20Cr13马氏体不锈钢表面粗糙分析及改进

工业生成的20Cr13马氏体不锈钢钢卷在制造餐具刀叉时出现了表面粗糙缺陷,为了分析该缺陷的产生原因,通过对两个批次生产的20Cr13实验钢卷进行化学成分分析、对比加热炉残氧含量以及观察表面氧化皮的形貌与厚度,结果表明:当钢卷中的Cr和Si元素含量的较大时,会导致难以去除的FeCr2O4的尖晶石复合氧化物和铁橄榄石Fe2...     

粗糙表面流动与换热的数值模拟的新模型

考虑“粗糙元高度”和“粗糙元间距”两个因素的影响，对Van Drist阻尼函数进行了改进，然后将其引入边界层计算程序，对不同粗糙远间距、不同来流速度等工况下粗糙表面的换热进行了计算，结果与实验数据符合良好，说明改进后的Van Drist阻尼函数很好地反映了粗糙度雷诺数及粗糙元分布疏密度的影响，能适用于计算粗糙表面上的流动及换热。     

超声波垂直入射于固体粗糙平面时发生波型转换现象分析

本文以实际锻件探途中之异常回波为对象进行分析,证明超声波垂直入射于固体粗糙平面时,确有滤型转换现象发生,并试用棱边再生源原理对这一现象进行解释。     

地球变暖动态特性及滞后现象分析

工业生产过程中排放的温室气体会造成全球变暖现象,但全球变暖与工业排放在时间上具有一定的滞后效应.通过分析地球、大气、太阳三者热平衡体系的辐射换热,建立了地球及其大气的动态数学模型;利用此模型考察了造成地球温度变化的主要原因和变暖滞后的现象.结果表明:工业温室气体的过度排放会造成大气对地球辐射的吸收系数提高,导致地球温度升高;同时,太阳辐射能量增加,地球和大气对太阳辐射吸收增加,导致地球温度升高.结合近年来人为因素造成的地球温度升高现象进行了定量热分析,预测了温室气体CO2体积分数线性增加条件下的地球温度走势.     

微槽道热管低温启动滞后现象的分析

小型铜-水微槽道热管在水平或负倾角位置,低温状态下启动时,可能发生启动滞后现象。对此现象进行试验研究和理论分析,得出以下结论:在管内蒸汽温度20℃附近,Kn大于0.01,蒸汽进入不连续流动区,与微槽道侧壁面之间产生了速度滑移和温度突跃,导致蒸汽与壁面产生了明显的温度差,发生启动滞后现象。随着热管管内蒸汽温度的升高,Kn逐渐下降,蒸汽逐渐进入连续流动区,速度滑移和温度突跃消失,蒸汽与壁面之间换热状况改善,壁面温度回落,进入正常启动过程。理论分析与实验结果基本吻合。     

换热表面直接与间接测温的对比性实验研究

进行了沸腾换热表面的间接测温与直接测温的对比性实验研究。以水和乙醇作工质,对大气压力下的池沸腾换热平表面,用间接测温法和直接测温法同时测量壁面过热度,对两种测温方法的结果进行了比较研究。同时,用直接测温法进行三种不同管径的光管沸腾试验,并把实验结果与Rohsenow公式进行了比较。大量实验结果表明,在一定操作条件下,直接测温法测量沸腾换热表面过热度误差较小,是一种简便而又行之有效的方法。     

Marangoni凝结表面传热系数的影响因素分析

对水和酒精混合蒸气在竖直平板上的凝结传热进行了研究。利用液膜覆盖率,分别按考虑和忽略浓度边界层扩散热阻计算了凝结表面传热系数,并将计算结果与实验值进行了比较。结果发现同时考虑液膜导热热阻及浓度边界层扩散热阻的计算值在极小酒精质量分数时高于实验值,在较小酒精质量分数时和实验值接近,但是在高酒精质量分数时低于实验值。由此可见计算凝结表面传热系数时,在小酒精质量分数条件下扩散热阻可以忽略,但在高酒精质量分数条件下扩散热阻对整个热阻的贡献较大,必须考虑其对传热的影响。     

