ZrO2纳米流体的对流换热系数测定及机理浅析

建立了测量圆管内纳米流体流动与传热性能的实验系统,测量了不同粒子浓度的ZrO2/水纳米流体在雷诺数为3 000～18 000范围内的管内对流换热系数以及不同位置处纳米流体对流换热系数的变化情况.实验结果显示,在液体中添加纳米粒子显著增大了液体的管内对流换热系数,例如,在相同雷诺数时,与纯水相比,如果纳米粒子的质量浓度从1.6%增大到4.1%,则纳米流体的对流换热系数增加的比例从1.09增大到1.2.此外,从颗粒的浓度、粒径两方面分析纳米流体强化传热的机理.     

纳米颗粒聚集形态对纳米流体导热系数的影响

纳米流体中悬浮的纳米颗粒可以增强其导热性能已经得到广泛认可,然而纳米流体颗粒增强传热的机理目前尚不清楚.研究表明,纳米颗粒的聚集是纳米流体导热系数增大的重要机制,而且纳米颗粒聚集的形态对纳米流体的导热系数有重要影响,但是目前的导热系数模型大多是建立在Maxwell有效介质理论的"静态"和"均匀分散"假设基础上.本文用平衡分子动力学模拟Cu-Ar纳米流体,采用Green-Kubo公式计算导热系数,采用Schmidt-Ott关系式计算不同聚集形态下的分形维数.对比导热系数与分形维数可以发现:在相同体积分数下,较低的分形维数会有更高的导热系数,分析了分形维数与导热系数的定量关系.此外,通过径向分布函数可以看出纳米颗粒紧密聚集与松散聚集的差异,基液分子在纳米颗粒附近的纳米薄层中处于动态平衡状态.研究结果有助于理解纳米颗粒聚集形态对导热系数的影响机理.     

方腔内Cu/Al2O3水混合纳米流体自然对流的格子Boltzmann模拟

纳米流体作为一种较高的导热介质,广泛应用于各个传热领域.鉴于纳米颗粒导热系数和成本之间的矛盾,本文提出了一种混合纳米流体.为了研究混合纳米流体颗粒间相互作用机理和自然对流换热特性,在考虑颗粒间相互作用力的基础上,利用多尺度技术推导了纳米流体流场和温度场的格子Boltzmann方程,通过耦合流动和温度场的演化方程,建立了Cu/Al2O3水混合纳米流体的格子Boltzmann模型,研究了混合纳米流体颗粒间的相互作用机理和纳米颗粒在腔体内的分布.发现在颗粒间相互作用力中,布朗力远远大于其他作用力,温差驱动力和布朗力对纳米颗粒的分布影响最大.分析了纳米颗粒组分、瑞利数对自然对流换热的影响,对比了混合纳米流体(Cu/Al2O3-水)与单一金属颗粒纳米流体(Al2O3-水)的自然对流换热特性,发现混合纳米流体具有更强的换热特性.     

方腔内Cu/Al2O3水混合纳米流体自然对流的格子Boltzmann模拟

纳米流体作为一种较高的导热介质, 广泛应用于各个传热领域. 鉴于纳米颗粒导热系数和成本之间的矛盾, 本文提出了一种混合纳米流体. 为了研究混合纳米流体颗粒间相互作用机理和自然对流换热特性, 在考虑颗粒间相互作用力的基础上, 利用多尺度技术推导了纳米流体流场和温度场的格子Boltzmann方程, 通过耦合流动和温度场的演化方程, 建立了Cu/Al₂O₃水混合纳米流体的格子Boltzmann模型, 研究了混合纳米流体颗粒间的相互作用机理和纳米颗粒在腔体内的分布. 发现在颗粒间相互作用力中, 布朗力远远大于其他作用力, 温差驱动力和布朗力对纳米颗粒的分布影响最大. 分析了纳米颗粒组分、瑞利数对自然对流换热的影响, 对比了混合纳米流体(Cu/Al₂O₃-水)与单一金属颗粒纳米流体(Al₂O₃-水)的自然对流换热特性, 发现混合纳米流体具有更强的换热特性.     

高压过冷池沸腾换热机理的研究

目前还没有一种被广泛承认的理论能够解释高压过冷池沸腾换热,其机理尚不明确。为了揭示高压池内过冷核沸腾的物理传热机理,并获得气泡脱离频率与活化穴半径的函数关系,根据池内过冷核沸腾加热表面活化穴的分布,在统计方法的基础上,提出了高压池内过冷核沸腾的一个数学模型。从该模型中发现,池内过冷核沸腾热流密度是壁面过热度、液体过冷度、活化穴尺寸、流体的接触角以及流体物理特性的函数。对不同的过冷度,将模型预测的结果与实验数据进行了比较,两者吻合得极好,从而证明了数学模型的可靠性。该解析模型更深刻地揭示了过冷池沸腾换热的物理机理,且没有增加新的经验常数。     

流体在微多孔介质内对流换热实验研究

本文对空气流过烧结微多孔介质内部对流换热进行了实验研究,分析了不同颗粒直径下对流换热努谢尔特数随流量的变化.结果表明:当颗粒直径为200～40μm时,实验得到的对流换热努谢尔特数与已有研究结果符合很好;当颗粒直径为20μm和10 μm时,实验结果略小于已有研究结果,说明空气在微多孔介质中的对流换热需要考虑微尺度效应的影响.同时,根据实验结果给出了微多孔介质内对流换热努谢尔特数与雷诺数的经验关联式,并提出了考虑努森数的修正关联式.     

