核态池沸腾中气泡生长和脱离的动力学特征：气泡的脱离直径与？…

在前人工作的基础上提出了表征核态池沸腾中气泡脱离和生长过程的特征时间和特征尺度,并进而得到了气泡生长时间和气泡脱离直径的通用关系式,应用传热学类比方法建立了计算气泡脱离直径的一般公式。本文的研究配前人的实验结果甚为相符。     

核态池沸腾中气泡生长和脱离的动力学特征_气泡的脱离直径与脱离频率

在前人工作的基础上提出了表征核态池沸腾中气泡脱离和生长过程的特征时间和特征尺度,并进而得到了气泡生长时间和气泡脱离直径的通用关系式,应用传热学类比方法建立了计算气泡脱离直径的一般公式。本文的研究结果与前人的实验结果甚为相符。     

基于水基SiO_2纳米流体PV/T系统辐射性能研究

为研究辐射在水基SiO_2纳米流体PV/T系统的传递特性,用两步法制备不同的水基SiO_2纳米流体。利用分光分度仪测试不同工质在不同光程和体积分数下的透射率,结合Mie和Rayleigh散射理论建立系统的辐射传递模型,同时对该系统进行了实验测试。结果表明:水基SiO_2纳米流体对光电可用辐射有高透过性,对低频辐射有高吸收性,与Si-PV光伏响应曲线基本匹配;当流体体积分数小于0.5%时,透射率实验值与理论值吻合度好;当光程大于10 mm时,90%的低频辐射被工质吸收,低于80%的光电可利用辐射传递到Si-PV表面;以水基SiO_2纳米流体作为PV/T辐射分频工质,可有效降低Si-PV的温升速率,且其光热总效率可达75%以上。     

Al2O3-H2O纳米流体池内沸腾强化传热实验的曲面响应优化

为了提高余热回收效率,强化沸腾换热。在池内沸腾强化换热实验中运用Al_2O_3-H_2O纳米流体,研究了Al_2O_3纳米流体浓度、工件壁厚、热流密度对强化率的影响及最优强化条件。单因素分析结果显示,随着Al_2O_3纳米流体浓度、热流密度增大,强化率先增大后减小;随着工件壁厚增大,强化率逐渐减小。在单因素分析结果上,采用响应曲面法中Box-Behnken Design(BBD)模型对池内沸腾传热条件优化,得出三个因素对强化率的影响大小为:Al_2O_3纳米流体浓度工件壁厚热流密度。并且Al_2O_3纳米流体浓度与热流密度交互作用对强化率最为显著。通过曲面响应拟合最佳实验条件为:质量浓度1.2 wt%、热流密度83 543 W/m~2、壁厚0.45 mm,模拟结果强化率为107%,实验测得最优条件下强化率为106%,与预测接近。     

中高温导热油基SiO2纳米流体比热容实验研究

实验采用两步法制备了6种不同类型导热油基SiO₂纳米流体样本,使用Zeta电位法进行了稳定性检验。采用高温流体比热容测试系统,对6种不同类型导热油基SiO₂纳米流体的比热容在30～290℃的中高温区间进行了测量。结果表明：实验温度升高,导热油基SiO₂纳米流体的比热容也会随之升高。在30～240℃,比热容与质量分数成正相关,240℃之后,纳米颗粒由于聚合沉降,比热容的相对增大率不再上升。加入纳米颗粒粒径分别为15 nm和30 nm时,前者比热容比后者比热容有明显增大,而基液对纳米流体比热容影响较小。     

Nucleate Pool Boiling of Pure Liquids and Binary Mixtures：PartI－Analytical Model for Boiling Heat Transfer of Pure Liquids on Smooth Tubes

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核态池沸腾中气泡生长和脱离的动力学特征_气泡动力学研究回顾

系统地总结和分析了近半个世纪以来气泡动力学的研究进展情况,指出了气泡生长脱离研究中在的不足及今后的发展方向,以促进气泡动力学研究的深入发展。     

Nucleate boiling in two types of vertical narrow channels

To explore the mechanism of boiling bubble dynamics in narrow channels, we investigate 2-mm wide I- and Z-shaped channels. The influence of wall contact angle on bubble generation and growth is studied using numerical simulation. The relationships between different channel shapes and the pressure drop are also examined, taking into account the effects of gravity, surface tension, and wall adhesion. The wall contact angle imposes considerable influence over the morphology of bubbles. The smaller the wall contact angle, the rounder the bubbles, and the less time the bubbles take to depart from the wall. Otherwise, the bubbles experience more difficulty in departure. Variations in the contact angle also affect the heat transfer coefficient. The greater the wall contact angle, the larger the bubble-covered area. Therefore, wall thermal resistance increases, bubble nucleation is suppressed, and the heat transfer coefficient is lowered. The role of surface tension in boiling heat transfer is considerably more important than that of gravity in narrow channels. The generation of bubbles dramatically disturbs the boundary layer, and the bubble bottom micro-layer can enhance heat transfer. The heat transfer coefficient of Z-shaped channels is larger than that of the I-shaped type, and the pressure drop of the former is clearly higher.     