不凝结气体对多壁碳纳米管换热表面的池沸腾性能的影响

为揭示不凝结气体对多壁碳纳米管（Multi-walled Carbon Nanotube, MWCNT）纳米结构表面核态池沸腾过程的影响,使用气体沉积法（Chemical Vapor Deposition, CVD）在硅表面制作MWCNT纳米结构表面,并使用光滑硅表面进行对比实验研究。实验操作中,将驱气前后的工作液体应用于两种表面的池沸腾实验,换热表面过热度控制在0℃-35℃,工作液体过冷度分为40℃和50℃。实验结果表明,液体中含气量的变化对MWCNT纳米结构表面影响较小,而对光滑硅表面的影响较大;对比硅表面,MWCNT纳米结构表面能够有效提升沸腾传热效果,对于驱气后的工作液体提升效果更为明显。     

有无切削液铣削B65A-S时表面粗糙度经验模型的建立与分析

利用均匀试验方法在有无切削液条件下对B65A—S进行了铣削试验,利用最小二乘法对试验数据进行了回归分析,建立了干切削和湿切削时表面粗糙度的经验模型,得出了切削参数及切削液对表面粗糙度的影响规律。     

高电压直流复合绝缘子表面电荷积聚的有限元数值模拟及影响分析

空间直流电场和伞裙表面电荷静电将造成高电压直流工程用复合绝缘子伞裙表面快速并大量积聚电荷,引起绝缘子沿面电场畸变且闪络电压降低,从而导致高电压直流工程用复合绝缘子的绝缘性能和耐老化性能下降。以FXBW6-10/70型号的高电压直流输电线路用复合绝缘子为例,建立复合绝缘子二维仿真模型,采用有限元法对表面存在有电荷的高电压直流复合绝缘子进行电气特性分析,研究表面积聚电荷的正、负极性以及电荷积聚量对伞裙轴向和径向电场分布的影响规律,采用改进的电容探头测量伞裙表面积聚电荷随时间的动态变化过程,验证有限元仿真的有效性。结果表明：导线侧三交汇点处场强最大值与伞裙表面积聚的负极性电荷量呈负相关,与伞裙表面积聚的正极性电荷量呈正相关;伞裙表面积聚电荷存在由伞裙内侧逐步向伞裙边沿扩散的动态特性。     

Contact angle hysteresis on rough solid surfaces

With the introduction of an additional interfacial tension (or hysteresis tension) to describe the effect of the surficial roughness on the liquid‐solid contact angle hysteresis, both contact angle and its hysteresis were derived from mechanical equilibrium and were perfectly consistent with thermodynamics. This unifies the mechanical and thermodynamic performance, or removes the contradiction existing in classical mechanical and thermodynamic understanding. By combining the sessile drop method with CCD camera and the digital imaging techniques, the contact angle hysteresis of distilled water, diethylene glycol, and 1‐butyl alcohol on stainless steel surfaces was measured. The results of the experiments agree very well with the predictions of the hysteresis tension model. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(4): 201–210, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20013     

Droplet behaviors on inclined surfaces with dynamic contact angle

Dynamic contact angle (DCA) is of crucial importance in the numerical investigation of water management problems in proton exchange membrane fuel cells (PEMFCs). In order to well predict the gas-liquid two-phase flow in the complex flow field in PEMFCs, first, it is very critical to build a robust DCA model that is capable of simulating droplet behaviors on a single surface under various conditions. In our previous research work, an advancing-receding DCA (AR-DCA) model is developed and has been successfully validated against a series of experiments from the available literature for droplet impact on surfaces. In this study, the AR-DCA model is further applied to simulate droplet behaviors on inclined surfaces with different droplet impact velocities, impact angles and viscosities. It is found that the droplet spreading and deformation from the simulations have excellent agreement with those captured in the corresponding experiments. The results also indicate that higher impact velocity and impact angle can facilitate the spreading trend at the droplet trailing edge and have no notable effects on the leading edge. In addition, the increase of droplet viscosity leads to a transition from droplet deposition phenomenon to partial rebound on the surface.     

