滴状冷凝传热的理论模型与计算

滴状冷凝传机理已有一些报道，作者提出了一个新的观点并建立了一个计算滴状冷凝传热的新模型，模型通过计算液滴之间的液膜传热和液滴传热并结合液滴分布，获得了滴状冷凝的总传热系数。该模型仅含少量几个主要参数，如温差，液膜厚度，脱离液膜半径及液滴尺寸分布，滴状冷凝的传热系数和热通量的理论计算结果与文献值符合较好，说明该模型能较好地描述滴状冷凝传热的实际行为。     

疏水表面冷凝的可控毛细力微对象操作方法与实验

提出一种疏水表面冷凝的可控毛细力微操作方法,所研制的液滴操作手可实现操作液滴的动态控制,相应地调控液桥毛细力.建立疏水表面冷凝的单液滴生长模型,分析最小液滴半径、过冷度、饱和温度等参数的影响;通过拾取和释放理论模型,讨论微对象的拾取和释放进程;搭建微操作实验系统,实验分析疏水探针端面液滴冷凝及影响毛细力变化的因素.1 mm×1 mm×0.52 mm微型硅片(重力12.1μN)和直径200μm、壁厚4μm薄壁微球(重力5.069 n N)的操作实验验证了该方法的有效性.     

气化驱动的液滴在湿润性表面上的仿真研究

为了研究液滴在高温锯齿形表面时各参数对液滴摩擦学性能的影响,运用了计算流体力学研究了表面润湿性对蒸发驱动的Leidenforst液滴自推进运动的影响.结果 表明,三线接触角对液滴的悬浮和运动特性有很大的影响.表面微结构参数H/W有助于控制液滴表面的润湿性,从而提高液滴的运动速度.数值模拟结果表明,高宽比H/W=7/12时的超疏水表面具有快速运动特性.研究了温度和液滴直径等各种参数的影响,对液滴的蒸发驱动机理有了深入的认识.     

内混式空气雾化喷嘴雾化性能的实验及模拟研究

费托合成装置气液分离器的分离性能研究中,雾滴粒径的准确测量和控制是关键。本文对内混式空气雾化喷嘴的喷雾性能进行了实验研究,考察了液滴索特尔直径D_(32)的轴向分布,以及气液比、液体流量与黏度对其的影响规律;同时结合CFD数值模拟,进一步研究了气液相对速度、雾化锥角及液体表面张力对液滴D_(32)的影响。结果表明:液滴D_(32)沿喷雾方向不断增大;液体雾化主要通过喷嘴内气流对液膜的剪切、冲击作用,因此增大气液比可增强雾化效果,但存在"饱和"现象;液量的增大不利于液膜破碎雾化;增大气液相对速度可显著提高雾化效果;同时,液体黏度和表面张力对雾化效果影响很大,黏度或表面张力越大,雾化显得更为困难。此外,液滴轴向速度沿喷雾方向先迅速下降随后缓慢降低。     

气泡雾化喷嘴雾化射流场性能仿真

研究气泡雾化喷嘴外部射流场的特性问题,由于喷嘴出口处液膜破碎过程的复杂性,有时会影响雾化质量.为了提高雾化性能,提出将雾化过程细分两个子过程,利用FORTRAN语言程序计算液膜破碎过程,得出液滴初始数据,再用计算流体仿真软件Fluent,建立描述气流-液滴两相流动的合理模型,用Realizable κ-ε湍流模型与DPM离散相组合,进行二次雾化过程,并考虑气液两相间的耦合作用以及液滴破碎和碰撞模型,对雾化射流中的液滴粒径SMD沿喷嘴轴向的分布进行仿真,计算结果与实验数据进行比较吻合较好,可为雾化性能的优化提供有效的数据支持.     

