微通道流动沸腾气泡受限特性实验研究

本文利用可视化手段实验研究矩形截面微通道(1 mm×0.5 mm)内发生流动沸腾时气泡的生长及受限现象。实验使用去离子水为工质,气泡受限过程由高速CCD相机观察并记录。研究发现,气泡高度方向通道壁面的存在对气泡在该方向的生长产生强烈的限制作用,气泡生长后期顶端气液界面在并未与限制壁面接触时其曲率便明显减小,壁面对气泡生长的限制作用通过其对气泡界面施加一个壁面限制力而体现.通过对比不同运行工况条件下的气泡受限现象,分析讨论了气泡受限过程中其界面形状变化规律及影响因素。     

多孔介质流动沸腾微观模型实验研究

多孔通道内流动沸腾广泛应用于热管、高效换热器、航天热防护等领域,认识孔隙尺度相变典型行为及特点有助于理解多孔介质内沸腾传热机理,进而改进多孔介质内部流体相变模型。基于此,本文搭建了二维多孔介质内流动沸腾实验台,对微米级多孔通道内部流动沸腾现象及气泡行为进行研究。实验发现在毛细力、黏性力和惯性力的共同作用下,多孔介质内的流动沸腾形态显著区别于直通道内的沸腾,存在局部气泡堵塞、合并和液膜蒸干、再润湿等行为。该研究有助于增强对相变过程的理解,并为多孔结构优化设计提供指导。     

小通道内液氮流动沸腾的可视化研究

通过高速CCD可视化实验,在气体表观速度0.01～26.5m/s,液体表观速度0.01～1.2m/s范围内,对内径为1.931mm垂直向上圆管内液氮流动沸腾的流型特性进行了研究.所观测的主要流型为:泡状流,弹状流,搅拌流和环状流.并绘制了流型图,发现环状流占了大部分的区域,干度大于0.15的区域基本上都是环状流.分析了流量对流型转变的影响,流量越大,相应的流型转变干度越低,而且流量大于820kg/m2s时,没有发现泡状流.通过与相同水力直径的空气-水的流型图比较,发现本文中的弹状流区域要小很多.通用的流型转变模型预测结果与实验结果相差较大.     

微通道内流动沸腾气泡生长特性数值研究

利用FLUENT软件,采用VOF多相流模型对水在矩形微通道内流动沸腾过程中气泡的生长进行了数值模拟,分析了壁面热流密度、入口质量通量和接触角对气泡生长速率的影响,并得到了气泡周围的温度和压力分布。结果表明,壁面热流密度和质量通量的增大导致气泡生长速度的增加,而最小的接触角有最大的气泡生长速度,气泡边界上的温度梯度和压力梯度都较大。     

HFE-7100平行微通道流动沸腾实验

微通道流动沸腾冷却技术兼具相变潜热和微尺度效应的诸多优点,是解决微电子器件热致失效问题的重要方法之一. HFE-7100是一种安全环保的电子氟化液,特别适用于微电子器件的冷却.本文在水力直径为0.5 mm的矩形平行微通道内,对HFE-7100的流动沸腾传热和两相流动特性进行了实验研究,测量范围为常压下质量流率88.9—277.8 kg·m^–2·s^–1、入口过冷度20.5—35.5℃和有效热流密度12—279 kW·m^–2.本文分析了质量流率、入口过冷度、有效热流密度和干度对传热系数和压降的影响,发现在较低的入口过冷度下HFE-7100出现了沸腾迟滞现象,且增大入口过冷度和质量流率会延缓沸腾起始点的发生,且会提高传热系数和临界热流密度.两相压降受有效热流密度影响较大,且在定干度下不同质量流率的两相压降在塞状流和环状流阶段有明显差异.同时,通过观测两相流型,对流动沸腾传热现象进行了分析.本文还将两相压降实验数据与文献关联式预测值进行了对比,与Lockhart提出的关联式预测值偏差为19.6%.本文研究结果可为微电子器件散热设...     

