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.     

羧基化碳纳米管/水纳米流体核沸腾传热研究

将羧基基团引入多壁碳纳米管,改善了碳纳米管在水中的分散性及稳定性。同时研究了不同质量浓度纳米流体的导热系数、加热表面颗粒沉积、接触角变化对核沸腾传热性能的影响。结果表明;羧基化碳纳米流体可强化核沸腾传热。在测试浓度范围内,强化率在低热通时,随着热通量的增加急剧增大,高热通时,趋于稳定;当质量比ω为0.10%,功率为210.6 kW.m~(-2)时,强化率达到最大为138.3%;流体的导热系数随着质量浓度的增大而增大,0.15%浓度导热系数是纯水的1.18倍。分析认为纳米流体表面张力,纳米颗粒沉积,纳米颗粒扰动和导热系数的变化均是影响水基羧基化碳纳米流体沸腾的因素。结论由0.05%的纳米流体沸腾过程高速成像得到验证。     

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.     

基于基液连续假设的大体系Cu-H2O纳米流体输运特性的模拟研究

分子动力学模拟是研究纳米流体的输运特性的重要手段, 但计算量庞大. 为研究能体现流动传热过程的大体系纳米流体的输运特性, 本文对基液采用连续介质假设, 将基液的势能拟合在纳米团簇的势能中, 大幅度减小了计算量, 使得大体系输运特性的模拟成为可能, 且模拟结果与多组实验结果吻合较好. 采用此方法模拟研究了速度梯度剪切对Cu-H₂O纳米流体颗粒聚集过程和聚集特性的影响, 进而对Cu-H₂O纳米流体在流动传热过程中的热导率和黏度进行了模拟计算, 定量揭示了宏观流动传热过程中不同的速度梯度、速度、平均温度和温度梯度对于Cu-H₂O纳米流体热导率和黏度的影响.     

以R22为冷却介质的机载闭式喷雾冷却传热性能关联式研究

以机载应用为研究背景,搭建了使用R22的闭式喷雾冷却实验系统,考察了加热表面在介质沸腾状态和临界失效状态的传热性能,为机载高热流密度武器热管理系统的设计提供参考。定义了表征液滴撞击、液膜流动、蒸发换热和沸腾换热的无量纲数,分别为雷诺数Re、韦伯数We、雅可比数Ja和无量纲温度ε,并给出了适用于沸腾换热和临界后失效状态的传热性能关联式,关联式结构简单,实验值与拟合值的相对误差均在±15%以内,满足航空工程设计计算要求。拟合过程中分析了各无量纲数对关联式精度的影响,得出在沸腾换热阶段,韦伯数We的影响不可忽略;雅可比数Ja较之无量纲温度ε更适合用来反映沸腾换热阶段流体和表面的温度变化;在过临界失效阶段主要影响传热性能的是韦伯数We。     

矩形微槽道纳米流体饱和沸腾临界热流密度特性

针对纳米流体在微小尺度传热领域的应用,在常压下对微槽道中纳米流体的流动沸腾临界热流密度进行实验研究。分别以体积浓度为0.2%、0.5%的水基Al2O3纳米流体为工质进行试验,研究不同质量流速、槽道尺寸以及体积浓度等因素对沸腾CHF的影响。对比水为工质实验结果,表明:槽道尺寸、质量流速对于水-Al2O3纳米流体和纯水的CHF影响一致。其它参数一定的工况下,纳米流体CHF比纯水大,且随着纳米流体体积浓度增大,出口壁面过热度会增大。最后介绍一个微槽道沸腾CHF的预测模型,在评价其不足的基础上提出一个关于CHF的预测公式,与实验数据进行对比,验证该公式的适用性。     

浮升力和流动加速对超临界CO2管内流动传热影响

基于单相流体的概念,超临界流体的异常传热行为已经被研究很多年了,但是关于其流动传热机理仍没有统一的认识.本文通过理论分析和实验研究了超临界二氧化碳在竖直管内向上流动过程中,浮升力和流动加速效应对其流动结构和传热过程的影响.结果表明,没有确凿的实验证据表明超临界流体的异常传热行为是浮升力和流动加速直接导致的,存在的估计浮升力和流动加速效应准则均是在常物性流体的基础上,做了大量假设得出的,不同的研究者采用浮升力和流动加速准则分析超临界流体的传热恶化得出的结论不一致.最后,基于拟沸腾理论分析超临界流体的传热恶化过程,提出超临界沸腾数区分了超临界流体正常传热与恶化传热的转换边界,为超临界流体流动传热研究提供新思路,超临界沸腾数对建立用于不同技术的超临界流体动力循环的最佳运行条件具有重要意义.     