变物性参数对微通道内纳米流体强制对流换热的影响

对二维微通道内Al_2O_3-水纳米流体的强制对流换热进行了数值研究。主要研究纳米流体的变热物性参数、纳米粒子体积分数φ和Re数对纳米流体强制对流换热的影响。研究表明:在Re数和纳米颗粒体积分数φ一定时,变热物性参数纳米流体比定热物性参数纳米流体在微通道内的强制对流换热强。在Re数一定时,随着纳米粒子体积分数φ的增加,纳米流体换热性能增强。在纳米粒子体积分数φ一定时,随着Re数的增加,纳米流体的换热能力也随之增加。     

格子Boltzmann方法模拟填充纳米流体三角形腔内的自然对流

采用格子Boltzmann方法研究填充水-氧化铝纳米流体的等腰直角三角形腔体中的自然对流。讨论瑞利数、颗粒体积分数、热源位置等因素对对流换热的影响,以及不同纳米流体模型对模拟结果的影响。结果表明：在低瑞利数下,随着热源在左壁面向上移动,换热效率逐渐增加。而在高瑞利数（Ra=10⁶）时,观察到相反的现象;采用单相纳米流体模型,模拟表明热壁面平均努塞尔数比率随着体积分数的增加近似线性增加。采用改进的纳米流体模型,结果显示平均努塞尔数比率随着体积分数的增加而增大,但是平均努塞尔数比率的变化斜率逐渐减小。改进模型模拟的换热效率比单相模型高,这是因为改进模型考虑了粒子间作用力及换热,更符合实际情况。     

池沸腾传热的数学分析

在统计方法的基础上,对于池沸腾换热的传热机理提出了一个数学模型. 在没有增加新的经验常数的条件下,从该模型中可得到池沸腾热流密度是壁面过热度、活化穴最小与最大尺寸、流体的接触角与流体物理特性的函数. 该模型可以较好地解释润湿性如何影响沸腾热流密度. 对不同的接触角,模型预测的结果与实验相符合. 关键词： 池沸腾 传热 数学模型     

纳米流体对流换热的实验研究

建立了测量纳米流体对流换热系数的实验系统,测量了不同粒子体积份额的水-Cu纳米流体在层流与湍流状态下的管内对流换热系数,实验结果表明,在液体中添加纳米粒子增大了液体的管内对流换热系数,粒子的体积份额是影响纳米流体对流换热系数的因素之一。综合考虑影响纳米流体对流换热的多种因素,提出了计算纳米流体对流换热系数的关联式。     

A fractal model for heat transfer of nanofluids by convection in a pool

Based on the fractal distribution of nanoparticles, a fractal model for heat transfer of nanofluids is presented in the Letter. Considering heat convection between nanoparticles and liquids due to the Brownian motion of nanoparticles in fluids, the formula of calculating heat flux of nanofluids by convection is given. The proposed model is expressed as a function of the average size of nanoparticle, concentration of nanoparticle, fractal dimension of nanoparticle, temperature and properties of fluids. It is shown that the fractal model is effectual according to a good agreement between the model predictions and experimental data.     

A fractal study for nucleate pool boiling heat transfer of nanofluids

In this paper, a fractal model for nucleate pool boiling heat transfer of nanofluids is developed based on the fractal distribution of nanoparticles and nucleation sites on boiling surfaces. The model shows the dependences of the heat flux on nanoparticle size and the nanoparticle volume fraction of the suspension, the fractal dimension of the nanoparticle and nucleation site, temperature of nanofluids and properties of fluids. The fractal model predictions show that the natural convection stage continues r...     

Prediction of heat transfer of nanofluid on critical heat flux based on fractal geometry

Analytical expressions for nucleate pool boiling heat transfer of nanofluid in the critical heat flux (CHF) region are derived taking into account the effect of nanoparticles moving in liquid based on the fractal geometry theory. The proposed fractal model for the CHF of nanofluid is explicitly related to the average diameter of the nanoparticles, the volumetric nanoparticle concentration, the thermal conductivity of nanoparticles, the fractal dimension of nanoparticles, the fractal dimension of active cavities on the heated surfaces, the temperature, and the properties of the fluid. It is found that the CHF of nanofluid decreases with the increase of the average diameter of nanoparticles. Each parameter of the proposed formulas on CHF has a clear physical meaning. The model predictions are compared with the existing experimental data, and a good agreement between the model predictions and experimental data is found. The validity of the present model is thus verified. The proposed fractal model can reveal the mechanism of heat transfer in nanofluid.     