Wetting on a plate with three‐dimensional random roughness and heterogeneity

A theoretical study was conducted to investigate the wetting behavior of liquid meniscus on a vertical plate with three‐dimensional random characteristics of heterogeneity and roughness. The thermodynamic stable condition was derived by considering the minimum of system free energy. The local stable condition leads to a result similar to that obtained for a plate with two‐dimensional characteristics, that is, the system has many meta‐stable states. For the stable condition of the whole system, a relation was derived between the macroscopically observed contact angle and the surface characteristics. The product of cosine of the contact angle and liquid surface tension is equal to the energy difference for the liquid to wet the plate by apparent unit area. If the liquid wets the solid surface reversibly, there is only one contact angle observed macroscopically. This fact suggests that the contact angle hysteresis is caused by the irreversible motion when the liquid advances or recedes on the solid surface. The well‐known Cassie's and Wenzel's contact angles are explained as those corresponding to a thermodynamically stable condition when the liquid wets the solid reversibly. © 2001 Scripta Technica, Heat Trans Asian Res, 30(5): 371–382, 2001     

An improved sessile drop method for assessing the wettability of heterogeneous coal surface

To assess the wettability of heterogeneous coal surface, a new modified sessile drop technique named volume–length method is proposed to calculate the contact angle according to the spreading ability of droplet on the coal surface. The tested contact angles were compared with the predicted contact angles using Cassie–Baxter model. Low ash coal and kaoline powders are mixed with different mass ratios and mixing degrees. The mixtures were pressed to plates for sessile drop contact angle tests. The surface chemical component and nonuniform degree determine the droplet spreading on the coal surface, thereby affecting the contact angle.     

Comparisons and validations of contact angle models

In the numerical simulation of water management for proton exchange membrane fuel cells (PEMFCs), the static contact angle (SCA) model is generally used. However, an empirical correlation for dynamic contact angle (DCA), known as Hoffman function or Kistler's law, was recently employed to numerically simulate the droplet behaviors either in a microchannel or on a surface. In this paper, for the first time, a DCA evolution map is created based on Hoffman function and related experiments to better understand the DCA evolving mechanism; based on this evolution map, the Advancing-Receding DCA (AR-DCA) model is proposed and explained, in addition to the Advancing DCA (A-DCA) model that is based on the original Hoffman's experiments; using user defined function (UDF), the A-DCA and AR-DCA models are implemented with Volume of Fluid (VOF) method in ANSYS Fluent; a series of numerical simulations are conducted with the SCA, A-DCA and AR-DCA models for droplet impact on horizontal and inclined surfaces; the validations of these contact angle models are performed, qualitatively and quantitatively, by comparing the numerical simulation results with the corresponding experimental results from the literature. It is indicated that the AR-DCA model can better simulate the droplet deformation and evolvement, showing its potential for the DCA simulations in a more complex gas-liquid flow domain such as the cathode of PEMFCs.     

Effect of contact angle on wetting limit temperature

The effect of surface wettability on evaporation of a water drop has been examined experimentally using surfaces with various contact angles. To greatly change the surface wettability, TiO₂ superhydrophilicity, plasma irradiation, and super‐water‐repellent surface are adopted as the heating surface. The range in contact angle achieved by these methods was between 0^° and 170^°. The relationship between the contact angle and the wetting limit temperature was obtained and it was found that the lifetime of a water drop dramatically decreases with contact angle in the lower temperature region, and that the wetting limit temperature increases with the contact angle. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(7): 513–526, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20128     

Improvement and further investigation on Hoffman-function-based dynamic contact angle model

