沸腾表面凹坑的尺度分布特征

在沸腾表面上储气(或汽)凹坑是形成活化核心的关键因素。文中从类似Sierpinski地毯的经典分形表面出发，成功构造了凹坑以不同尺度分布的表面。研究结果表明凹坑的尺度分布雄数与表面形貌的分形维数是两个含义不同的概念，机械表面凹坑(或凸点)的尺度分布特征符合分形理论。分析活化核心的测量结果，表明活化核心的尺度分布同表面上凹坑的尺度分布一样具有分形特征。     

对"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"一文的评论

郁伯铭教授对本刊 1999年第 5期发表“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”文章中的分形维数的定义提出了质疑。本着学术上百家争鸣的原则 ,对某一理论和观点进行交流 ,为提高学术水平 ,促进科学技术的发展 ,现将郁伯铭教授的评论文章发表于此 ,以飨读者     

多壁碳纳米管纳米流体的大容积沸腾

通过在直径为12 mm的沸腾表面进行的多壁碳纳米管阿拉伯树胶水溶液的大容积沸腾实验,研究其沸腾传热特性。纳米流体的沸腾传热效果弱于阿拉伯树胶水溶液,烧毁点的过热度增加而临界热流密度减小,同时,阿拉伯树胶水溶液的传热效果劣于水。纳米颗粒在沸腾加热表面富集、结垢引起液体密度、沸腾表面上活化核心数目的变化,随传热时间的延长,垢层结构包括毛细孔直径、空隙率、垢层厚度不断发生变化,进而引起蒸汽在毛细孔中的流动阻力不断增加、加热表面和垢层间热阻增加,沸腾表面的活化核心数目减小,阿拉伯树胶在蒸发表面的局部富集、黏度大大增加,最终导致沸腾传热恶化。     

一个水基SiO2纳米流体核态池沸腾数学模型

基于水基SiO_2纳米流体沸腾实验研究结果,在双流体多相流模型和热流分区模型(RPI模型)的基础上建立了一个水基SiO_2纳米流体核态池沸腾数学模型。结果表明:汽化核心密度、气泡脱离直径和纳米流体润湿角是纳米流体换热性能提升的主要原因,所建立的新的汽化核心密度、气泡脱离直径模型预测结果与实验数据吻合非常好,证明该模型的可靠性,给精确预测水基SiO_2纳米流体核态沸腾换热特性提供了重要参考。     

冻融损伤混凝土梁抗力性能演化的裂缝分形特征

为分析冻融循环作用下钢筋混凝土梁受压区的表面裂缝分布特征,采用自主设计的混凝土梁反力试验架对受压区受冻融循环作用的梁进行抗弯试验,取得各级荷载作用下梁表面的裂缝图像。依据分形理论计算表面裂缝分布的分形维数,并讨论分形维数与梁的荷载、跨中挠度、屈服荷载和冻融循环次数之间的关系。研究表明,受压区冻融损伤的钢筋混凝土梁表面裂缝的分布符合分形特征,其分形维数与荷载、跨中挠度、屈服荷载和冻融循环次数之间均有一定关系,裂缝的分形维数可作为钢筋混凝土构件安全性能预测的指标。研究成果可为冻融损伤混凝土梁安全预测提供参考。     

竖直矩形窄通道内流动沸腾局部换热特性实验研究

为了明确竖直矩形窄通道内各阶段流动沸腾的换热特性,优化换热器性能,以去离子水为工质,对尺寸为720 mm×250 mm×3.5 mm的单面电加热竖直矩形窄通道内的流动沸腾换热进行实验研究,分析了质流密度、进口温度、热流密度对流动沸腾局部换热特性的影响。并在已有流动沸腾传热关联式的基础上,对实验数据进行非线性回归分析,得到适用于实验工况下的新流动沸腾传热关联式。结果表明：质流密度增大对流动沸腾段换热特性有强化作用,对核态沸腾段换热特性有削弱作用;热流密度对核态沸腾影响剧烈,但对流动沸腾的影响不明显;入口温度越高,流体会越早进入过冷沸腾阶段,但对局部传热系数的影响不明显;新流动沸腾传热关联式与实验值的平均相对误差为23.87%,其中74.19%的预测值在±25%内,83.87%的预测值在±50%以内,能很好地预测本实验工况下矩形窄通道内流动沸腾的局部传热系数。     

