临界热流密度物理机理的实验研究

本文通过实验研究了池沸腾和液膜沸腾条件下的临界热流密度。实验结果表明,初始液膜厚度对于临界热流密度没有影响,并且液膜沸腾的临界热流密度值和相同条件下的池沸腾的临界热流密度值相当。这一实验结果验证了‘尺度分离'假设。通过高速摄像机对液膜沸腾进行直接观测,发现在高热流密度时,绝大多数的气泡底下有微液膜存在,只有少量气泡底下出现干斑,但是由于高频的气泡生长过程,干斑很快被润湿而消失。接近临界热流密度时,干斑的面积占据整个加热表面的比例不超过10%。当热流密度达到临界值时,干斑由于不能被润湿而渐渐扩大,最后导致加热壁面烧毁。     

流态化固体颗粒对对流沸腾传热的强化

在对流沸腾系统中引入固体颗粒,固体颗粒在液体中呈流态化,从而形成气液固三相对流沸腾过程。对气液固三相流对流沸腾过程的传热特性进行了实验研究,结果表明流态化固体颗粒对液体的对流沸腾传热具有显著的强化作用。基于固体颗粒撞击沸腾气泡时的受力分析,获得了固体颗粒穿透气泡并使气泡破碎的条件,分析了流态化固体颗粒强化沸腾传热的机理。实验结果与理论分析符合良好。     

载气汽油在水平螺旋扁管管束中的沸腾换热

以汽油-空气为介质,在不凝性气体质量含量不超过5%时,对导程为200mm的螺旋扁管管束中的沸腾换热进行了实验研究,分析了沸腾换热系数随两相质量流量的变化规律以及流动压降随空气流量的变化规律。得到了相应条件下,载气汽油在该导程的螺旋扁管管束中的沸腾换热实验关联式。     

采用抑泡孔板的沸腾传热

液体沸腾时泡底微层液膜的导热蒸发热阻低,在一定热流密度范围内,采取措施适当延迟汽泡的脱离就可能强化沸腾传热。本文据此提出了一种强化核沸腾传热的新方法——抑泡孔板法,并对采用抑泡孔板的池沸腾传热进行了实验研究。实验结果表明,采用抑泡孔板可以使沸腾传热系数增加几倍乃至十几倍。同时也得到了采用抑泡孔板时沸腾传热的准则方程式和最佳板距的准则方程式。     

FC-72在浸泡于液池中的微小圆管内的沸腾传热

本文对浸泡于FC-72液池中的两个微小圆管进行了沸腾实验研究,得到了沸腾曲线和传热系数,并用DV摄像机拍摄到了圆管出口处的沸腾状况,研究了管道尺寸对沸腾传热特性的影响。实验结果显示,管道尺寸对沸腾传热特性有显著的影响。传热系数和CHF随着管道尺寸的缩小而减小.直径为1.10mm的圆管出口处在低热负荷加热时发生了汽泡阻塞,并导致了剧烈的沸腾滞后现象。     

工质对微通道沸腾液膜瞬态变化的影响

本文采用去离子水和无水乙醇两种工质,利用微通道流动沸腾同步测量实验系统,研究了液膜厚度的瞬态变化规律,实验发现流动沸腾形成的初始液膜厚度在毛细数Ca很宽的范围内都遵循Taylor流动原理;液膜形成后,在蒸发和蒸汽流动携带的耦合作用下,厚度迅速减薄直至蒸干;由于水的汽液黏度比小,速度梯度小,剪切作用带来的液膜厚度减少量小,且水的汽化潜热大,吸收相同热量时蒸发量小,导致水的液膜厚度变化斜率较小,通过理论分析提出了沸腾液膜厚度变化的计算模型,计算结果与实验结果的误差小于20%。     

圆锥形传热面上的薄液膜沸腾

液体在水平传热面上沸腾时,液位越低传热系数越高的现象,见文献[1—3]。液位降到一定限度后,稳定沸腾遭到破坏,传热面上出现的干燥斑因得不到液体的补充逐渐扩大,直到传热面烧毁。为解决零液位附近薄液膜滞腾的稳定性问题,文献[4]研究了倾斜传热面的液膜沸腾。指出:当倾斜传热面的一端保持零液位,沿长度方向液位逐渐增加的状     

超急速温升沸腾热流密度研究

本文以不干涉测温系统的激光为热源,以高响应、高测量精度的铂薄膜电阻为测温元件和加热元件,从而保证了研究丙酮液体在超急速温升条件下的沸腾热流密度时瞬态测量的精确性。利用显微镜和数码相机对沸腾过程进行照相,计算了丙酮液池的温度场、热流密度及其蒸汽量,得出了沸腾曲线,发现了过冷沸腾中汽泡并不淹没的现象。     