微通道内沸腾流动与传热特性耦合机理研究

以水为工质,模拟研究不同条件下水平矩形微通道沸腾流动过程中气泡发生发展及流型演变与温度、压力、传热系数的耦合关系。结果表明：持续吸热的弹状流会占据通道大部分流动区域,易造成堵塞和局部高温;泡状流区域压力波动幅度较小,较长弹状气泡和大雷诺数均会导致较大的局部压降;升高热流密度减小了单相区长度,强化了核态沸腾,提高了通道整体传热性能;增大雷诺数使得通道内气泡尺寸减小,减少了两相流动的不稳定与堵塞现象,通道整体传热性能得以提升。     

三角形微通道流动冷凝的实验研究

实验研究了三角形硅微通道中的流动冷凝.通道中的冷凝流型沿程主要有珠状流、环状流、喷射流和弹状-泡状流等.在同一通道中,喷射流位置随着工质流量的增大而延后;在相同蒸气入口雷诺数下,喷射流位置则随着通道尺度的增大而延后.喷射流频率随着蒸气入口雷诺数和冷凝液韦伯数的增大而增大.较小水力直径的三角形通道中的流动冷凝不稳定性较高.冷凝通道的壁面温度呈沿程下降趋势.在同一通道中,流动冷凝的平均冷凝传热系数和平均努塞尔数,皆随着蒸气入口雷诺数的增大而增大,通道尺度的减小显著强化冷凝传热.     

微米线结构表面的过冷流动沸腾实验研究

以去离子水为工质,配合高速摄像观测,研究了截面为0.5 mmx5mm的微细窄矩形通道内氧化锌微米线结构表面的竖直流动过冷沸腾.流量范围200～400 kg·m-2s-1,过冷度为10 K,热流密度最高为200 kW·m-2.分析了不同工况下过冷沸腾的沸腾曲线、平均换热系数、局部换热系数和流型特征.     

基于流动沸腾传热机理的分段式微通道结构优化研究

列车牵引变流器功率模块IGBT的散热问题近年来备受关注.本文以微通道内流动沸腾换热的"M"型曲线峰值点传热强化理论为依据,通过实验研究微通道长度和结构对冷板表面温度的影响,发现短通道能够有效控制通道内蒸汽干度水平,使得微通道冷板内以弹状流或薄膜环状流为主流流型,从而获得较高传热系数.在一定面积的冷板内设置短通道组合的分...     

微通道热沉内液氮流动沸腾的换热特性

本文研究了流最为50.1～880.5 kgm-2s-1,干度为0.01～0.25范围内微通道热沉内液氮流动沸腾的换热特性.热沉基材为一块长宽厚为50 min×30 mm×4 mm的不锈钢板,钢板上加工有宽1.0 mm,深2.0 mm的9个通道.实验结果表明在定热流密度条件下,热沉表面温度分布很不均匀,这主要是由微通道内...     

Experimental Investigation of Heat Transfer in Two-phase Flow Boiling

In this article, an experimental investigation is performed to measure the boiling heat transfer coefficient of water flow in a microchannel with a hydraulic diameter of 500 μm. Experimental tests are conducted with heat fluxes ranging from 100 to 400 kW/m², vapor quality from 0 to 0.2, and mass fluxes of 200, 400, and 600 kg/m²s. Also, this study has modified the liquid Froude number to present a flow pattern transition toward an annular flow. Experimental results show that the flow boiling heat transfer coefficient is not dependent on mass flux and vapor quality but on heat flux to a certain degree. The measured heat transfer coefficient is compared with a few available correlations proposed for macroscales, and it is found that previous correlations have overestimated the flow boiling heat transfer coefficient for the test conditions considered in this work. This article proposes a new correlation model regarding the boiling heat transfer coefficient in mini- and microchannels using boiling number, Reynolds number, and modified Froude number.     

烧结/堆积床多孔介质流动沸腾换热实验研究

本文实验对比研究了0.3 mm、0.5mm、0.7 mm三种粒径的铜颗粒烧结与堆积床多孔介质中的流动沸腾换热,主要研究了入口流速、热流密度、加热方位及粒径对流动沸腾换热的影响,以及多孔介质中的沸腾滞后。实验结果表明:大入口流速、低热流密度、下方加热以及小粒径时加热壁面的过热度较低,即有利于沸腾换热;本实验所用烧结多孔介质壁面过热度高于堆积床多孔介质,其原因是内部含有闭孔。     