内置弹簧线圈平板通道流动传热实验研究

提出了在平行平板通道中等间隔布置轴线垂直于流动方向的弹簧线圈以强化传热的方法,并对其换热和流动阻力特性进行了实验研究。实验分别对水-水换热和水-油换热进行了测试,得到了所测参数范围内水侧和油侧的换热及阻力关联式。通过与典型人字形板式换热器实验结果的对比,分析了弹簧线圈对不同黏性流体流动换热的影响,得出平板间填充弹簧线圈的结构更适用于高黏性流体的流动换热。     

碳纳米流体相变特性实验研究

通过"两步法"制备分散均匀、稳定的单壁碳纳米管/水、多壁碳纳米管/水碳纳米流体,通过差示扫描量热法(DSC)测试分析了碳纳米流体的凝固、熔化相变过程,实验结果表明在凝固相变过程中随着碳纳米管质量分数的增加,碳纳米流体的起始凝固温度及凝固温度逐渐升高,单壁碳纳米流体的凝固温度显著提高,在熔化相变过程中,碳纳米流体的质量分数对于纳米流体的熔点无明显影响,但对于碳纳米流体的潜热影响较大,碳纳米流体的潜热随着质量分数的增加而明显减小。     

纳米流体对流换热机理分析

考虑在纳米流体中纳米颗粒做布朗运动引起的对流换热, 基于纳米颗粒在纳米流体中遵循分形分布, 本文得到纳米流体对流换热的机理模型. 本解析模型没有增加新的经验常数, 从该模型发现纳米流体池沸腾热流密度是温度、纳米颗粒的平均直径、 纳米颗粒的浓度、纳米颗粒的分形维数、沸腾表面活化穴的分形维数、基本液体的物理特性的函数. 对不同的纳米颗粒浓度和不同的纳米颗粒平均直径与不同的实验数据进行了比较, 模型预测的结果与实验结果相吻合. 所得的解析模型可以更深刻地揭示纳米流体对流换热的物理机理.     

Comparative Study on Heat Transfer Enhancement of Low Volume Concentration of Al2o3–Water and Carbon Nano-Tube–Water Nano-Fluids in Transition Regime Using Helical Screw Tape Inserts

This study reports the comparison of heat transfer and friction factor characteristics of helical screw inserts in Al₂O₃–water and carbon nano-tube–water nano-fluids through a straight pipe in transition regime with constant heat flux boundary condition. Experiments were carried out by using 0.15% volume concentration of Al₂O₃–water and carbon nano-tube–water nano-fluid with helical tape inserts of twist ratio, TR = 1.5, 2.5, and 3. The thermal performance of helical screw tape inserts with the carbon nano-tube–water nano-fluid is found -to be higher when compared to the Al₂O₃–water nano-fluid. In addition, the maximum enhancement in heat transfer was obtained for the carbon nano-tube–water nano-fluid with helical tape inserts of twist ratio 1.5. The increase in pressure drop of the Al₂O₃–water nano-fluid with helical screw tape inserts is found to be higher compared to the carbon nano-tube–water nano-fluid helical screw tape inserts at lower value of twist ratio.     

微细管碳纳米管悬浮液强制对流换热实验研究

测量了水平微细圆管内蒸馏水和不同质量浓度的水基多壁碳纳米管纳米流体在低雷诺数下的强制对流换热特性。实验结果表明,与蒸馏水相比,纳米流体的对流换热系数显著提高,且随质量浓度和管内雷诺数的增大而增大;并且研究了流体管内流动阻力特性,得到的泊肃叶数f·Re值随着雷诺数的变化不明显,但纳米流体的f·Re值要明显小于纯水。     

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...     