电导率测量合成氨冷凝液中氨浓度的应用

测定合成氨生产过程中的工艺冷凝液的回收利用是一项很重要的节能措施。但是合成氨系统的冷凝液往往由于管壁渗漏,使少量氨以游离状或铵盐形式存在于冷凝液中。而为了确保合成氨系统工艺冷凝液回收装置回收的工艺冷凝液不影响后续工序的正常运行,工艺冷凝液中氨等杂质的含量必须控制在一定的范围内,因此精确测定冷凝液中氨含量是回收利用冷凝液的先决条件。测定工艺冷凝液中氨离子用选择电极等测定法,该方法在工艺中受温度、压力和膜头密封等因素的影响使其不能正常与运行、且仪器价格昂贵。我公司采用电导率和氨根离子之间的浓度关系诺模图快速准确地反映合成氨冷凝液浓度值,满足合成氨冷凝液回收装置的运行要求。     

接触角滞后对变截面微通道内水银液滴流动特性影响的数值仿真研究

微通道中气-液两相流的流动特性复杂,影响因素众多,表面效应成为了微流体流动的主要影响因素。在表面张力作用下,静态接触角由固相和液相物性决定,而动态接触角和接触角滞后则受表面粗糙度、表面不均匀性、表面污染等很多因素的影响。本文利用FLUENT中的VOF-CSF模型研究了惯性微流体开关中水银微液滴在变截面微通道内两相流的流动特性。通过动态和静态接触角滞后的UDF函数对变截面微通道中内流动特性的影响进行了数值仿真分析。结果表明,接触角滞后对水银微液滴的流动特性有重要影响,接触角滞后性越大,水银液滴越难通过微阀而进入储液槽闭合信号电极。由于水银液滴在加速度作用下接触线运动速度较低,动态接触角对流动特性的影响可以忽略,动态接触角可由静态接触角代替。     

磁记录非牛顿液膜润滑仿真

随着记录密度的不断提高，头盘浮动间隙不断减小。目前近场光、磁混合记录已经在研究10nm以下的浮动系统。由于浮动间隙已远小于空气分子平均自由程(室温下约65nm)而有使气膜浮动技术有失效的危险。非牛顿液膜浮动技术就是在这种背景下在近几年备受关注的。该文提出了一个新的非牛顿润滑液膜，建立了考虑表面粗糙度影响的非牛顿液膜浮动系统模型，对不同表面粗糙度形状系数进行了仿真研究。研究表明，非牛顿液膜浮动系统的性能与气膜的情况有很大的差异。     

用串级控制器回收冷凝液

输入到再沸器或加热器的热量可由调节蒸汽流量或调节露出表面面积加以控制。当使用低压蒸汽时(50磅/时~2,表压),调节蒸汽流量可能不适用于冷凝液返回处于汽柜压力下的除氧器。由于过冷冷凝液的蒸汽压太低(冷凝液低于212°F)所以有时用泵使冷凝液返回。     

液滴微操作机械手的机理分析与实验

液滴微操作机械手由1根毛细微管和环绕在其周围的6根钨丝微棒组成,注入到毛细微管内的液体,在机械手末端形成液滴.基于液滴的表面张力吸取微小物体,通过控制液桥的形态改变液桥力,可以实现对微小物体姿态变换和释放等操作.建立液滴微操作机械手工作过程的力学模型,详细讨论了液滴微操作机械手对微小物体进行拾取、姿态调整、释放的方法和物理过程,分析了微操作过程中各种微力的作用机理、作用方式、作用条件以及影响液滴微操作机械手性能的因素;并通过实验验证了分析方法的正确性.     

Thin film evaporation effect on heat transport capability in a grooved heat pipe

A theoretical model predicting the heat transfer performance occurring in a grooved heat pipe is developed. The model includes the effects of groove geometry, thin film evaporation, contact angle, and film condensation. The numerical results show that the groove geometry significantly affects the thin film evaporation and condensation. The thin film evaporation plays a key role in the total effective thermal conductivity and determines a limit for the maximum amount of heat transport through the micro regions for a given evaporator geometry. While the contact angle can influence the capillary limitation, it significantly affects the thin film evaporation and the total effective thermal conductivity of a groove heat pipe. In order to verify the theoretical analysis, an experimental investigation on a grooved heat pipe was conducted. The current investigation will result in a better understanding of thin film evaporation and its effect on the maximum heat transport in a grooved heat pipe.     