狭缝通道内核态沸腾中的气泡动力学研究

为加深对狭缝通道内核态沸腾气泡动力学机理的探索,对宽度为2 mm的I-形和Z-形两种不同截面形状的狭缝通道内核态沸腾展开研究,通过数值模拟的方法探究不同壁面接触角对气泡生成及长大过程的影响,不同狭缝形状与流动压降的关系,计算中考虑了重力、表面张力和壁面黏附的作用.发现:壁面接触角的不同对气泡的形态有很大影响,壁面接触角...     

石墨烯纳米制冷剂核态池沸腾换热的实验研究

为分析单层石墨烯纳米片对核态池沸腾换热的影响机理,对基液为R141b、分散相为单层石墨烯纳米片的纳米制冷剂的核态池沸腾换热特征进行了测定,采用Hot Disk热物性分析仪和铂金板法分别测定了石墨烯纳米制冷剂的热导率和表面张力,采用接触角测量仪和扫描电子显微镜（SEM）观测了沸腾后加热表面的润湿性和形貌特征。实验中,单层石墨烯纳米片的质量百分含量（ω）为0.02%~0.50%,实验压力为一个标准大气压,热流密度为20~200 kW/m2。实验结果表明：单层石墨烯纳米片的加入,使制冷剂R141b的核态池沸腾换热得到强化;当ω=0.2%时,换热系数提高比例出现峰值,为57.7%。伴随ω的增加,石墨烯纳米制冷剂的热导率增大、表面张力减小,沸腾表面润湿性增强且微腔数先增后减,综合作用的结果导致存在一个最佳的单层石墨烯纳米片浓度（即ω=0.2%）使换热系数最高。     

Experiment Analysis of Pool Boiling Heat Transfer in Bead Packed Porous Layers

The heat transfer of pool boiling in bead packed porous layers was experimentally investigated to analyze the effects of the bead material, bead diameter and the layer number of the porous bed on the transport of flux and the heat transfer coefficients. The glass and copper bead, the bead sizes of 4 mm and 6 mm as well as the bead packed porous structures ranging from one to three layers were chosen in the experiments. The pool boiling heat transfer in the bead packed porous structures and that on the plain surface were compared to analyze the enhancement of pool boiling heat transfer while the bead packed porous layers were employed. The maximum relative error between the collected experimental data of the pure water on a plain surface and the theoretical prediction of pool boiling using the Rohsenow correlation was less than 12%. Besides, the boiling bubble generation, integration and departure have a great effect on the pool boiling and were recorded with a camera in the bead stacked porous structures of the different layers and materials at different heat flux. All these results should be taken into account for the promotion and application of bead packed porous structures in pool boiling to enhance the heat transfer.     

高热流密度下R113核态沸腾中汽化核心密度的识别

1引言核态沸腾是一种被工业界广泛应用的传热技术,在冶金、化工、动力等领域有广泛的发展前景。其优点是能以较低的温差传递较多的热量。在核态沸腾中有相变发生,相变发生的地方是成核地点[1],称为汽化核心。汽化核心的研究对于核态沸腾技术的发展具有非常重要的意义。传统的拍     

用饱和核态池沸腾换热机理模型预测加热壁面活化核心密度

活化核心密度是核沸腾过程的重要参数,但其描述方法至今尚未统一,为了避免核心密度直接测量的困难,提出根据核沸腾换热机理模型预测沸腾表面的活化核心密度,预测得到的六种表面的核心尺度分布曲线与表面活化核心的分形分布相一致,证明了活化核心尺度分布具有分形特征这一结论具有普遍意义。     

二氧化硅与石墨烯混合纳米流体沸腾换热实验研究

设计并搭建带冷凝回流的闭合回路沸腾换热实验装置,研究了质量分数为0.000 5％、0.001％、0.002％和0.003％的水基二氧化硅(SiO2)纳米流体、水基石墨烯(GNs)纳米流体、水基和乙二醇水基SiO2与GNs混合(SiO2/GNs-DW,SiO2/GNs-EG-DW)纳米流体沸腾换热性能.结果表明:纳米流体...