加热表面粗糙度对高热流密度沸腾传热影响的多尺度数值模拟

本文以沸腾固气液界面为研究对象,建立了包括孔穴活化、液膜蒸发、气泡生长与脱离、壁面热传导等子过程的耦合模型,以探究固气液界面传热对高热流密度沸腾过程的影响。为了能够分辨微米量级的孔穴,模型中10 mm×10 mm的沸腾表面被划分为诸多子区域,每一个子区域中孔穴大小和数量随机分布,当子区域的过热度大于孔穴活化的临界过热度时,一部分孔穴活化生成气泡。进一步结合大液膜蒸发模型获得沸腾传热热流密度,并将其作为边界条件分析加热器热传导特性,从而通过对不同过程的多尺度耦合模拟不同表面粗糙度条件下高热流密度区的核态沸腾曲线,并进一步分析了孔穴数量及分布对加热壁面温度的影响。结果表明:预测所得沸腾曲线与实验结果基本相符,加热表面孔穴数量的增加使沸腾曲线左移,同时,孔穴数目的增多还会使活化点密度对壁面温度波动更为敏感,从而产生交替出现的长短周期。     

新型内嵌式热管流型演化与传热特性实验研究

本文面向电蓄热采暖应用设计了一种新型内嵌式热管,通过可视化和传热实验研究了充液率在5%至70%之间变化时热管内部的流型演化规律和传热特性。根据流型演化过程可以将充液率分为小、中、大三个类别。小充液率时,热管内部出现间歇剧烈沸腾过程;中等充液率时,热管内部出现持续沸腾现象,冷凝段受到搅混流的周期性冲刷;大充液率时,气液混合工质在绝热段往复振荡,冷凝段始终存在液态工质。对于充液高度小于加热段高度和充液高度大于加热段高度两种情况,随着充液率的增大,热阻均先降低后升高,但充液高度大于加热段高度时的热阻普遍大于充液高度小于加热段高度时的热阻。10%充液率时热阻最小,80 W加热功率时约为0.017℃/W,此时蒸发段为持续薄液膜蒸发传热,冷凝段为周期性扰动冷凝和强制对流耦合传热。     

微液膜动力学特性与稳定性实验研究

气-液两相流设备的性能受限于临界热流密度,开展流动微液膜动力学特性及其稳定性的相关研究是深入理解沸腾危机及临界热流密度机理的关键。采用光学玻璃制成的矩形通道作为实验段,使用微流量齿轮泵驱动去离子水,使其在实验通道入口处与在其上部流动的压缩空气接触形成同向流动的分层流。利用共轭光学探测器对流动微液膜的厚度进行了测量,利用高速摄像机对气-液两相分层流波动特性进行了可视化观测。研究表明,在绝热情况下,当液速一定时,液膜的平均厚度随着气速增加而减小,当气速增加到某一阈值时会导致液膜破裂。     

微重力下凝结和沸腾着的汽泡周围流场

1引言尽管对沸腾换热已有大量的研究，但对沸腾过程中驱动汽泡外流动的机理仍没有透彻理解，通常认为，汽泡外流动是由液体自然对流引起的。但是，近来的微重力实验表明［‘］；在没有自然对流的微重力场中，沸腾换热同样很剧烈。为了弄清正常重力和微重力环境中驱动换热的机理，很有必要作进一步的研究。本文数值分析了微重力环境下驱动壁面汽泡周围液体流动的基本机理，深入分析了Marangoni效应的影响，同时考虑表面凝结和蒸发过程的作用。2理论模型汽泡外流动可由二维层流N－S方程及烙方程来描述，流动是由Marangoni边界条件驱动，该条…     

Experimental data on the mechanism of subcooled water boiling: high-speed video shooting

The article provides statistical data on the characteristics of subcooled liquid boiling and behavior of vapor and air bubbles resulted from high-speed shooting. The authors have described a number of new phenomena going along with water boiling.     

混合润湿性柱状纳米结构对铜板上纳米氩膜沸腾传热的影响

混合润湿性对固/液相互作用有显著影响,因此对提高相变过程中的传热速率有积极作用.采用分子动力学模拟方法研究了柱状纳米结构表面混合润湿性对池沸腾传热的影响.分析了混合润湿性和纳米结构柱高对液体起始沸腾时间和温度的影响及其机理.结果表明,疏水比例和柱高会影响爆沸的起始温度和时间.与纯亲水壁相比,增加疏水比改变了固液界面性质,可以降低沸腾温度,更容易突破势能壁垒,使液体起始沸腾时间提前,并且随着疏水比的增加,不同柱高下的沸腾温度降低;当疏水比相同时,增加柱高扩大了混合润湿性的影响,也能降低沸腾起始温度并使液体起始沸腾时间提前.这为设计微纳粗糙结构和混合润湿表面以强化沸腾传热提供了思路.     