微尺度水平单管内流动沸腾特性实验研究

对非共沸混合工质R134a/R32(75/25)在水平微尺度管道内的流动沸腾换热实验结果进行了分析和讨论,以探究微细通道内流动沸腾换热的主导机制。对影响其换热的多种因素(热流密度、质量流量和质量干度)进行了分析,实验得出,当质量干度较低时,热流密度和质量流量共同控制着微尺度管内的换热方式,当热流密度的影响占主导地位时,管道内的换热以核态沸腾为主;当质量流量的影响占主导地位时,管道内的换热以强制对流为主。     

微通道局部加热气泡动力学实验研究

实验研究了在水平放置的低高宽比聚二甲基硅氧烷(polydimethylsiloxane,PDMS)微槽道(300μm×60μm)内的局部位置给予恒定热流密度条件下气泡的核化沸腾、生长和运动情况,实验中的工质采用FC-72,并用真空泵抽至室温下的饱和气压。研究发现微通道内起始沸腾需要比常规槽道更大的壁面过热度,液体流量和加热速率对气泡的生长和运动有很大的影响。     

Experimental Investigation of Subcooled Vertical Upward Flow Boiling in a Narrow Rectangular Channel

Accurate models for the onset of nucleate boiling, density of active nucleation sites (N_a), bubble departure size (D_d), and departure frequency (f_d) are essential to the success of computational fluid dynamics analysis of two-phase thermal-hydraulics involving subcooled flow boiling in nuclear reactor systems. This work presents an experimental study of subcooled flow boiling in a vertical upward narrow rectangular channel that mimics the flow passage in the plate fuel assembly of boiling water reactors. The experiments are conducted over a range of mass flux (G = 122–657 kg/m²s), inlet subcooling (ΔT_sub = 4.7–33.3?C), and heat flux (q″ = 1.7–28.9 W/cm²). Based on the experimental data, empirical correlations are developed for the prediction of onset of nucleate boiling, N_a, D_d, and f_d for given flow conditions. These correlations are valid in the nucleate boiling regime when the wall superheat is less than 12°C and can be incorporated in the computational fluid dynamics codes to enable more precise simulation of subcooled flow boiling heat transfer and two-phase flow in nuclear energy applications.     

Experimental Study on Flow Boiling Pressure Drop and Flow Instabilities of Reentrant Micro-channels

Flow boiling pressure drop and flow instabilities of Ω-shaped reentrant copper micro-channels were experimentally explored. Tests were conducted in deionized water and ethanol at inlet subcoolings of 10°C and 40°C, mass fluxes of 125–300 kg/m²·s, and a wide range of heat fluxes and exit vapor qualities. The operational parameters effects, i.e., heat flux, mass flux, inlet subcooling, and coolants, on pressure drop and flow instabilities were systematically explored. The two-phase pressure drop of reentrant micro-channels were found to generally increase monotonically with increasing heat fluxes and exit vapor qualities. Nevertheless, the roles of mass flux and inlet subcooling were dependent on the test coolant.     

Effect of Bubble Contact Diameter on Bubble Departure Size in Flow Boiling

The growth and departure of bubbles on a vertical heated wall of an annular channel in flow boiling were observed by a high-speed camera, and the bubble contact diameter and bubble departure diameter were measured. It was found that bubble departure diameters were different among different nucleation sites in the same boiling area (same test condition, same surface roughness). Force balance on a single bubble attached to the heated surface was also analyzed to explain this phenomenon. The theoretical research results show that bubble contact diameter has a significant influence on bubble departure diameter, and the corresponding departure diameter is larger as the bubble contact diameter is larger. This agrees with the visual experimental results.     

PTFE疏水修饰法消除多孔表面的沸腾迟滞现象

微纳多孔结构表面在提高沸腾传热性能的同时,通常也会伴随着沸腾迟滞现象出现.以底部树林状阵列结构、上部蜂窝状微纳双尺度多孔结构为基础的双层多孔表面在展示出良好性能的同时,也出现了沸腾迟滞现象。本文通过电泳沉积PTFE修饰的方法,降低多孔表面成核壁面过热度,从而基本消除双层多孔表面的沸腾迟滞效应。另外,修饰后的双层多孔表面的CHF和最大HTC与未修饰的双层多孔表面相比提高了20%和19%;和光滑铜表面相比,CHF提高了97%,HTC提高了400%,展现出优异的沸腾传热性能.