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     

柴油机缸内团聚态颗粒氧化过程中的破碎

基于全气缸取样系统采集不同燃烧时刻的柴油机碳烟,使用粒数粒径测试分析仪和透射电子显微镜测量了碳烟的粒径分布、数密度、分形维数和团聚度,进而获得团聚态颗粒的破碎速率,在上述工作的基础上,分析了柴油机缸内碳烟氧化主导阶段团聚态颗粒物的破碎现象.结果表明:碳烟氧化主导阶段初期,团聚态颗粒破碎速率高,碳烟颗粒总粒数密度和核态颗粒数密度增加,同时分形维数和团聚度明显减小.随氧化主导阶段燃烧反应的进行,破碎速率逐渐降低,总颗粒数密度逐渐减小,核态颗粒数密度先增加后减小,而分形维数和团聚度呈现上升的趋势.     

微细管内R290流动沸腾换热与干涸特性研究

对内径为1、2、3 mm的水平不锈钢圆管内R290两相流动沸腾换热特性进行了理论与实验研究。分析了热流密度为15~35 kW/m~2、质量流率为76~200 kg/(m~2·s)、饱和温度为16~36℃、干度为0~1时的管内传热特性。研究结果表明:热流密度的增加促进管内核态沸腾,换热得到强化,从而导致换热系数随之增加;质量流率的增加促进管内由核态沸腾换热向对流换热转化,换热系数也随之增加;饱和温度的增加促进管内气泡核心的形成速率加快,强化管内沸腾换热;管径的减小导致微尺度效应增加,从而导致换热系数随之增加;在整个换热过程中干涸前平均换热系数、干涸过程中的平均换热系数分别占总换热系数的40%、37%。     

微细通道CO_2流动沸腾换热临界热流密度研究

CO2在微细通道内流动沸腾换热过程所具有的临界热流密度(CHF)对于其换热系数有着重要影响。根据国内外现有发表的公开文献的实验数据分析了质量流量、饱和温度、管径等对临界热流密度的影响,并对理论模型与试验数据进行误差分析。发现Bowring预测关联式对小于3 mm管径内临界热流密度预测精度较高,在30%误差范围内可以达到70%预测精度,Wojtan预测关联式具有较小的平均绝对误差。提出了今后CO2在微细通道内沸腾换热CHF的研究方向。     

Enhanced heat transfer performance of alumina sponge-like nano-porous structures through surface wettability control in nucleate pool boiling

For several decades, a porous surface has been recognized as an efficient medium to increase boiling performance in a nucleate boiling regime. Most feasible porous surfaces have been studied in millimeter and micron-sized domains. It has been believed that a higher wall superheat is required to commence incipient nucleate boiling under a submicron regime. In this study, we demonstrate that a significantly enhanced pool boiling heat transfer is observed in a submicron regime through three dimensionally interconnected hybrid pores: the Alumina sponge-like nano-porous structure (ASNPS). The structural uniqueness of the ASNPS leads to an enlarged surface area, increases the potential number of the active nucleation site density, and improves the vapor–liquid menisci through the reentrant pore. Simultaneously, by changing the surface wettability with a hydrophobic self-assembled monolayer (SAM) coating, the number of active nucleation site density is improved. Eventually, the combination of the ASNPS and hydrophobic SAM coating can achieve substantial heat transfer coefficient (HTC) enhancement in the nucleate boiling. Also, the thickness of the ASNPS is a critical issue to adequately augment the HTC in pool boiling. The thickness of the ASNPS is optimized by examining the boiling performance of the ASNPS fabricated in different amounts of anodizing times. A classical mechanistic model from literature was modified and compared with the experimentally obtained data. The modified mechanistic model – with the combination of forced-convection and thin liquid film evaporation – showed reasonable predictions.     

