纳米流体传热强化技术

主要汇总了国内外纳米流体传热强化技术的研究成果,对纳米流体传热强化技术的国内外研究发展状况进行了综述;针对纳米流体的物性参数及流动情况,分析了纳米流体的强化传热机理;并具体阐述了纳米流体的主要物性参数——导热系数和粘度的影响因素;叙述了纳米流体的在各个领域中的应用并对其未来进行了展望。     

纳米流体强化传热研究分析

文中综述了目前国内外对于纳米流体强化传热技术的研究情况,分析了纳米流体的强化传热机理及添加纳米粒子后对液体的物性参数--粘度、比热、密度、流体流动的影响;说明了石墨/水纳米流体及Fe3O4/水纳米流体导热系数和对流换热系数测量实验的原理及结果,并对结果进行了分析,实验结果表明纳米流体强化了传热.     

内燃机冷却系统中应用纳米流体强化传热研究综述

内燃机工作时依赖冷却系统将多余热量及时带走以保证燃烧室核心部件及润滑油膜的正常工作温度。常规内燃机冷却介质导热系数偏低,而新一代强化传热工质纳米流体具有明显提升的传热性能,应用于内燃机冷却系统有利于强化内燃机传热及提高热管理性能。且由于纳米流体的传热性能受纳米粒子的种类、大小、浓度、形状等因素影响,可以通过改变这些因素控制内燃机冷却水腔的传热量。综述了国内外研究者针对纳米流体导热系数与对流换热性能开展的试验测试、理论分析和计算机模拟研究工作,以及纳米流体应用于内燃机冷却系统中强化传热的进展,最后指出当前研究工作的不足及未来工作方向。     

纳米流体在直接吸收式太阳能集热器上的应用研究

综述了提高集热器集热效率的措施及目前的研究进展,说明了直接吸收式太阳能集热器的优越性。对纳米流体稳定性的影响因素进行了总结,并从太阳辐射吸收、导热性能及对流换热性能三个方面阐述了纳米流体作为集热工质的优越性。表明了纳米流体作为集热工质在直接吸收式太阳能上应用的可行性,并展望了纳米流体作为传热工质在在供暖上的应用。     

十四烷纳米流体导热系数影响因素探究

十四烷是工业中最常用的液态烷烃之一,常被用于有机溶剂，有重要的应用价值。相比于纯烷烃，烷烃基纳米流体具有许多 优异的性质，特别是导热系数的增强。本文采用实验与理论模型对比的方法，对一些影响十四烷基纳米流体导热系数的因素进行研究，包括纳米颗粒种类、浓度、温度以及稳定性。结果表明，本文中纳米流体的有效导热系数随纳米颗粒体积分数的增加而增加，随温度的升高而下降；在各种纳米颗粒中，碳纳米管对导热的增强最为显著，且碳纳米管流体具有最好稳定性。     

纳米流体在螺旋通道中数值模拟的研究进展

纳米流体作为一种新型高效的换热介质,其在螺旋通道中的应用被广泛研究。尤其是通过单相和两相模型对其流动及传热特性进行数值模拟,是研究热点。介绍了单相模型中纳米流体物理参数的计算关联式,包括密度、比热、黏度及导热系数等,并分析了不同关联式对同一参数计算结果的影响;总结了单相模型下纳米流体在螺旋通道中的流动及传热特性,并与两相Lagrangian-Eulerian及Mixture模型下得到的结论进行对比分析;展望了未来研究的发展趋势。     

纳米流体毛细弯液蒸发界面热质迁移特性分析

基于增广杨拉普拉斯方程的毛细弯液面薄膜蒸发区的传热传质模型,数值分析了过热度和纳米流体工质对毛细弯液面薄膜蒸发区热质迁移特性的影响。结果表明,过热度增大导致薄膜区范围减小,蒸发界面热流密度增大,薄膜区总换热量增大,但同时减弱了薄膜界面的稳定性。在传统流体工质中添加合适的纳米颗粒,纳米流体运动粘性系数随体积分率增大而减小,导热系数随体积分率增大而增大,影响其传热传质效果。较大体积分率的纳米流体,其薄膜厚度更小,薄膜区热流密度和蒸发质量流率更大,但同时蒸发界面的稳定性减弱。不同种类的纳米流体对毛细弯液蒸发界面的影响也较为明显,具有较低运动粘性系数和较高导热系数的纳米流体能够迁移更多的热量。     

纳米流体在微通道换热中的研究进展

随着微尺度应用需求日益增长,纳米流体与微通道分别作为强化传热流动介质与强化传热结构获得学者们的广泛关注。主要概述了纳米流体的制备方法与稳定性,以纳米颗粒及基液类型、纳米颗粒的浓度、粒径以及强化传热机理为类别,综述了纳米流体在不同结构微通道中传热与流动性能的研究进展。通过分析已发表的研究成果,总结了纳米流体在微通道换热中的研究难点,提出了研究纳米流体在微通道中流动与传热特性的主要方向。     

