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.     

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.     

纳米流体应用于强化传热的研究进展及面临的挑战

纳米流体是近年来新兴的一种强化传热工质,其在强化传热领域有着广阔的应用前景,然而目前对纳米流体强化传热应用的研究还存在一些障碍。本文从纳米流体的制备和传热特性方面总结了近几年的研究进展,并提出了研究过程中需要克服的几个问题,以促进今后进一步的研究工作。     

Natural convective heat transfer of Ag-water nanofluid flow inside enclosure with center heater and bottom heat source

A numerical study on natural convective heat transfer inside an enclosure with center heater using nanofluid has been carried out. The effect of different length of center heater on the flow and temperature fields is analysed for different Rayleigh numbers. Results are displayed in terms of streamlines, isotherms, mid height velocity profile and average Nusselt number. The numerical results reveal heat transfer increases with increasing heater length at both vertical and horizontal positions for increasing values of Rayleigh numbers. In particular, a higher increase in heat transfer is obtained with heater situated with vertical position of maximum length. Also it is obtained that enhancement of heat transfer is high for Ag - water nanofluid than CuO -water and Al₂O₃ -water nanofluids.     

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.     

纳米流体对流换热系数增大机理

纳米流体流动换热能力优于传统流体介质.研究了纳米流体热物性的提升和热散射对其对流换热系数的影响.结果表明,纳米颗粒的加入,优化了介质的热物性,增大了导热系数,强化了纳米流体内颗粒、流体以及流道管壁碰撞和相互作用,同时加强了流体的混合脉动和湍流,从而增大了对流换热系数. 关键词： 纳米流体 换热系数 热散射     

Intriguingly high convective heat transfer enhancement of nanofluid coolants in laminar flows

We reported on investigation of the convective heat transfer enhancement of nanofluids as coolants in laminar flows inside a circular copper tube with constant wall temperature. Nanofluids containing Al₂O₃, ZnO, TiO₂, and MgO nanoparticles were prepared with a mixture of 55 vol.% distilled water and 45 vol.% ethylene glycol as base fluid. It was found that the heat transfer behaviors of the nanofluids were highly depended on the volume fraction, average size, species of the suspended nanoparticles and the flow conditions. MgO, Al₂O₃, and ZnO nanofluids exhibited superior enhancements of heat transfer coefficient, with the highest enhancement up to 252% at a Reynolds number of 1000 for MgO nanofluid. Our results demonstrated that these oxide nanofluids might be promising alternatives for conventional coolants.     

Analytical prediction of forced convective heat transfer of fluids embedded with nanostructured materials (nanofluids)

Nanofluids are a new class of heat transfer fluids developed by suspending nanosized solid particles in liquids. Larger thermal conductivity of solid particles compared to the base fluid such as water, ethylene glycol, engine oil etc. significantly enhances their thermal properties. Several phenomenological models have been proposed to explain the anomalous heat transfer enhancement in nanofluids. This paper presents a systematic literature survey to exploit the characteristics of nanofluids, viz., thermal conductivity, specific heat and other thermal properties. An empirical correlation for the thermal conductivity of Al₂O₃ + water and Cu + water nanofluids, considering the effects of temperature, volume fraction and size of the nanoparticle is developed and presented. A correlation for the evaluation of Nusselt number is also developed and presented and compared in graphical form. This enhanced thermophysical and heat transfer characteristics make fluids embedded with nanomaterials as excellent candidates for future applications.      

Numerical study of heat transfer enhancement of counter nanofluids flow in rectangular microchannel heat exchanger

This paper reports a numerical analysis of the performance of a counter-flow rectangular shaped microchannel heat exchanger (MCHE) using nanofluids as the working fluids. Finite volume method was used to solve the three-dimensional steady, laminar developing flow and conjugate heat transfer in aluminum MCHE. The nanofluids used were Ag, Al₂O₃, CuO, SiO₂, and TiO₂ and the performance was compared with water. The thermal, flow fields and performance of the MCHE were analyzed using different nanofluids, different Reynolds numbers and different nanoparticle concentrations. Temperature profile, heat transfer coefficient, pressure profile, and wall shear stress were obtained from the simulations and the performance was discussed in terms of heat transfer rate, pumping power, effectiveness, and performance index. Results indicated enhanced performance with the usage of nanofluids, and slight penalty in pressure drop. The increase in Reynolds number caused an increase in the heat transfer rate and a decrease in the overall bulk temperature of the cold fluid. The increase in nanoparticle concentration also yielded better performance at the expense of increased pressure drop.     

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.     