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.     

Convective Heat Transfer of a Cu/Water Nanofluid Flowing Through a Circular Tube

Abstract

Fluids in which nanometer-sized solid particles are suspended are called nanofluids. These fluids can be employed to increase the heat transfer rate in various applications. In this study, the convective heat transfer for Cu/water nanofluid through a circular tube was experimentally investigated. The flow was laminar, and constant wall temperature was used as thermal boundary condition. The Nusselt number of nanofluids for different nanoparticle concentrations, as well as various Peclet numbers, was obtained. Also, the rheological properties of the nanofluid for different volume fractions of nanoparticles were measured and compared with theoretical models. The results show that the heat transfer coefficient is enhanced by increasing the nanoparticle concentrations as well as the Peclet number.     

Experimental investigation into the pool boiling heat transfer of aqueous based γ-alumina nanofluids

This paper is concerned about pool boiling heat transfer using nanofluids, a subject of several investigations over the past few years. The work is motivated by the controversial results reported in the literature and the potential impact of nanofluids on heat transfer intensification. Systematic experiments are carried out to formulate stable aqueous based nanofluids containing γ-alumina nanoparticles (primary particle size 10–50 nm), and to investigate their heat transfer behaviour under nucleate pool boiling conditions. The results show that alumina nanofluids can significantly enhance boiling heat transfer. The enhancement increases with increasing particle concentration and reaches ∼ ∼40% at a particle loading of 1.25% by weight. Discussion of the results suggests that the reported controversies in the thermal performance of nanofluids under the nucleate pool boiling conditions be associated with the properties and behaviour of the nanofluids and boiling surface, as well as their interactions.     

几种氧化物纳米流体强化传热性能研究

以水/乙二醇混合液为基液,加入A12O3,MgO和ZnO纳米颗粒配制得到纳米流体.在自制对流传热性能测试平台上进行基液及纳米流体传热性能的测试.结果表明:同基液相比,随流体流速增大,A12O3纳米流体的传热系数变化不明显,MgO和ZnO纳米流体的传热系数均有提高.层流状态下,随雷诺数增大,三种纳米流体的传热系数都不断增...     

Microscopic Boiling of Liquid Nitrogen Induced by a Pulsed Laser Irradiation

An experimental investigation was conducted to explore the characteristics of microscopic boiling induced by firing a microsecond pulsed laser beam on a thin platinum (Pt) film that immerged in the liquid nitrogen (LN₂) cryostat. High-speed photography aided by a high-voltage lighting system was employed to visually observe the bubble formation and the dynamical boiling process of LN₂. A rapid transient temperature-measuring system was designed to record the temperature evolution of the heating surface. Explosive boiling, characterized by bubble cluster, was observed within LN₂ at the early stage of laser heating, and conventional boiling followed after a certain time. The transition time, therefore, was introduced for separating these two different boiling modes. The temperature of Pt film rose sharply to its maximum during laser pulse, with a very high rising rate of about 10⁷ K/s, and then dropped rapidly after laser irradiation. A model of bubble cluster was proposed to describe the explosive boiling heat transfer, and the latent heat released by bubble collapse in explosive boiling was explored as an important mechanism considerably influencing the boiling heat transfer.     

Experimental investigation into the pool boiling heat transfer of aqueous based γ-alumina nanofluids

This paper is concerned about pool boiling heat transfer using nanofluids, a subject of several investigations over the past few years. The work is motivated by the controversial results reported in the literature and the potential impact of nanofluids on heat transfer intensification. Systematic experiments are carried out to formulate stable aqueous based nanofluids containing γ-alumina nanoparticles (primary particle size 10–50 nm), and to investigate their heat transfer behaviour under nucleate pool boiling conditions. The results show that alumina nanofluids can significantly enhance boiling heat transfer. The enhancement increases with increasing particle concentration and reaches ∼ ∼40% at a particle loading of 1.25% by weight. Discussion of the results suggests that the reported controversies in the thermal performance of nanofluids under the nucleate pool boiling conditions be associated with the properties and behaviour of the nanofluids and boiling surface, as well as their interactions.This revised version was published online in August 2005 with a corrected issue number.