纳米流体强化内燃机冷却系统流动的基础研究

利用高导热率、传热性能好的传热工质(纳米流体)替代传统冷却介质应用于内燃机冷却系统中,通过纳米流体流动特性的基础研究,为其在内燃机冷却系统中的应用提供理论基础支持.因此,利用试验方法对纳米流体在波壁管内的流动进行可视化研究,以期对纳米流体的流动机理进行详细的探讨,从而推动纳米流体在内燃机冷却系统中的应用.研究发现:纳米流体的黏度增加值不大,且随着温度的升高,增加值降低;而相同入口速度状态下,纳米流体在波壁管内的流动比纯水更为活跃,漩涡数量增多,质量传递特性增强,且随纳米颗粒浓度的增加,流动湍流效应增大.通过分子动力学方法发现纳米颗粒在纳米流体流动过程中存在强烈的旋转作用,从而出现微湍流流动效应,进一步强化了纳米流体的湍流流动效果.     

活塞内冷油腔中纳米流体振荡流动与传热特性影响因素研究

采用计算流体力学的方法,研究了分别含Al₂O₃、Cu O、Si O₂的3种纳米机油在纳米颗粒体积分数为1%、3%、5%时相对于传统机油的振荡传热能力和机油在油腔内流动的规律。结果表明,纳米颗粒的加入改变了流体的物性参数,纳米流体的传热效果比传统机油更好,且内冷油腔的传热系数随着纳米流体体积分数的增加而增加,但对内冷油腔内瞬态机油的瞬态分布和充油率的影响不大;纳米流体的黏度、密度、导热系数、比热容都能影响内冷油腔的传热性能,密度的增加会使流体对壁面的冲击作用更强,从而增强油腔的传热能力;在纳米颗粒体积分数为5%时CuO纳米机油的传热系数比Al₂O₃、SiO₂纳米机油分别高8.2%和14.6%。     

Experimental and theoretical studies of thermal conductivity,viscosity and heat transfer coefficient of titania and alumina nanofluids

Thermal conductivity, viscosity and heat transfer coefficient of water-based alumina and titania nanofluids have been investigated. The thermal conductivity of alumina nanofluids follow the prediction of Maxwell model, whilst that of titania nanofluids is slightly lower than model prediction because of high concentration of stabilisers. None of investigated nanofluids show anomalously high thermal conductivity enhancement frequently reported in literature. The viscosity of alumina and titania nanofluids was higher than the prediction of Einstein–Batchelor model due to aggregation. Heat transfer coefficients measured in nanofluids flowing through the straight pipes are in a very good agreement with heat transfer coefficients predicted from classical correlation developed for simple fluids. Experimental heat transfer coefficients in both nanofluids as well as corresponding wall temperatures agree within ±10% with the values obtained from numerical simulations employing homogeneous flow model with effective thermo-physical properties of nanofluids. These results clearly shows that titania and alumina nano-fluids do not show unusual enhancement of thermal conductivity nor heat transfer coefficients in pipe flow frequently reported in literature.     

Numerical study of nanofluid heat transfer for different tube geometries – A comprehensive review on performance

The heat transfer performance of a system can be improved using a combination of passive methods, namely nanofluids and various types of tube geometries. These methods can help enhance the heat transfer coefficient and consequently reduce the weight of the system. In this paper, the effect of tube geometry and nanofluids towards the heat transfer performance in the numerical system is reviewed. The forced convective heat transfer performance, friction factor and wall shear stress are studied for nanofluid flow in different tube geometries. The thermo-physical properties such as density, specific heat, viscosity and thermal conductivity are reviewed for the determination of nanofluid heat transfer numerically. Various researchers had measured and modelled for the determination of thermal conductivity and viscosity of nanofluids. However, the density and specific heat of nanofluids can be estimated with the mixture relations. The different tube geometries in simulation work are analyzed namely circular tube, circular tube with insert, flat tube and horizontal tube. It was observed that the circular tube with insert provides the highest heat transfer coefficient and wall shear stress. Meanwhile, the flat tube has greater heat transfer coefficient with a higher friction factor compared to the circular tube.     

Buoyancy-Driven Heat Transfer in Nanofluid-Filled Trapezoidal Enclosure with Variable Thermal Conductivity and Viscosity

Heat transfer performance utilizing nanofluids in a trapezoidal enclosure is investigated taking into account variable thermal conductivity and viscosity. Transport equations are modelled by a stream-vorticity formulation, and are solved numerically by the finite difference method. The effects of the Rayleigh number, base angle, volume fraction, and size of nanoparticles on flow and temperature patterns as well as the heat transfer rate are presented. We found that the effect of the viscosity was more dominant than the thermal conductivity, and there is almost no improvement in heat transfer performance utilizing nanofluids.     