Investigation of coalesced droplet vertical jumping and horizontal moving on textured surface using the lattice Boltzmann method

The coalesced droplet vertical jumping and horizontal moving on conical posts textured surface are numerically studied using the three-dimensional (3D) multi-relaxation-time (MRT) pseudopotential lattice Boltzmann model. The influences of wettability gradient and roughness gradient are investigated systematically. It is found that the coalesced droplet on the flat and conical posts textured surfaces can move horizontally from superhydrophobic bend to hydrophobic bend without the roughness gradient. Moreover, the coalesced droplet is able to spontaneously move from lower conical post density region to higher conical post density region without the wettability gradient. Specifically, the in-line array textured surface is more beneficial to the coalesced droplet horizontal moving than the staggered array at the same wettability parameter. However, the staggered array textured surface is more beneficial to the coalesced droplet vertical jumping than the in-line array. The hybrid effect of wettability gradient and roughness gradient plays critical roles in coalesced droplet vertical jumping and horizontal moving. The present work demonstrates that the dropwise condensation heat transfer can be enhanced in a self-sustained manner if the wettability and roughness of the textured surface are properly designed. It is also confirmed that the 3D MRT pseudopotential lattice Boltzmann model is of potential to simulate coalesced droplet behaviors on textured surface.     

Laminar film condensation on a horizontal disk with suction at the wall

The problem of two-dimensional, steady-state film condensation on an isothermal horizontal disk with suction at the porous wall is studied for the case in which a cold plate faces upward. The dimensionless film thickness along the disk is found to be a function of parameter Ja/Pr (Jakob number/Prandtl number) and the suction parameter S_w. An essential part of the present analysis is the use of the condition that the boundary layer depth at the disk edge is equal to a critical(minimum) depth. The dimensionless heat transfer coefficients are also found to be functions of parameters Ja/Pr and Ra/Ja. Furthermore, the dimensionless heat transfer coefficient increases as suction parameter S_w increases.     

Evaporative heat transfer characteristics of industrial safety helmets

Thermal discomfort is one of the major complaints from the wearers of industrial safety helmets. While studies have been reported on dry heat transfer (conduction, convection and radiation) in safety helmets, the investigation of wet heat dissipating (evaporation) properties has not been found in the literature. To evaluate experimentally the evaporative heat transfer characteristics of industrial safety helmets, a method was developed to simulate sweating of a human head on a thermal head manikin, and to use this manikin to assess the wet heat transfer of five industrial safety helmets. A thermal head manikin was covered with a form-fitting cotton stocking to simulate 'skin'. The skin was wetted with distilled water to simulate 'sweating'. A form-fitting perforated polyethylene film was used to cover the wetted stocking to control the skin wettedness at two levels, 0.64 and 1.0. Experiments were conducted in a climatic chamber, under the following conditions: the ambient temperature = head manikin surface temperature = 34 +/- 0.5 degrees C; ambient relative humidity 30% and 60%. Also, the effects of wind and a simulated solar heat load were investigated. The five helmets showed statistically significant difference in evaporative heat transfer under the experimental conditions. Skin wettedness, ambient humidity, wind and solar heat showed significant effects on evaporative heat transfer. These effects were different for the different helmets.     

升压汽水喷射器的理论模型及输出量的解耦控制

针对汽水喷射器内所产生的凝结和激波现象,使其升压过程和机理非常复杂,提出利用直接接触凝结理论建立升压汽水喷射器的理论模型,并对求解该模型的一些关键问题进行论述,如采用平均凝结换热系数计算相间质量传递及利用汽羽确定各相体积分数等.根据该理论模型给出升压式汽水喷射器出口温度和流量的调节方案,理论分析表明出口温度及流量的调节...     