Phase change heat transfer of liquid nitrogen upon injection into aqueous based TiO<Subscript>2</Subscript> nanofluids

This paper reports an experimental study on the cryogenic phase change behaviour of liquid nitrogen upon injection into a relatively large pool of aqueous based titanium dioxide nanofluids under ambient temperatures. Stable TiO₂ are formulated at concentrations between 1% and 4% by particle weight, and characterized in the experiments. Both transient pressure and temperature profiles are measured and analysed in the range of liquid nitrogen Reynolds number of 2,200–11,000 and Webber number of 9–220. The results, however, do not show obvious influence of nanoparticles’ concentration on the pressure build up and heat transfer under the condition of this work. Very similar results are observed for liquid nitrogen injection into pure distilled water and into different concentrations of TiO₂ nanofluids; both the pressure and the rate of pressure rise during the injection process increase linearly with injection velocity irrespective of nanoparticle concentrations. Further discussion shows that a higher rate of pressure rise could be achieved if operating conditions were optimised to induce the fragmentation and subsequent vapour explosion.     

On the role of structural disjoining pressure to boiling heat transfer of thermal nanofluids

Research on nanofluids has progressed rapidly since their enhanced thermal conductivities were identified about a decade ago. For boiling heat transfer with nanofluids, however, many contradictory results have been reported, which cannot be explained by conventional theories developed for pure fluids. Recent progress in colloidal science shows that the presence of nanoparticles could enhance the spreading and wettability of base fluids through a long-range structural disjoining pressure. This article explores theoretically the influence of structural disjoining pressure to the nucleate boiling heat transfer through a four-zoned microlayer evaporation model. The influence of particle size, particle concentration, and heat flux on the structural disjoining pressure and the interfacial shape of the microlayer are investigated. The calculated equilibrium interfacial shape shows that the meniscus is displaced toward the vapor phase in the presence of nanoparticles, an implication of enhanced wettability. Such an improved wettability affects the number of active nucleate sites and bubble dynamics significantly, which could be one of the important parameters that is responsible for the controversy of boiling heat transfer with nanofluids reported in the literature.     

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...     

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.     

Analysis of the convective heat transfer characteristics in a two-dimensional cavity subjected to an external magnetic field

Natural convection heat transfer in the presence of a magnetic field has received considerable attention in the past few decades because of its various applications in industrial installations. In particular, a large number of numerical studies analyzing the effect of the magnetic field on natural convection in a two-dimensional cavity have been performed. In this work, we propose to study the main characteristics of the convective heat transfer of pure fluids and nanofluids in a two-dimensional cavity differentially heated and subjected to an external magnetic field. The scale analysis method is used first to obtain a correlation giving the heat transfer rate, which is then developed to predict the behavior of the heat transfer rate for pure fluids and nanofluids. To verify the reliability of the theoretical predictions, a numerical study is also carried out. The results show that the proposed correlation predicts well the convective heat transfer characteristics obtained numerically.     

New trends of fractional modeling and heat and mass transfer investigation of (SWCNTs and MWCNTs)-CMC based nanofluids flow over inclined plate with generalized boundary conditions

In this paper, we have considered unsteady MHD viscous fluid flow of Carboxyl methyl cellulose (CMC) as based fluid and Carbon nanotubes CNTs (SWCNTs, MWCNTs) nanoparticles passing through an inclined plate of infinite length. Furthermore, the effects of heat source, chemical reaction, porosity and MHD are considered. Fractional model is developed by Caputo time fractional derivatives though recent trends of fractional modeling. The semi exact solutions are obtained for the governing equations in dimensionless form by Laplace transform method. Influence of fractional and other flow parameters on temperature, concentration and velocity fields are graphically illustrated. The Nusselt number, Sherwood number and skin friction are computed for fixed values of flow parameters and presented in tabular form. As a result, for larger values of fractional parameters temperature, concentration and velocity fields can be enhanced. A comparison has been drawn between SWCTs-CMC and MWCTs-CMC based nanofluids and found that MWCTs based nanofluids are more efficient in heat transfer than SWCTs-CMC based nanofluids. Further, in the absence of nanoparticles the obtained results are reduced to recently published results and this fact have been proved graphically and they are in good agreement.     

