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.     

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...     

固体表面振荡液滴接触角演化

固体表面液滴振荡存在于很多实际生产当中,了解和掌握其中的界面特性对于实际的生产具有指导意义。利用可视化实验台对吹风条件下固体表面液滴的振荡现象进行了观察。实验在不同的条件下进行,着重风速、液滴尺寸和表面粗糙度等因素对实验现象的影响。实验观察到液滴与固体壁面接触角在振荡过程中有一定的变化规律,而接触线在振荡过程中始终保持不动。通过对实验结果的分类整理和对比,发现固体表面振荡液滴接触角变化和风速、液滴尺寸及表面粗糙度存在一定的关系。最后,通过接触线力平衡条件初步分析解释了接触角的演化规律。     

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。     

粗糙表面非极性辐射器近场热光伏系统性能

研究粗糙表面效应对由非极性辐射器石墨以及砷化铟（In As）电池构成的近场热光伏系统性能的影响,分析其中的系统性能与辐射换热过程、光电转换过程的量化关系。针对辐射换热过程,利用等效多层近似法以及涨落电动力学求得不同粗糙度下的辐射换热量。对于光电转换过程,使用解析近似法获得系统的功率密度及效率。分析表明,一定的表面粗糙度可提升系统的性能,在50～200 nm的平均距离下,5 nm以内的粗糙度最多能提升10.7%的功率密度以及1.5%的效率。     

水平管降膜冷态流动形态及滞后现象实验研究

以25.00 mm铝黄铜管水平管外降膜流动过程为研究对象,对以水为工质的管间流动形态及管间距对临界雷诺数的影响进行研究,基于观察到的流动形态变化的五个阶段分析了流型转换的滞后现象。计算了泰勒波长理论值并与实验测量值进行对比分析。研究结果表明,流动形态的五个阶段,临界雷诺数随管间距增大而增大,滞后现象存在于各个临界流态转化点中,柱帘-帘阶段滞后区间最大,实验泰勒波长较理论值略低,且在理论值附近波动。     

纳米颗粒的流化特征及滞后现象研究

为研究纳米颗粒的流化特征以及滞后现象,通过实验考察SiO_2、Al_2O_3和TiO_2纳米颗粒的流化行为,详细比较了在纳米颗粒流化过程中正向和反向的流化状态及床层压降之间的差异.结果表明:在稳定流化状态下,3种纳米颗粒的床层压降和流化状态有所差异,SiO_2表现为聚团散式流态化,Al_2O_3和TiO_2表现为聚团鼓泡流态化;3种纳米颗粒的正向流化均出现了显著滞后,在正向流化过程中床层压降和膨胀高度波动较大,而反向流化时相对平稳;通过设计"正-反-正"流化实验,发现二次正向流化曲线与反向流化曲线几乎重合,没有出现滞后现象,说明初始床层纳米颗粒的堆积结构和内聚力是引起流化滞后的原因.     

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。     

加热板液滴蒸发的理论分析及实验研究

液滴蒸发是由气-液浓度差驱动的一种常见而复杂的扩散现象.通过实验与理论相结合对去离子水在玻璃表面和有机硅油表面的蒸发特性进行研究,测量了液滴接触角和接触直径随时间的动态演变过程.结果 发现:玻璃表面的液滴蒸发为典型的定底半径模式和混合模式;而液滴在有机硅油表面较为特殊,除了定底半径模式和混合模式还有周期性的黏滑模式.出...     

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。     

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     

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.     

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     

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.     

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     

Experimental investigation of the effects of orientation angle on heat transfer performance of pin-finned surfaces in natural convection

Natural convection heat transfer in air from a pin-finned surface is investigated experimentally by considering the effect of radiation heat transfer. The plate was oriented as the pin arrays facing either downwards or upwards from vertical axis with different angles and the experiments were performed for different values of heater power input. From the results of the experiments it is observed that the pin fins increase the heat transfer considerably when compared to the unpinned surface. The upfacing pins are more enhancing heat transfer than the downfacing pins and the enhancement is decreasing with increasing orientation angle from the vertical axis.     

Motion of droplets induced by the Marangoni force on a wall with a temperature gradient

Motion of silicone oil and water droplets induced by the Marangoni force was numerically simulated by using two‐ and three‐dimensional second‐order finite difference methods with the CIP and the level set methods. The surface tension was introduced by the continuum surface force (CSF) method. The results clearly showed the flow induced by the Marangoni force and the dependence of droplet velocity on droplet size, contact angle, temperature gradient, and fluid properties. The Marangoni force balanced with the viscous force in the small contact angle case; on the other hand, in the large contact angle case, it balanced with the normal component of surface tension. As for the effect of fluid properties on droplet motion, the temperature coefficient of surface tension had a much larger effect than did viscosity, thermal diffusivity, or surface tension. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(2): 81–93, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20004     

Numerical simulation of two-phase cross flow in microstructure of gas diffusion layer with variable contact angle

For a gas diffusion layer (GDL) with hydrophobic treatment, its hydrophobicity (contact angle) may change along the through-plane direction, and lead to different two-phase transport characteristics. In this study, such variable contact angle is implemented in a three-dimensional unsteady two-phase model based on the microstructure of GDL to study the liquid water transport characteristics along the in-plane direction caused by cross flow. It is found that during a liquid water intrusion process, the liquid water first moves through some of the pores that are easy to penetrate, forming a “fingering transport” mode; and after that, with more liquid water accumulated, the rest of the pores can also be filled, forming a “steady transport” mode. Increasing the differential pressure or decreasing the contact angle of GDL accelerates the liquid water intrusion, and this effect is weakened at higher differential pressures and contact angles. For a GDL with variable contact angle, the water transport characteristics in different cross sections normal to the through-plane direction with different contact angles are similar to the corresponding fixed contact angle cases in these cross sections, and the overall process of water intrusion with variable contact angle is similar to its corresponding average fixed contact angle case.