Micromechanism of sulfurizing activated carbon and its ability to adsorb mercury

The effect of ambient environment (dry or wet) and overlapping laser pulses on the laser ablation performance of brass has been investigated. For this purpose, a Q-switched, frequency doubled Nd:YAG laser with a wavelength of 532 nm, pulse energy of 150 mJ, pulse width of 6 ns and repetition rate of 10 Hz is employed. In order to explore the effect of ambient environments, brass targets have been exposed in deionized water, methanol and air. The targets are exposed for 1000, 2000, 3000 and 4000 succeeding pulses in each atmosphere. The surface morphology and chemical composition of ablated targets have been characterized by using Scanning Electron Microscope (SEM), Atomic Force Microscope (AFM) and Attenuated Total Reflection (ATR) techniques. In case of liquid environment, various features like nano- and micro-scale laser-induced periodic surface structures with periodicity 500 nm–1 μm, cavities of size few micrometers with multiple ablative layers and phenomenon of thermal stress cracking are observed. These features are originated by various chemical and thermal phenomena induced by laser heating at the liquid–solid interfaces. The convective bubble motion, explosive boiling, pressure gradients, cluster and colloid formation due to confinement effects of liquids are possible cause for such kind of features. The metal oxides and alcohol formed on irradiated surface are also playing the significant role for the formation of these kinds of structure. In case of air one huge crater is formed along with the redeposition of sputtered material and is ascribed to laser-induced evaporation and oxide formation.     

毛细微槽内的相变传热的实验研究

本文对矩形毛细微槽竖直板的相变传热特性进行了实验研究。结果表明毛细微槽对相变换热具有很大的促进作用。当壁面过热度较小时,相变换热形式主要是三相接触线附近的蒸发换热机制。而当过热度较大时,微槽内发生剧烈的沸腾。微槽内相变换热的临界热负荷有两种产生机理:其一是当微槽长度较大时微槽内由于流动阻力而产生的液体输运临界;另一机理是当微槽长度较小时的池内沸腾临界现象,亦即由动态微液层模型决定的临界机理。实验还得到了微槽强化传热的最佳优化尺寸。     

ESCA applied to liquids

The conditions for applying ESCA on liquid samples are discussed. A “liquid beam” technique is developed which meets the various requirements. The first spectrum of a liquid is presented, namely from formamide. It is shown that one can adjust the liquid beam so that a complete separation between the ESCA signals from the liquid and the vapour is achieved. The binding energies of the core levels of the elements in the molecule differ from each other in the liquid and the vapour phase by about 1.6 eV. One can also obtain electron spectra from dissolved chemical species. This is illustrated by potassium iodide dissolved in formamide.     

Change of Volume of Air Dissolved in Water Imposed to Millimeter Waves of Low Intensity

We discuss an experimental evidence for a decrease in the volume of air dissolved in water caused by millimeter electromagnetic waves of low intensity. The changes obtained were much greater than the expected conditioned by a pure thermal effect. The result observed is explained by air bubbles that take an active part in a mechanism of withdrawing dissolved air from water. The variation of restoring the dissolved air volume up to an equilibrium level after ceasing irradiation of water by millimeter waves was investigated as well.     

The effect of temperature-dependent thermal conductivity in heat transfer simulations of frozen biomaterials

The thermal conductivity value of pure water ice is inversely proportional to the temperature and decreases about 5-fold as the temperature increases from the liquid nitrogen boiling temperature (77 K to the freezing point of pure water. The temperature dependency of the thermal conductivity is typically overlooked in bioheat transfer simulations. A closed-form solution of the one-dimensional temperature distribution in frozen water and blood is presented in this study, based on a new thermal conductivity model. Results indicate that temperatures are overestimated up to 38K, and heat fluxes through the frozen region boundaries are underestimated by a factor of 2, when the temperature dependency of the thermal conductivity is neglected.     

火焰原子吸收光谱法测定多种矿石中的砷

在氯酸钾作氧化剂的条件下用浓硝酸将硫或碳氧化成硫酸或二氧化碳,用王水在沸水浴中溶解多种矿石样品中的砷,用火焰原子吸收光谱法测定。方法检出限为0.0032%,精密度为1.60%—3.65%。