Experimental Investigation of Saturated Flow Boiling Heat Transfer to TiO2/R141b Nanorefrigerant

Heat transfer in two-phase flow boiling of a dilute mixture of TiO₂ nanoparticles in R141b base fluid in a smooth tube is investigated experimentally. Examining the obtained results reveals that enhancement of the convective heat transfer coefficient for the particle volume fractions of 0.01% and 0.03% in comparison with pure R141b is more pronounced for a higher volume fraction. The measured data also show that at higher vapor qualities, the improvement in heat transfer coefficient is greater. Moreover, the heat transfer coefficient decreases substantially with mass flux while an increase in saturation temperature leads to an improvement in this coefficient.     

Flow Boiling Heat Transfer in Micro-Channel Heat Sinks under Sub-Atmospheric Pressures

A closed-loop two-phase mini-channel-based heat sink driven by a micro-gear pump was developed in this work. Using water as an example, experiments were performed in two micro-channel heat sinks under the conditions of initial pressure of P_in = 34–113 kPa, mass velocity of G = 19–468 kg/m₂s, outlet quality of x_e,out = ?14–66%, and heat flux of q″ = 0–230 W/cm², which covered single-phase flow, subcooled flow boiling, and saturated flow boiling regions. The results showed distinctive differences between the subcooled and saturated boiling regime and revealed that the influence of the system pressure. The experimental data were also compared to a boiling mechanism demarcation map and assessed against some empirical correlations, which suggests some uniqueness of the current heat sink associated with flow boiling at the mesoscale.     

微尺度水平管内沸腾换热特性研究

对不同质量分数下非共沸混合工质(R134a/R32)在微尺度管道内的流动沸腾换热特性进行了比较和分析,阐述了热流密度、质量流量和质量干度对换热的影响。结果表明:热流密度对换热的影响随着质量流量的增加而愈加明显;在质量分数为75%/25%和65%/35%时,换热系数随着质量流量的增大而增大;而质量分数为85%/15%时,换热系数随质量流量的变化先增加后减小;随着质量干度的增加,换热系数在各质量分数下基本上都呈上升趋势。     

多孔材料微通道流动沸腾可视化实验研究

微通道换热器因其结构紧凑、换热能力高、工质消耗少等优点成为解决微小空间"散热难"问题的有效途径之一,采用多孔材料制作的微通道热沉能够极大地增加换热面体比,因而可以进一步提高其换热能力.本文利用可视化手段对槽道翅片顶部与盖板间留有狭缝、通道截面为矩形(400 μm×600 μm)的开放型多孔微通道的流动沸腾现象进行实验研...     

乙烷／丙烷混合工质流动沸腾传热研究

对乙烷、丙烷纯质及三种不同浓度比例的乙烷／丙烷二元混合物在内径为8mm的水平管内进行了饱和流动沸腾传热特性的实验研究,重点分析了热流密度、质量流量的变化对二元混合工质传热系数的影响。选用了4种专用于计算混合物流动换热的关联式,并与实验数据进行了比较,其中Gungor—Winterton and Thome关联式和Zou...     

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.     

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...     

Subcooled Flow Boiling Heat Transfer to Water and Ethylene Glycol/Water Mixtures in a Bottom-heated Tube

Subcooled flow boiling heat transfer experiments were performed with water and ethylene glycol/water mixtures at five flow velocities and four fluid inlet temperatures. Both turbulent flow and laminar flow were tested in a special test facility with its experimental test section heated from the bottom surface only. Boiling curves and subcooled flow boiling heat transfer coefficients of the tested fluids were determined from the experimental measurements. Predictive correlations of the subcooled flow boiling heat transfer coefficients were developed based on the experimental data. Comparisons are presented of wall temperatures between the experimental measurements and the correlation predictions.     

