Heat transfer and flow behaviour of aqueous suspensions of titanate nanotubes (nanofluids)

Titanate nanotubes of an aspect ratio of ~ 10 are synthesized, characterised and dispersed in water to form stable nanofluids containing 0.5, 1.0 and 2.5 wt.% of the nanotubes. Experiments are then carried out to investigate the effective thermal conductivity, rheological behaviour and forced convective heat transfer of the nanofluids. The results show a small thermal conductivity enhancement of ~ 3% at 25 °C and ~ 5% at 40 °C for the 2.5 wt.% nanofluid. The nanofluids are found to be non-Newtonian with obvious shear thinning behaviour with the shear viscosity decreasing with increasing shear rate at low shear rates. The shear viscosity approaches constant at a shear rate higher than ~ 100-1000 s^− 1 depending nanoparticle concentration. The high shear viscosity is found to be much higher than that predicted by the conventional viscosity models for dilute suspensions. Despite the small thermal conduction enhancement, an excellent enhancement is observed on the convective heat transfer coefficient, which is much higher than that of the thermal conductivity enhancement. In comparison with nanofluids containing spherical titania nanoparticles under similar conditions, the enhancement of both thermal conductivity and convective heat transfer coefficient of the titanate nanotube nanofluids is considerably higher indicating the important role of particle shape in the heat transfer enhancement. Possible mechanisms are also proposed for the observed enhancement of the convective heat transfer coefficient.     

纳米流体对流传热的研究进展

将纳米颗粒加入到传统换热介质中形成的纳米流体是一种新型的强化传热介质。它不仅具有较高的单相对流传热系数,而且分散稳定,不容易磨损和堵塞管道。文中综述了纳米流体在对流条件下强化传热的实验研究进展及强化传热模型,并对实验结果和强化传热的机理及模型进行了简要分析。发现:纳米流体强化对流传热的效果与颗粒和基液的属性等有关;强化传热主要是由于导热系数的增加和纳米颗粒的运动及重新分布引起的物性变化等影响。同时提出了纳米流体对流传热研究中存在及有待改进的一些问题。     

Effect of alumina nanoparticles in the fluid on heat transfer in double-pipe heat exchanger system

This study was performed to investigate the convective heat transfer coefficient of nanofluids made of several alumina nanoparticles and transformer oil which flow through a double pipe heat exchanger system in the laminar flow regime. The nanofluids exhibited a considerable increase of heat transfer coefficients. Although the thermal conductivity of alumina is not high, it is much higher than that of the base fluids. The nanofluids tested displayed good thermal properties. One of the possible reasons for the enhancement on heat transfer of nanofluids can be explained by the high concentration of nanoparticles in the thermal boundary layer at the wall side through the migration of nanoparticles. To understand the enhancement of heat transfer of nanofluid, an experimental correlation was proposed for an alumina-transformer oil nanofluid system.     

Natural convection heat transfer of nanofluids along a vertical plate embedded in porous medium

The unsteady natural convection heat transfer of nanofluid along a vertical plate embedded in porous medium is investigated. The Darcy-Forchheimer model is used to formulate the problem. Thermal conductivity and viscosity models based on a wide range of experimental data of nanofluids and incorporating the velocity-slip effect of the nanoparticle with respect to the base fluid, i.e., Brownian diffusion is used. The effective thermal conductivity of nanofluid in porous media is calculated using copper powder as porous media. The nonlinear governing equations are solved using an unconditionally stable implicit finite difference scheme. In this study, six different types of nanofluids have been compared with respect to the heat transfer enhancement, and the effects of particle concentration, particle size, temperature of the plate, and porosity of the medium on the heat transfer enhancement and skin friction coefficient have been studied in detail. It is found that heat transfer rate increases with the increase in particle concentration up to an optimal level, but on the further increase in particle concentration, the heat transfer rate decreases. For a particular value of particle concentration, small-sized particles enhance the heat transfer rates. On the other hand, skin friction coefficients always increase with the increase in particle concentration and decrease in nanoparticle size.     