Thermo-physical properties of hybrid nanofluids and hybrid nanolubricants: A comprehensive review on performance

Hybrid nanofluids and hybrid nanolubricants are very new types of research which can be prepared by suspending two or more than two dissimilar nanoparticles either in a mixture or composite form in the base fluids. The term hybrid can be considered as different materials which are a combination of physical and chemical properties to form a homogeneous phase. The main objective of synthesizing hybrid nanofluids/nanolubricants is to improve the properties of single materials where it has great enhancement in thermal properties or rheological properties that are better than individually conventional nanofluids/nanolubricants. This review summarizes the previous research on the thermo-physical properties of hybrid nanofluids/nanolubricants including methods of preparation, instrumentations, development and current progress, and hybrid performance in terms of heat transfer and pressure drop. Challenges and several applications using hybrid nanofluids/nanolubricants were also discussed. Recent studies showed that the hybrid nanofluids/nanolubricants improved the performance of the single type suspended nanoparticles. Various studies of hybrid nanofluids have been carried out to investigate the heat transfer performance and thermal conductivity; however, other thermo-physical properties such as viscosity, density and specific heat have been neglected. In addition, few studies on hybrid nanolubricants were done only for thermo-physical properties. Thus, a comprehensive study on heat transfer and the other thermo-physical properties are necessary to show the potential of hybrid in engineering applications.     

A Review of Thermal Conductivity Models for Nanofluids

Nanofluids, as new heat transfer fluids, are at the center of attention of researchers, while their measured thermal conductivities are more than for conventional heat transfer fluids. Unfortunately, conventional theoretical and empirical models cannot explain the enhancement of the thermal conductivity of nanofluids. Therefore, it is important to understand the fundamental mechanisms as well as the important parameters that influence the heat transfer in nanofluids. Nanofluids’ thermal conductivity enhancement consists of four major mechanisms: Brownian motion of the nanoparticle, nanolayer, clustering, and the nature of heat transport in the nanoparticles. Important factors that affect the thermal conductivity modeling of nanofluids are particle volume fraction, temperature, particles size, pH, and the size and property of nanolayer. In this paper, each mechanism is explained and proposed models are critically reviewed. It is concluded that there is a lack of a reliable hybrid model that includes all mechanisms and influenced parameters for thermal conductivity of nanofluids. Furthermore, more work needs to be conducted on the nature of heat transfer in nanofluids. A reliable database and experimental data are also needed on the properties of nanoparticles.     

Numerical Investigation on Conjugate Heat Transfer Performance of Microchannel Using Sphericity-Based Gold and Carbon Nanoparticles

Numerical study has been carried out on the laminar forced convection flow of nanofluids in a wide rectangular microchannel. The flow and heat transfer characteristics of gold and of single-walled carbon (SWCNT) nanofluids are investigated in order to find an efficient and cost-effective heat transfer fluid. The effects of nanoparticle volume concentration and of spherical and cylindrical particulate sizes on the conjugate heat transfer performance of the microchannel are reported. The effective thermal conductivity of a nanofluid is evaluated on the basis of particle sphericity by considering the volume and surface area of the nanoparticles. The average convective heat transfer coefficient increases with increase in Reynolds number and volume concentration. Moreover, sphericity-based thermal conductivity evaluation showed that increasing the length of the SWCNT nanoparticle has significant effect on the heat transfer performance, concluding that axial heat conduction dominates the radial heat conduction within the nanoparticle. The carbon nanofluid is identified as an optimized heat transfer fluid with better heat transfer characteristics in comparison with the gold nanofluid. It also reduces the cost of the working fluid. The variations in the interface temperature between solid and fluid regions are reported for nanofluids with different concentrations at different Reynolds numbers. The diameter and length of the SWCNT nanoparticle show a significant effect on heat transfer characteristics.     

Numerical investigation on supercritical turbulent heat transfer of copper/n-decane nanofluid inside a miniature tube

To evaluate the potential benefits of kerosene-based nanofluids as coolants for regenerative cooling system, a detailed numerical study of the turbulent heat transfer of copper/n-decane nanofluid flowing inside a miniature cooling tube at supercritical pressures has been conducted. Numerical results reveal that copper nanoparticles can significantly improve heat transfer performance in the entire cooling tube. This can be explained by the fundamental mechanism that within the near-wall turbulent flow region, the reduction of nanofluid kinematic viscosity leads to increased turbulent thermal conductivity and consequently causes heat transfer enhancement. Moreover, heat transfer deterioration phenomenon is delayed or weakened by nanoparticles, and the overall heat transfer performance of the base fluid has been improved. Results indicate potential advantages of kerosene nanofluids as coolants for regenerative engine cooling applications.