泡沫金属对含油制冷剂管内流动沸腾换热特性的影响

实验研究了制冷剂-润滑油混合流体在内嵌泡沫金属圆管内流动沸腾的换热特性。泡沫金属为10ppi、90%孔隙率;制冷剂为R410A,润滑油为VG68,油浓度为0～5%。实验结果表明:纯制冷剂工况下,泡沫金属强化流动沸腾换热系数,换热系数提高30%～120%;含油工况下,泡沫金属只强化流动沸腾换热系数20%以下,在低质流密度或者高质流密度的高干度情况下出现恶化换热的情况。润滑油总是恶化制冷剂在内嵌泡沫金属圆管内流动沸腾的换热系数,换热系数最多恶化71%,且在低质流密度下对换热的恶化比在高质流密度工况下严重。     

微通道换热公式应用于水和制冷剂适用性评价

水是微通道流动沸腾常用的换热工质,由于其饱和温度较大,目前的沸腾实验大多采用制冷剂替代.然而,水和制冷剂在物性参数上依然具有较大差异,应用于制冷剂的换热公式并不一定能有效应用于水.因此,本文收集1084个工质为水和1002个工质为制冷剂的实验数据,比较两者在无量纲数上的差异,并选取6个常用换热公式,比较其应用于两者的预...     

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。     

用强化构造强化沸腾传热的实验及可视化研究

重点研究的是水和乙醇在放置了强化元件的光滑铜表面上的池核沸腾传热性能。该强化元件对部分加热表面上方的空间进行限制,使液体存在的空间就被分为交替存在的受限空间和自由空间。本研究中所探讨的主要参数是热通量(13～400 kW/m~2)和受限空间尺度(0.2 mm,0.5 mm,无限制结构)。实验结果表明,对水和乙醇而言,这样的强化元件都可获得良好的沸腾传热特性.可视化结果表明,几乎所有有效核化区域都在受限空间内,即使在高过热度下自由空间内也很少出现有效核化点。     

微重力下微结构表面强化沸腾换热研究

为了提高航天器中电子器件的冷却效率,本文利用干式腐蚀方法形成方柱微结构,通过控制加热电流方法,在北京落塔进行了持续3.6 s有效微重力时间的过冷FC-72池沸腾强化换热实验研究。研究结果表明:相比光滑表面,方柱微结构表面利用其独立于重力水平的毛细作用力,显著地强化了微重力沸腾换热。     

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。     

New Pool Boiling Heat Transfer in the Presence of Low-Frequency Vibrations Into a Vertical Cylindrical Heat Source

A vertical cylinder was applied as a heat source into a water pool; the vibrations were imposed into the heater with different heat fluxes, and the frequencies were adjusted at 10, 15, 20, and 25 Hz. An imaging system was employed to observe the produced bubbles around the cylindrical heat source. The results showed that the boiling heat transfer was enhanced under the vibrations with a shorter transient process, and the wall temperature also decreased. The best enhancement ratio was achieved at the frequency of 25 Hz and a heat flux value of 30 kW/m² as a consequence of imposed vibrations.     

低温液体流动沸腾数值计算中的相间传热模型

采用双流体模型预测了液氮在垂直管内的流动沸腾过程,着重考察和评价了五个常用的相间传热模型对数值计算结果的影响,找出了最优的相间传热模型;同时,研究还发现相间传热模型对流动压降的预测并无明显影响.     

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。     

An Experimental Investigation of Forced Convection Heat Transfer from a Coiled Heat Exchanger Embedded in a Packed Bed

Forced convection heat transfer from a helically coiled heat exchanger embedded in a packed bed of spherical glass particles was investigated experimentally. With dry air at ambient pressure and temperature as a flowing fluid, the effect of particle size, helically coiled heat exchanger diameter, and position was studied for a wide range of Reynolds numbers. It was found that the particle diameter, the helically coiled heat exchanger diameter and position, and the air velocity are of great influence on the convective heat transfer between the helically coiled heat exchanger and air. Results indicated that the heat transfer coefficient increased with increasing the air velocity, increasing helically coiled heat exchanger diameter, and decreasing the particle size. The highest heat transfer coefficients were obtained with the packed-bed particle size of 16 mm and heat exchanger coil diameter of 9.525 mm (1/4 inch) at a Reynolds number range of 1,536 to 4,134 for all used coil positions in the conducted tests. A dimensionless correlation was proposed for Nusselt number as a function of Reynolds number, particle size, coil size, and coil position.     

窄缝通道中液氮的沸腾传热研究第一部分:窄缝通道尺寸对传热特性的影响

窄缝通道中的液氮沸腾传热特性有别于开放空间中的情况.文中对常压下液氮在矩形窄缝通道内的沸腾传热进行了实验研究,系统地研究了窄缝几何结构参数和倾角对液氮的沸腾传热特性的影响.     

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.