Dynamic contact angle (DCA) plays a critical role in the numerical investigation of gas-liquid two-phase flow problem in proton exchange membrane fuel cells (PEMFCs). In our previous study, an advancing-receding DCA (AR-DCA) model was developed based on Hoffman function and has been successfully validated. This study aims to further modify the contact line velocity evaluation method in the AR-DCA model and an improved-AR-DCA (i-AR-DCA) model is proposed. The simulations of droplet impact on inclined surface and liquid water behaviors in microchannel are conducted based on these two models and the results show that the improved methodology has no significant effects on general droplet spreading process on inclined surface; whereas it can facilitate the liquid water detachment in the microchannel. In addition, the Hoffman function is modified into three different forms based on the original experimental data points in order to test the sensitivity of different formulae on the numerical results. It is indicated that the modified forms of Hoffman function can induce notable changes in the liquid water slug elongation in the microchannel.     

Technical challenges in numerical simulation of droplet behaviors with dynamic contact angle in microchannels

Droplet behaviors play a major role in water management of proton exchange membrane fuel cells. Contact angle, as one of the critical parameters in the boundary conditions for the droplet dynamics, can greatly affect the simulation results for droplet deformation and evolvement. Recently, the dynamic contact angle (DCA) model implemented with Hoffman function has been successfully validated in the simulation of droplet impact on surfaces. In this paper, the Hoffman function is further applied to simulate liquid water slug flow in a straight microchannel with the volume of fluid (VOF) method. It is found that the numerical results are difficult to well match the corresponding experimental results under the same reported experimental conditions. However, the numerical results with lower gas inlet velocity can significantly improve the comparison. It is indicated that the DCA model coupled with Hoffman function has limitations in the simulation of liquid water behaviors with surrounding flows and needs to be further developed. In addition, a series of numerical simulations are conducted with different air inlet velocities, surface tensions, and viscosities to investigate the effects of these factors on the droplet behaviors. The technical challenges in the current research progress for DCA simulation with Hoffman function and the VOF method are also proposed and discussed.     

Transient characteristics of thermal contact conductance between isotropic rough surfaces of metals

Transient measurements of thermal contact conductance are made on the interface between isotropic rough surfaces of metals in air. We present an analytical solution for temperature distribution of the one‐dimensional symmetric system with the condition of time‐dependent temperatures at two points in each body, and thereby interface temperature drops and heat fluxes can be obtained without the condition of heat‐flux continuity at the interface. Contacting surfaces of rod samples (Naval brass, JIS?SK5 carbon tool steel, and JIS?SUS 304 stainless steel) of 25‐mm diameter are uniformly polished using an emery 320 paper. Transient characteristics of both temperatures and heat fluxes at the interface are experimentally determined using the analytical solution. It is revealed through the transient experiment that the thermal contact conductances are not constant at the early stage, but rapidly increase from zero and that the discontinuity of interface heat‐flux is observed by about 20 percent for all metal pairs. For the contact between dissimilar metals, the dependence of thermal contact conductance on the direction of heat flow is not distinguishable. © 2001 Scripta Technica, Heat Trans Asian Res, 30(4): 341–356, 2001     

Numerical study of flow regimes in microchannel with dynamic contact angle

Liquid water flow behavior in the microchannel is of crucial importance to the water management in proton exchange membrane fuel cells (PEMFCs). In this study, the liquid water flow regimes in a single straight microchannel are numerically investigated using the volume of fluid (VOF) method with the dynamic contact angle (DCA). Various air and liquid water inlet flow velocities are considered in the simulation to study their effects on the gas-liquid behaviors and flow structure. It was found that the liquid water injection rate is the dominant factor for the formation of different flow regimes: the increase of water inlet velocity will lead to the transition from squeezing flow to partial-jetting flow and jetting flow. Meanwhile, the air inlet velocity can also significantly affect the flow patterns: the higher inertial air flow will facilitate the detachment of liquid water under the squeezing flow, and greatly accelerate the transition process from liquid water blob to the film under the jetting flow.