纳米流体圆管内的湍流流动特性

采用Eulerian-Eulerian模型和Eulerian-Lagrange模型研究了TiO₂-水纳米流体在水平管内的湍流流动特性,并与实验结果进行对比分析,探讨了不同模型中各种相间作用力的影响。从微流动角度探索纳米流体的流动本质,从而进一步揭示其传热强化机理。结果表明：在壁面附近,纳米颗粒与水存在着明显的速度差异,相间的动量交换十分明显,从而强化了局部微流动,导致边界层变薄。纳米颗粒在整个流场内部是不均匀分布的,使得边界层内部换热能力得到大幅度增强。纳米流体流动特性的改变是影响其强化换热的主要因素。     

Heterogeneous nanofluids: natural convection heat transfer enhancement

Convective heat transfer using different nanofluid types is investigated. The domain is differentially heated and nanofluids are treated as heterogeneous mixtures with weak solutal diffusivity and possible Soret separation. Owing to the pronounced Soret effect of these materials in combination with a considerable solutal expansion, the resulting solutal buoyancy forces could be significant and interact with the initial thermal convection. A modified formulation taking into account the thermal conductivity, viscosity versus nanofluids type and concentration and the spatial heterogeneous concentration induced by the Soret effect is presented. The obtained results, by solving numerically the full governing equations, are found to be in good agreement with the developed solution based on the scale analysis approach. The resulting convective flows are found to be dependent on the local particle concentration φ and the corresponding solutal to thermal buoyancy ratio N. The induced nanofluid heterogeneity showed a significant heat transfer modification. The heat transfer in natural convection increases with nanoparticle concentration but remains less than the enhancement previously underlined in forced convection case.     

A review of experimental investigations on thermal phenomena in nanofluids

Nanoparticle suspensions (nanofluids) have been recommended as a promising option for various engineering applications, due to the observed enhancement of thermophysical properties and improvement in the effectiveness of thermal phenomena. A number of investigations have been reported in the recent past, in order to quantify the thermo-fluidic behavior of nanofluids. This review is focused on examining and comparing the measurements of convective heat transfer and phase change in nanofluids, with an emphasis on the experimental techniques employed to measure the effective thermal conductivity, as well as to characterize the thermal performance of systems involving nanofluids.     

Toward nanofluids of ultra-high thermal conductivity

The assessment of proposed origins for thermal conductivity enhancement in nanofluids signifies the importance of particle morphology and coupled transport in determining nanofluid heat conduction and thermal conductivity. The success of developing nanofluids of superior conductivity depends thus very much on our understanding and manipulation of the morphology and the coupled transport. Nanofluids with conductivity of upper Hashin-Shtrikman (H-S) bound can be obtained by manipulating particles into an interconnected configuration that disperses the base fluid and thus significantly enhancing the particle-fluid interfacial energy transport. Nanofluids with conductivity higher than the upper H-S bound could also be developed by manipulating the coupled transport among various transport processes, and thus the nature of heat conduction in nanofluids. While the direct contributions of ordered liquid layer and particle Brownian motion to the nanofluid conductivity are negligible, their indirect effects can be significant via their influence on the particle morphology and/or the coupled transport.     

Boundary layer flow past a stretching/shrinking surface beneath an external uniform shear flow with a convective surface boundary condition in a nanofluid

The problem of a steady boundary layer shear flow over a stretching/shrinking sheet in a nanofluid is studied numerically. The governing partial differential equations are transformed into ordinary differential equations using a similarity transformation, before being solved numerically by a Runge-Kutta-Fehlberg method with shooting technique. Two types of nanofluids, namely, Cu-water and Ag-water are used. The effects of nanoparticle volume fraction, the type of nanoparticles, the convective parameter, and the thermal conductivity on the heat transfer characteristics are discussed. It is found that the heat transfer rate at the surface increases with increasing nanoparticle volume fraction while it decreases with the convective parameter. Moreover, the heat transfer rate at the surface of Cu-water nanofluid is higher than that at the surface of Ag-water nanofluid even though the thermal conductivity of Ag is higher than that of Cu.     

利用3ω法同时测量纳米流体热导率和热扩散系数

提出了应用基于谐波探测技术的3ω法进行液体导热性能测量的方法。设计了3ω测试系统，测试了不同浓度和不同温度下纳米流体的热导率和热扩散系数，与文献中的测试结果进行了对比。实验中测试的热波信号较好地满足频域内的导热方程，说明采用交流电流加热可使流体的微对流作用得到有效减弱。采用基于多颗粒布朗运动的微对流（MSBW）模型预测了纳米流体的热导率。浓度比较低时TiO₂+蒸馏水、Al₂ O₃+蒸馏水纳米颗粒流体的热导率随温度增加呈线性增大，并且与液体的Prandtl数有关，在测试温度为18～65℃范围内，水的热导率随温度升高以及纳米颗粒的布朗运动所引起周围基液的微对流作用是纳米流体强化传热的两个重要机理。     