Subcooled pool boiling heat transfer in fractal nanofluids:A novel analytical model

A novel analytical model to determine the heat flux of subcooled pool boiling in fractal nanofluids is developed. The model considers the fractal character of nanofluids in terms of the fractal dimension of nanoparticles and the fractal dimen- sion of active cavities on the heated surfaces; it also takes into account the effect of the Brownian motion of nanoparticles, which has no empirical constant but has parameters with physical meanings. The proposed model is expressed as a function of the subcooling of fluids and the wall superheat. The fractal analytical model is verified by a reasonable agreement with the experimental data and the results obtained from existing models.     

Flow boiling heat transfer of alumina nanofluids in single microchannels and the roles of nanoparticles

This study investigates flow boiling heat transfer of aqueous alumina nanofluids in single microchannels with particular focuses on the critical heat flux (CHF) and the potential dual roles played by nanoparticles, i.e., (i) modification of the heating surface through particle deposition and (ii) modification of bubble dynamics through particles suspended in the liquid phase. Low concentrations of nanofluids (0.001–0.1 vol.%) are formulated by the two-step method and the average alumina particle size is ~25 nm. Two sets of experiments are performed: (a) flow boiling of formed nanofluids in single microchannels where the effect of heating surface modification by nanoparticle deposition is apparent and (b) bubble formation in a quiescent pool of alumina nanofluids under adiabatic conditions where the role of suspended nanoparticles in the liquid phase is revealed. The flow boiling experiments reveal a modest increase in CHF by nanofluids, being higher at higher nanoparticle concentrations and higher inlet subcoolings. The bubble formation experiments show that suspended nanoparticles in the liquid phase alone can significantly affect bubble dynamics. Further discussion reveals that both roles are likely co-existent in a typical boiling system. Properly surface-promoted nanoparticles could minimize particle deposition hence little modification of the heating surface, but could still contribute to the modification in heat transfer through the second mechanism, which is potentially promising for microchannel applications.     

含不同形状内热源的圆管内纳米流体自然对流及换热数值研究

采用格子玻尔兹曼方法对有三种恒温热源（圆形、三角形、方形）参与的圆管内纳米流体（铜-水）自然对流进行数值研究。主要研究瑞利（Ra）数,纳米颗粒体积分数以及热源几何形状等控制参数对纳米流体的流动与传热的影响。结果发现纳米颗粒体积分数的增加有利于强化传热,且在Ra数较小时,平均努塞尔（Nu）数增加的幅度要优于Ra数较大的情况。在所研究的控制参数范围内,方形热源的平均Nu数最大。根据数值结果给出不同热源表面的平均Nu数、纳米颗粒体积分数、Ra数三者之间的函数关系式,该函数关系可为此类工程的设计提供理论指导。     

Thermal conductivity of nanofluids and size distribution of nanoparticles by Monte Carlo simulations

Nanofluids, a class of solid–liquid suspensions, have received an increasing attention and studied intensively because of their anomalously high thermal conductivites at low nanoparticle concentration. Based on the fractal character of nanoparticles in nanofluids, the probability model for nanoparticle’s sizes and the effective thermal conductivity model are derived, in which the effect of the microconvection due to the Brownian motion of nanoparticles in the fluids is taken into account. The proposed model is expressed as a function of the thermal conductivities of the base fluid and the nanoparticles, the volume fraction, fractal dimension for particles, the size of nanoparticles, and the temperature, as well as random number. This model has the characters of both analytical and numerical solutions. The Monte Carlo simulations combined with the fractal geometry theory are performed. The predictions by the present Monte Carlo simulations are shown in good accord with the existing experimental data.     

The mechanism of heat transfer in nanofluids: state of the art (<Emphasis Type="Italic">review</Emphasis>). Part 1. Synthesis and properties of nanofluids

Here is the review of experimental and theoretical results on the mechanism of heat transfer in nanofluids. A wide scope of problems related to the technology of nanofluid production, experimental equipment, and features of measurement methods is considered. Experimental data on heat conductivity of nanofluids with different concentrations, sizes, and material of nanoparticles are presented. Results on forced and free convection in laminar, and turbulent flows are analyzed. The available models of physical mechanisms of heat transfer intensification and suppression in nanofluids are presented. There are significant divergences in data of different researchers; possible reasons for this divergence are analyzed.     

细圆管内纳米颗粒悬浮液强化对流换热的探讨

实验研究了细圆管内去离子水和氧化铜纳米颗粒悬浮液的对流换热特性。根据实验数据,得到纳米颗粒悬浮液相对于去离子水的对流换热强化特性。结果表明,氧化铜纳米颗粒的加入强化了去离子水的换热性能,其强化程度随Re的变化因管径而异,并且与流态有关。