiment.By using with-gravity model, we obtained a three-dimensional topography of the droplet including the height of the droplet, the shape of the baseline and the circumference of the baseline.Comparing the with-gravity model with the ideal spherical model, our model considered the measurement error caused by gravity effect in the contact angle experiment which is a key point to influence the three-dimension topography of the droplet.From the calculation of our model, we found that there were two important points to enhance the measurement error: the size of the droplet and the contact angle.With the droplet and the contact angle became larger, measurement error was obviously increased.

Water droplet impact on superhydrophobic surfaces with microstructures and hierarchical roughness

Quantitative correlation between the critical impact velocity of droplet and geometry of superhydrophobic surfaces with microstructures is systematically studied.Experimental data shows that the critical impact velocity induced wetting transition of droplet on the superhydrophobic surfaces is strongly determined by the perimeter of single micropillar,the space between the repeat pillars and the advancing contact angle of the sidewall of the micropillars.The proposed model agrees well with the experimental results,and clarifies that the underlying mechanism which is responsible for the superhydrophobic surface with hierarchical roughness could sustain a higher liquid pressure than the surfaces with microstructures.

Evaporating behaviors of water droplet on superhydrophobic surface

We investigated the dynamic evaporating behaviors of water droplet on superhydrophobic surfaces with micropillars.Our experimental data showed that receding contact angles of the water droplet increased with the decreasing of the scale of the micropillars during evaporation,even though the solid area fractions of the microstructured substrates remained constant.We also experimentally found that the critical contact diameters of the transition between the Cassie-Baxter and Wenzel states are affected not only by the geometrical parameters of the microstructures,but also by the initial volume of the water droplet.The measured critical pressure is consistent with the theoretical model,which validated the pressure-induced impalement mechanism for the wetting state transition.

Rebound behaviors of droplets impacting on a superhydrophobic surface

The rebound behaviors of droplets impacting on a self-fabricated superhydrophobic brass surface （WCA=I64.5°） were ob- served and studied by using high-speed-camera. In accordance with energy conversion, theoretical analysis of different behav- iors and rebound mechanism were given. At lower velocities, three behaviors in different velocity ranges were observed： par- tial rebounding, entire rebounding and ejecting during rebounding. At higher velocities, such two behaviors as rebound after splashing and rebound, ejecting after splashing, occurred alternately and exhibited certain periodicity. A function to predict the critical impact velocity is derived from energy conservation condition, and the prediction values tally with the experimental values, with the maximum relative error about 14%.

An experimental investigation into the icing and melting process of a water droplet impinging onto a superhydrophobic surface

The freezing and melting process of a small water droplet on a superhydrophobic cold surface was investigated using the Laser Induced Fluorescence(LIF)technique.The superhydrophobic surface was prepared using a sol-gel method on a red copper test plate.From the obtained fluorescence images,the phase transition characteristics during the freezing and melting process of a water droplet were clearly observed.It was found that,at the beginning of the droplet freezing process,liquid water turned into ice at a very fast rate.Such phase transition process decreased gradually with time and the volume of frozen ice approached a constant value at the end of the icing process.In addition,the freezing time was found to reduce with the decrease of the test plate temperature.Besides,when the test plate temperature is relatively high,the effect of droplet volume on the freezing time is very significant.Over all,we provide some tentative insights into the microphysical process related to the icing and melting process of water droplets.