基于反扰动非平衡分子动力学的纳米流体导热增强机理研究

相较于水、乙二醇等常规流体,纳米流体出色的传热效果使其成为近十年来研究的热点之一。利用一种反扰动非平衡分子动力学方法对纳米流体的导热增强机理进行了模拟研究。在基液Ar 中加入 Cu 纳米颗粒后, 纳米流体的热通量和热导率均发生了不同程度的改变,纳米颗粒体积分数的变化,在一定程度上改变了纳米流体内部的能量传递过程。进一步分析了纳米流体热导率强化的微观作用机理,发现纳米颗粒的加入,使得纳米流体的微观结构具有了类似晶体的微观结构特性,在颗粒尺寸较小的情况下,流体内部受温度梯度作用效应明显。     

Stability of nanofluids in quiescent and shear flow fields

An experimental study was conducted to investigate the structural stability of ethylene glycol-based titanium dioxide nanoparticle suspensions (nanofluids) prepared by two-step method. The effects of particle concentration, fluid temperature, shear rate and shear duration were examined. Particle size and thermal conductivity measurements in quiescent state indicated the existence of aggregates and that they were stable in temperatures up to 60°C. Shear stability tests suggested that the structure of nanoparticle aggregates was stable in a shear interval of 500-3000 s^-1 measured over a temperature range of 20-60°C. These findings show directions to resolve controversies surrounding the underlying mechanisms of thermal conduction and convective heat transfer of nanofluids.     

A review on boiling heat transfer enhancement with nanofluids

There has been increasing interest of late in nanofluid boiling and its use in heat transfer enhancement. This article covers recent advances in the last decade by researchers in both pool boiling and convective boiling applications, with nanofluids as the working fluid. The available data in the literature is reviewed in terms of enhancements, and degradations in the nucleate boiling heat transfer and critical heat flux. Conflicting data have been presented in the literature on the effect that nanofluids have on the boiling heat-transfer coefficient; however, almost all researchers have noted an enhancement in the critical heat flux during nanofluid boiling. Several researchers have observed nanoparticle deposition at the heater surface, which they have related back to the critical heat flux enhancement.     

Lattice Boltzmann simulation of alumina-water nanofluid in a square cavity

A lattice Boltzmann model is developed by coupling the density (D2Q9) and the temperature distribution functions with 9-speed to simulate the convection heat transfer utilizing Al₂O₃-water nanofluids in a square cavity. This model is validated by comparing numerical simulation and experimental results over a wide range of Rayleigh numbers. Numerical results show a satisfactory agreement between them. The effects of Rayleigh number and nanoparticle volume fraction on natural convection heat transfer of nanofluid are investigated in this study. Numerical results indicate that the flow and heat transfer characteristics of Al₂O₃-water nanofluid in the square cavity are more sensitive to viscosity than to thermal conductivity.     

Convective heat transfer of alumina nanofluids in laminar flows through a pipe at the thermal entrance regime

The convective heat transfer characteristics of aqueous alumina nanofluids were investigated experimentally under forced laminar tube flows. The particles had different shapes of cylinders, bricks and blades, and particle loading was between 0?C5 volume%. The nanofluids were characterized rheologically, and the heat transfer system was validated by using water without particles. In calculating Nusselt and Peclet numbers to assess heat transfer enhancement of nanofluids, physical properties of water were used so as not to exaggerate the amount of heat transfer. It was found that heat transfer coefficients of nanofluids are almost the same or a little smaller than that of water. The heat transfer coefficient can be reduced by the lowering the thermal conductivity of the nanofluid under shearing conditions and particle depletion by the cluster migration from the wall to the tube center. The reduction in thermophysical properties also contributes to the reduction in heat transfer coefficient. It has been concluded that nanofluids from metal particles with appropriate stabilizing agents can satisfy the requirements to be a practically usable nanofluid.     

Discussion on the thermal conductivity enhancement of nanofluids

Increasing interests have been paid to nanofluids because of the intriguing heat transfer enhancement performances presented by this kind of promising heat transfer media. We produced a series of nanofluids and measured their thermal conductivities. In this article, we discussed the measurements and the enhancements of the thermal conductivity of a variety of nanofluids. The base fluids used included those that are most employed heat transfer fluids, such as deionized water (DW), ethylene glycol (EG), glycerol, silicone oil, and the binary mixture of DW and EG. Various nanoparticles (NPs) involving Al₂O₃ NPs with different sizes, SiC NPs with different shapes, MgO NPs, ZnO NPs, SiO₂ NPs, Fe₃O₄ NPs, TiO₂ NPs, diamond NPs, and carbon nanotubes with different pretreatments were used as additives. Our findings demonstrated that the thermal conductivity enhancements of nanofluids could be influenced by multi-faceted factors including the volume fraction of the dispersed NPs, the tested temperature, the thermal conductivity of the base fluid, the size of the dispersed NPs, the pretreatment process, and the additives of the fluids. The thermal transport mechanisms in nanofluids were further discussed, and the promising approaches for optimizing the thermal conductivity of nanofluids have been proposed.     