Generic features and puzzles of nucleate boiling

At high reduced pressures extremely high nucleate boiling heat transfer coefficients (HTC) were measured. A single mechanism, which presents a consistent explanation of such HTCs, is very high intensity of liquid evaporation at the periphery of dry spots (nucleation sites) at the heated wall. Due to very small size the nucleation sites can be considered as point heat sinks. Between them convective heat transfer occurs, which in its turn is governed by the inherent mechanisms of boiling. The above two mechanisms comprise a total heat flux from the heated wall in nucleate boiling. The predicting equation, which determines heat flux in boiling via the wall superheat and liquid properties, has been developed with accuracy to two universal numerical factors fitted to the experimental data. Although the equation developed is found to be in good agreement with numerous experimental data for different liquids and in the wide range of reduced pressures and heat fluxes there exists a problem in nucleate boiling, which has not been understood to the full even qualitatively. This problem is the dependence of nucleation site density on the physical properties of the liquid and on the controlling parameters. Some new experimental results by Theofanous et al. [T.G. Theofanous, T.N. Dinh, J.P. Tu, A.T. Dinh, The boiling crisis phenomenon. Part I: Nucleation and nucleate boiling heat transfer, Exp. Therm. Fluid Sci. 26 (2002) 775–792; T.G. Theofanous, T.N. Dinh, J.P. Tu, A.T. Dinh, The boiling crisis phenomenon. Part II: Dryout dynamics and burnout, Exp. Therm. Fluid Sci. 26 (2002) 793–810.] and Qi et al. [Y. Qi, J.F. Klausner, R. Mei, Role of surface structure in heterogeneous nucleation, Int. J. Heat Mass Transfer 47 (2004) 3097–3107; Y. Qi, J.F. Klausner, Heterogeneous nucleation with artificial cavities, J. Heat Transfer 127 (2005) 1189–1196; Y. Qi, J.F. Klausner, Comparison of nucleation site density for pool boiling and gas nucleation, J. Heat Transfer 128 (2006) 13–20.] require revising the traditional views on a nature of the active nucleation sites in boiling. These results remind the old question: why can the nucleation sites arise at low superheats of the absolutely wettable surface? Obtaining theoretical equation for nucleation site density remains the most significant challenge in nucleate boiling theory.     

A fractal model for critical heat flux in pool boiling

In this paper, a fractal model for the high heat flux nucleate boiling region and for the critical heat flux (CHF) is proposed. The expression for the critical heat flux (CHF) is derived based on the fractal distribution of nucleation sites on boiling surfaces. The proposed fractal model for CHF is found to be a function of wall superheat, the contact angle and physical properties of fluid. The relation between CHF and the number of active nucleation sites is obtained from the fractal distribution of active nucleation sites on boiling surfaces. The contact angle and the physical properties of fluid have the important effects on CHF. The predicted CHF from a boiling surface based on the proposed fractal model is compared with the existing experimental data. An excellent agreement between the proposed model predictions and experimental data is found.     

On the Relation between Nucleation Site Density and Critical Heat Flux of Pool Boiling

ABSTRACT

It is traditionally accepted that the critical heat flux (CHF) decreases with increasing nucleation site density (NSD). However, such a CHF-NSD relation was no longer observed in the BETA-B experiment performed on nano-film heaters; instead the increase of NSD resulted in a gain in CHF. To address this seeming contradiction in the relation between critical heat flux and nucleation site density, the present work employed probabilistic analysis to reveal the different tendencies. A concept of effective NSD was proposed, which concerns the active nucleation sites appear within a bubble lifetime, and the resulting bubbles have the chance of direct interaction. We assumed that the boiling crisis on a heater surface is mainly induced by two mechanisms: dry spot expanding in isolated bubble regime for low-NSD surface, coalescence of dry spots under multiple bubbles in fully developed nucleate boiling regime for high-NSD surface, or a combination of the two in the transition regime for medium-NSD surface. Accordingly, we estimated the critical heat flux of each boiling regime at which the boiling crisis occurs. The result indicated that there is a threshold of nucleation site density below which the increase of NSD is contributing to CHF enhancement, while the trend is inverted beyond the threshold.     

Modeling and numerical investigation of hydrogen nucleate flow boiling heat transfer