Fluid flow and heat transfer in the evaporating thin film region

The evaporating thin film region is an extended meniscus beyond the apparent contact line at a liquid/solid interface. Thin film evaporation plays a key role in a highly efficient heat pipe. A detailed mathematical model predicting fluid flow and heat transfer through the thin film region is developed. The model considers the effects of inertial force, disjoining pressure, surface tension, and curvature. Utilizing the order analysis, the model is simplified and can be numerically solved for the thin film profile, interfacial temperature, meniscus radius, heat flux distribution, velocity distribution, and mass flow rate in the evaporating thin film region. The prediction shows that while the inertial force can affect the thin film profile, interfacial temperature, meniscus radius, heat flux distribution, velocity distribution, and mass flow rate, in particular, near the non-evaporating region, the effect can be neglected. It is found that a maximum velocity, a maximum heat flux, and a maximum curvature exist for a given superheat, but the locations for these maximum values are different.     

Thermal conductivity measurement of submicrometer-scale silicon dioxide films by an extended micro-Raman method

The micro-Raman method is a noncontact and nondestructive method for thin film thermal conductivity measurements. To apply the micro-Raman method, however, the thickness of the film must be at least tens of micrometers. An analytical heat transfer model is presented in this work to extend the micro-Raman measurement method to measure the thermal conductivity of thin films with submicrometer- or nanometer-scale thickness. The model describes the heat transfer process in the thin film and substrate considering the effects of thin film thickness, interface thermal resistance, thermal conductivity of the thin film and substrate. From this heat transfer model, an analytical expression for the thermal conductivity of the thin film is derived. Experiments were successfully performed to measure the thermal conductivity of 200, 300 and 500 nm thickness silicon dioxide films using the extended micro-Raman measurement method, with results confirming the accuracy and validity of the extended model.     

复杂表面小尺度凝结现象真实感仿真

凝结是自然界中很常见的一种现象,其真实感仿真对虚拟现实、影视特效及游戏娱乐等领域都有重要的意义.针对传统的基于物理的仿真方法难以保持凝结小尺度液滴细节的问题,提出一种基于光滑粒子流体动力学与自适应流体隐式粒子法耦合的仿真方法.首先基于光滑粒子流体动力学离散建模空气的热传导,并辅以相对湿度模型和露点描述相变过程;然后结合基于八叉树的自适应背景网格和流体隐式粒子法仿真凝结液滴,突出了小尺度液滴高精度的细节;最后引入表面张力、黏附力与阻力,逼真地仿真了复杂固体表面上液滴的运动.实验结果表明,该方法可以真实、高效地反映复杂固体表面的凝结现象.     

A Novel Benzocyclobutene-Based Device for Studying the Dynamics of Heat Transfer During the Nucleation Process

A novel microelectromechanical device has been developed to study the details of the heat transfer mechanisms involved at the nucleation site for the nucleate boiling process. This device enables quantifying the magnitude, time period of activation, and specific areas of influence of different mechanisms of heat transfer from the surface with a resolution several times greater than previously reported. This is achieved through the use of an array of embedded temperature sensors within a carefully designed dual-layer (silicon and benzocyclobutene) wall which allows for the accurate calculation of local heat flux, circumventing difficulties encountered when using existing methods. The sensors are radially distributed around the nucleation site. Heat is supplied to the wall by a thin film heater fabricated on the outer nonwetted surface. Single bubbles are generated at the center of the array while the temperatures and the bubble images are recorded with a sampling frequency of 8 kHz. The temperature data provided the necessary thermal boundary conditions to numerically calculate the surface heat flux with an unprecedented radial resolution of 22-40 mum. Fabrication, characterization, and the ability of the developed device to elucidate the heat transfer aspects of the nucleation process are demonstrated.