Al2O3-based nanofluids: a review

Ultrahigh performance cooling is one of the important needs of many industries. However, low thermal conductivity is a primary limitation in developing energy-efficient heat transfer fluids that are required for cooling purposes. Nanofluids are engineered by suspending nanoparticles with average sizes below 100 nm in heat transfer fluids such as water, oil, diesel, ethylene glycol, etc. Innovative heat transfer fluids are produced by suspending metallic or nonmetallic nanometer-sized solid particles. Experiments have shown that nanofluids have substantial higher thermal conductivities compared to the base fluids. These suspended nanoparticles can change the transport and thermal properties of the base fluid. As can be seen from the literature, extensive research has been carried out in alumina-water and CuO-water systems besides few reports in Cu-water-, TiO₂-, zirconia-, diamond-, SiC-, Fe₃O₄-, Ag-, Au-, and CNT-based systems. The aim of this review is to summarize recent developments in research on the stability of nanofluids, enhancement of thermal conductivities, viscosity, and heat transfer characteristics of alumina (Al₂O₃)-based nanofluids. The Al₂O₃ nanoparticles varied in the range of 13 to 302 nm to prepare nanofluids, and the observed enhancement in the thermal conductivity is 2% to 36%.     

表面活性剂对水基纳米流体特性影响的研究进展

表面活性剂作为一种分散剂,广泛应用于新型换热工质——纳米流体中.研究纳米流体的各种特性对于其在实际的能量传递系统中的应用有重要意义.重点总结和比较了水基纳米流体中表面活性剂对体系的稳定性、热导率和黏度影响的实验研究,阐述了纳米流体中表面活性剂的作用机理,对目前的研究中存在的问题进行了分析.最后,提出了有助于完善表面活性剂对水基纳米流体特性影响的4点建议:混合表面活性剂的组合及其配比对纳米流体的稳定性、热导率和黏度的影响;使用分子动力模拟等方法来研究表面活性剂对纳米流体特性的影响;表面活性剂影响下的纳米流体的稳定性和热导率及黏度之间的关系;纳米流体中众多不确定因素的量化分析.     

水基石墨烯纳米流体在矩形小槽道内的流动换热特性

测试了水基石墨烯纳米流体的部分热物性,研究了不同浓度、雷诺数(Re)和加热功率条件下水基石墨烯纳米流体作为换热工质在设计的矩形结构小槽道内的对流换热性能。结果表明,层流状态(Re=500~2000)下,矩形槽道壁面温度随Re增大逐渐降低,随加热功率增大逐渐升高,与常规流体换热特性一致;在相同Re和换热功率条件下,随纳米流体浓度增大,壁温逐渐减小;水基石墨烯纳米流体的换热强度比基液去离子水提升较大,Re=2000、加热功率为210 W时,浓度为0.03wt%的水基石墨烯纳米流体的平均努塞尔数(Nu)为9.3,比基液水提升48.8%;受入口效应影响,沿槽道长度局部对流换热系数逐渐减小,最高可达25674.5 [W/(m2?℃)],较基液水最大可提高39.1%;Re=500~1400时,石墨烯纳米流体的流动换热强度随Re增大明显增强;由实验数据结合理论模型拟合了适用于石墨烯纳米流体对流换热强度的计算式,计算结果与实验结果最大相对误差不超过25%,平均相对误差仅为4.8%。     

Laminar heat transfer of non-Newtonian nanofluids in a circular tube

Forced convection heat transfer behavior of three different types of nanofluids flowing through a uniformly heated horizontal tube under laminar regime has been investigated experimentally. Nanofluids were made by dispersion of γ-Al₂O₃, CuO, and TiO₂ nanoparticles in an aqueous solution of carboxymethyl cellulose (CMC). All nanofluids as well as the base fluid exhibit shear-thinning behavior. Results of heat transfer experiments indicate that both average and the local heat transfer coefficients of nanofluids are larger than that of the base fluid. The enhancement of heat transfer coefficient increases by increasing nanoparticle loading. At a given Peclet number and nanoparticle concentration the local heat transfer coefficient decreases by axial distance from the test section inlet. It seems that the thermal entry length of nanofluids is greater than the base fluid and becomes longer as nanoparticle concentration increases.