The effect of velocity and dimension of solid nanoparticles on heat transfer in non-Newtonian nanofluid

In this investigation, the behavior of non-Newtonian nanofluid hydrodynamic and heat transfer are simulated. In this study, we numerically simulated a laminar forced non-Newtonian nanofluid flow containing a 0.5 wt% carboxy methyl cellulose (CMC) solutionin water as the base fluid with alumina at volume fractions of 0.5 and 1.5 as the solid nanoparticle. Numerical solution was modelled in Cartesian coordinate system in a two-dimensional microchannel in Reynolds number range of 10≤Re≤1000. The analyzed geometrical space here was a rectangular part of whose upper and bottom walls was influenced by a constant temperature. The effect of volume fraction of the nanoparticles, Reynolds number and non-Newtonian nanofluids was studied. In this research, the changes pressure drop, the Nusselt number, dimensionless temperature and heat transfer coefficient, caused by the motion of non-Newtonian nanofluids are described. The results indicated that the increase of the volume fraction of the solid nanoparticles and a reduction in the diameter of the nanoparticles would improve heat transfer which is more significant in Reynolds number. The results of the introduced parameters in the form of graphs drawing and for different parameters are compared.     

Experimental and numerical investigation of heat transfer in a miniature heat sink utilizing silica nanofluid

In this paper, heat transfer characteristics of a miniature heat sink cooled by SiO₂–water nanofluids were investigated both experimentally and numerically. The heat sink was fabricated from aluminum and insulated by plexiglass cover plates. The heat sink consisted of an array of 4 mm diameter circular channels with a length of 40 mm. Tests were performed while inserting a 180 W/cm² heat flux to the bottom of heat sink and Reynolds numbers ranged from 400 to 2000. The three-dimensional heat transfer characteristics of the heat sink were analyzed numerically by solving conjugate heat transfer problem of thermally and hydrodynamically developing fluid flow. Experimental results showed that dispersing SiO₂ nanoparticles in water significantly increased the overall heat transfer coefficient while thermal resistance of heat sink was decreased up to 10%. Numerical results revealed that channel diameter, as well as heat sink height and number of channels in a heat sink have significant effects on the maximum temperature of heat sink. Finally, an artificial neural network (ANN) was used to simulate the heat sink performance based on these parameters. It was found that the results of ANN are in excellent agreement with the mathematical simulation and cover a wider range for evaluation of heat sink performance.     

Studies on Al<Subscript>2</Subscript>O<Subscript>3</Subscript>, CuO,and TiO<Subscript>2</Subscript> water-based nanofluids: A comparative approach in laminar and turbulent flow

The Prandtl number, Reynolds number and Nusselt number are functions of thermophysical properties of nanofluids, and these numbers strongly influence the convective heat transfer coefficient. The thermophysical properties vary with volumetric concentration of nanofluids. Therefore, a comprehensive analysis was performed to evaluate the effects on the performance of nanofluids due to variations of density, specific heat, thermal conductivity and viscosity, which are functions of nanoparticle volume concentration. Three metallic oxides, aluminum oxide (Al₂O₃), copper oxide (CuO), and titanium dioxide (TiO₂), dispersed in water as the base fluid were studied. A convenient figure of merit, known as the Mouromtseff number, is used as a base of comparisonfor laminar and turbulent flows. The results indicated that the considered nanofluids can successfully replace water in specific applications for a single-phase forced convection flow in a tube.     

Physics and mechanics of heat exchange processes in nanofluid flows

Nanofluids present a new type of dispersed fluids consisting of a carrier fluid and solid nanoparticles. Unusual properties of nanofluids, particularly high thermal conductivity, make them eminently suitable for many thermophysical applications, e.g., for cooling of equipment, designing of new heat energy transportation and production systems and so on. This requires a systematic study of heat exchange properties of nanofluids. The present paper contains the measurement results for the heat transfer coefficient of the laminar and turbulent flow of nanofluids on the basis of distilled water with silica, alumina and copper oxide particles in a minichannel with circular cross section. The maximum volume concentration of particles did not exceed 2%. The dependence of the heat transfer coefficient on the concentration and size of nanoparticles was studied. It is shown that the use of nanofluids allows a significant increase in the heat transfer coefficient as compared to that for water. However, the obtained result strongly depends on the regime of flow. The excess of the heat transfer coefficient in the laminar flow is only due to an increase in the thermal conductivity coefficient of nanofluid, while in the turbulent flow the obtained effect is due to the ratio between the viscosity and thermal conductivity of nanofluid. The viscosity and thermal conductivity of nanofluids depend on the volume concentration of nanoparticles as well as on their size and material and are not described by classical theories. That is why the literature data are diverse and contradictory; they do not actually take into account the influence of the mentioned factors (size and material of nanoparticles). It has been shown experimentally and by a molecular dynamics method that the nanofluid viscosity increases while the thermal conductivity decreases with the decreasing dispersed particle size. It is found experimentally for the first time that the nanofluid viscosity coefficient depends on the particle material. The higher is the density of particles, the higher is the thermal conductivity coefficient of nanofluid.