Liquid hydrogen flow boiling heat transfer in tubes is of great importance in the hydrogen applications such as superconductor cooling, hydrogen fueling. In the present study, a numerical model for hydrogen nucleate flow boiling based on the wall partition heat flux model is established. The key parameters in the model such as active nucleation site density, bubble departure diameter and frequency are carefully discussed and determined to facilitate the modeling and simulation of hydrogen flow boiling. Simulation results of the numerical model show reasonably well agreement with experimental data from different research groups in a wide operation condition range with the means absolute error (MAE) of 10.6% for saturated and 5.3% for subcooled flow boiling. Based on the model, wall heat flux components and void fraction distribution of hydrogen flow boiling are studied. Effects of mass flow rate and wall heat flux on the flow boiling heat transfer performance are investigated. It is found that in the hydrogen nucleate flow boiling, the predominated factor is the Boiling number, rather than the vapor quality. A new simple correlation is proposed for predicting hydrogen saturated nucleate flow boiling Nusselt number. The MAE between the correlation predicted and experimentally measured Nusselt number is 13.6% for circular tubes and 12.5% for rectangular tubes. The new correlation is applicable in the range of channel diameter 4–6.35 mm, Reynolds number 64000–660,000, saturation temperature 22–29 K, Boiling number 8.37 × 10^?5–2.33 × 10^?3.     

A new comprehensive model for nucleate pool boiling heat transfer of pure liquid at low to high heat fluxes including CHF

Based on the fractal distribution of nucleation sites present on heating surfaces, a new comprehensive model is developed for the nucleate pool boiling of pure liquid at low to high heat fluxes including the critical heat flux (CHF). The proposed model is expressed as a function of total number, minimum and maximum sizes of active nucleation sites, fractal dimension, superheat temperature, and properties of fluids. No additional empirical constant is introduced in the proposed model. This fractal model contains less empirical constants than the conventional models. The model predictions are in good agreement with the available experimental data.     

Transient boiling heat transfer performances of subcooled water during quenching process

Transient boiling of subcooled water on a vertical flat surface during quenching process was visually observed using high-speed photography technology and analyzed by special boiling heat transfer models. After the quenching process began, the nucleation was initiated within a short time, and then followed by the outbreak transition boiling with extremely unstable bubbles and sharp heat flux increment which lasted only for about 1 s. According to the boiling curves, the nucleate boiling should be divided into two stages including the stable nucleate boiling and transition nucleate boiling with the effects of initial conditions. The outbreak transition boiling was first studied by special boiling curve corresponding to the temperature decrement, and was further analyzed using a theoretical model based on the combination of the boiling heat flux evolution and lumped parameter assumption. The results showed that the boiling curve of outbreak transition boiling predicted by the theoretical model had a good agreement with experimental data and the heat flux almost had linear relation with the square root of temperature decrement.     

Numerical simulation of nucleate boiling on a horizontal surface at high heat fluxes

Nucleate boiling at high heat fluxes has been studied numerically by solving the equations governing conservation of mass, momentum and energy in the liquid and vapor phases. The interface is captured by using the level set method based on a sharp-interface representation. The evaporative heat flux from the liquid microlayer is incorporated in the analysis. The effects of wall superheat, number density of nucleation sites and waiting period on the bubble dynamics and heat transfer in nucleate boiling are investigated. The heat fluxes obtained from the present numerical simulations are compared with the experimental data reported in the literature.     

The Evolution of Enhanced Surface Geometries for Nucleate Boiling

This paper surveys the evolution of special surface geometries that promote high-performance nucleate boiling. Early work by Jakob and Fritz in 1931 showed that emery paper roughening or machined grooves provided only temporary performance increase. However, this improvement dissipated after a few days to the flat surface value. There was little sustained interest in this unique, but apparently unuseful, phenomenon until the mid-1950s. During the period 1955-1965, supporting fundamental studies of the character and stability of nucleation sites provided a basis for renewed efforts to develop a high area density of stable, artificially formed nucleation sites whose performance does not deteriorate with time. Beginning in 1968 industrial research produced patented technology that achieved the long-sought goal. In 1980 at least six high-performance nucleate boiling surfaces were commercially available. The technology reported in this paper represents a dramatic advance in the field of heat transfer.     

Jet flow phenomena during nucleate boiling

Boiling phenomena are with highly complex nonlinear and nonequilibrium characteristics, which cause diversity and complexity of boiling nucleation. In the present paper, an experimental investigation was conducted to investigate the nucleate boiling behavior on a very fine heating wire. Using zoom routine and CCD camera system, the dynamical process of nucleate boiling was visually observed and several modes of jet flows were explored during nucleate boiling. This phenomenon is quite different from the usual observation of nucleate boiling. High-energy liquid jet, fog-like jet, cluster-like jet, bubble-forming jet, bubble-bunch jet and bubble-top jet were described in detail. The microscopic mechanism concerning the phenomena was discussed.