汽泡在电场作用下的变形

为探明外加电场对汽泡形状的影响规律,本文采用人工注射汽泡的方式,对均匀电场作用下单个汽泡的形状进行了可视化试验研究,计算了汽-液两相系统中的电场分布及汽泡所受的电应力。结果表明:电场作用下汽泡表面所受电应力分布的不均匀性导致汽泡沿与电场相平行的方向拉长,变成扁长椭球形。随着场强的增加,汽泡变形量加剧,汽泡与壁面的接触角逐渐变大。此外,讨论了汽泡变形对电场强化沸腾换热的影响。     

多孔表面开槽影响池沸腾传热的实验研究

本文报道了开槽密度对R11在烧结多孔表面池沸腾换热性能影响的实验研究。观察发现,多孔表面开槽,让蒸汽从槽道逸出、液体从多孔区吸入到受热面,将增强池沸腾换热。沸腾特征可分为液体灌注、槽道起泡、底部蒸干三个区。对特定的多孔层,合理开槽可获得较好的换热效果。带槽道的多孔表面实验件与均匀多孔表面相比,在相同壁面过热度条件下,热流密度提高2～10倍,临界热流密度提高2～4倍。     

螺旋管内高压汽液两相强制对流沸腾传热试验

螺旋管式蒸汽发生器或热交换器在核电站、直流锅炉和各种化工设备等领域中都有相当广泛的应用。在高压水回路上对螺旋管中汽液两相强制对流沸腾传热特性进行了试验研究,得到了立式上升流动螺旋管内过冷水紊流流动和过热蒸汽紊流流动时的放热系数。用修正L-M关系式整理了立式上升流动螺旋管内两相强制对流区放热系数,螺旋管在该区的放热系数大致范围为25-40 kw／(m2·℃)。     

超急速爆发沸腾传热的实验与理论研究

超急速爆发沸腾研究具有重大理论与实用价值。本文在实验研究的基础上,建立了脉冲激光作用下超急速爆发沸腾传热数学模型,并采用恰当的数值处理方法进行了数值模拟计算。结果表明,计算与实验结果基本吻合,在超急速爆发沸腾过程中,汽相、液相的升温吸热以及汽化潜热等是影响温度变化的重要因素,实际过程中应综合加以考虑。     

A new microstructure for pool boiling

Combining electrocoating and etching processes, we have developed a new type of microstructure for nucleate boiling. The basic elements of the structure are cylindrically shaped; their density ranges up to 10⁷ cm⁻².To test the efficiency of the structure, the outer surface of a tube has been provided with such a structure and used in pool boiling experiments with the refrigerant R141b at atmospheric pressure. The results obtained show the heat flux to remain independent of the wall superheat in the fully developed boiling region. The behavior is novel. It is most probably associated with the density of active bubble nucleation sites. Activated at a certain wall superheat, the density of the sites generating bubbles remains apparently unaffected by raising the heat flux. Given that these preliminary results should be confirmed by further experiments, the microstructure developed will be suitable for nucleate boiling in general, but for keeping the heating surface largely isothermal, despite the variation of the heat flux, in particular.     

小管道内汽泡生长过程特性

本文在大空间汽泡生长模型的基础上,结合小直径管道内空间尺度对汽泡生长过程的限制特点,推导出小直径管道内汽泡生长过程方程,为讨论小直径管道内沸腾换热特性打下理论基础。     

垂直窄缝流道内过冷沸腾时的汽泡行为

通过高速摄像可视化研究发现,在p=1.3～2MPa时,F-12质初始汽泡在壁面以小于0.1m/s的低速滑动中生长。热流密度和断面平均过冷度等参数对初始汽泡影响较大,热流密度越高,沸腾越早发生;小汽泡(d=0.01～0.07mm)运动速度在0.1～0.2 m/s左右,而较大(d=0.1～0.3 mm)汽泡的运动速度在0.25～0.7m/s左右。较大汽泡聚合小汽泡的过程是汽泡从小汽泡生长为大汽泡乃至于汽层的主要形式。     

Measurements and high-speed visualizations of flow boiling of a dielectric fluid in a silicon microchannel heat sink

Experiments were conducted to investigate flow boiling heat transfer to a dielectric fluid in a silicon chip-integrated microchannel heat sink. Twenty-four microchannels, each 389 μm × 389 μm in cross-section, were fabricated into the 12.7 mm × 12.7 mm silicon substrate. High-speed visualizations (at 12,500 frames per second) were performed simultaneously with heat transfer and pressure drop measurements to investigate the physics of flow boiling in parallel microchannel arrays. At low heat fluxes, bubbly flow is dominant, with the bubbles coalescing to form vapor slugs as the heat flux is increased. At high heat fluxes, the flow regimes in the downstream portion of the microchannels are characteristic of alternating wispy-annular flow and churn flow, while flow reversal is observed in the upstream region near the microchannel inlet. Local heat transfer measurements, obtained at three flow rates ranging from 35 to 60 ml/min, show that at lower heat fluxes, the heat transfer coefficient increases with increasing heat flux. The heat transfer coefficient in fully developed boiling is seen to be independent of flow rate in this range. At higher heat fluxes (exceeding 542, 673, 730 kW/m², respectively, for flow rates of 35, 47 and 60 ml/min), this trend is reversed, and the heat transfer coefficient decreases with further increases in heat flux due to partial dryout in some of the microchannels. Heat fluxes at which fully developed boiling is achieved depend on the flow rate. The pressure drop in fully developed boiling increases with increasing heat flux and is independent of flow rate for the test conditions considered in this work.     

Secondary atomisation produced by single drop vertical impacts onto heated surfaces

The paper reports an experimental analysis of the secondary atomisation produced by the impact of a single drop on a solid heated surface. Different wall temperatures were used to study different boiling regimes. The size of secondary drops produced by the impact was measured by two techniques, namely the phase Doppler anemometry (PDA) and the image analysis technique (IAT); this allowed to extend the measurable size range from 5.5 μm up to few mm. Two impacting walls with different surface roughness were used to show the effect of this parameter on different atomisation regimes. The liquid viscosity was also varied in a limited range by using water–glycerol mixtures. Image analysis allowed also to define the details of the morphology of drop spreading and break-up.     

Full-Core Test Method for Experimental Determination of Convective Boiling Heat Transfer Coefficients in Tubes of Crossflow Compact Evaporators

An experimental methodology is proposed in which localized convective boiling heat transfer coefficients inside the tubes of compact evaporators are determined by testing of full evaporator cores. The proposed technique makes use of a special test system having two main flow circuits. One of these flow loops is a conventional vapor compression system, which provides a steady, low-quality, two-phase flow of refrigerant to the tube side of the evaporator. The second primary flow loop provides a steady flow of the vapor of a second working fluid, which condenses on the finned side of the evaporator. Measured data from this system are analyzed using an iterative scheme. Trends in the variation of the refrigerant-side heat transfer coefficient determined by this method throughout a typical evaporator core are described, and the differences and similarities relative to previously published results for single round tubes are discussed.