氧化铝有机纳米流体的流动传热基础特性

以氧化铝为纳米粒子、丙二醇和水为基础液体制备了氧化铝有机纳米流体,分别测量了它的沸点、热导率、比热容和黏度。以1％～5％（体积分数）的氧化铝纳米流体作为冷介质,测试了在车用机油冷却器中的传热系数和流动阻力。试验结果表明,纳米粒子能够显著强化基础液体在机油冷却器中的换热能力,粒子体积分数和流体温度是影响纳米流体热物性的重要因素。氧化铝纳米流体的沸点高于120℃,比热容随体积分数增加而降低,热导率、黏度和在机油冷却器中的传热系数均随粒子体积分数的增加而提高。在试验Ⅱ中,5％（体积分数）纳米流体的平均传热系数比基础液体提高了124.56％,而流动阻力增幅较小。     

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.     

Numerical evaluation of laminar heat transfer enhancement in nanofluid flow in coiled square tubes

Convective heat transfer can be enhanced by changing flow geometry and/or by enhancing thermal conductivity of the fluid. This study proposes simultaneous passive heat transfer enhancement by combining the geometry effect utilizing nanofluids inflow in coils. The two nanofluid suspensions examined in this study are: water-Al2O3 and water-CuO. The flow behavior and heat transfer performance of these nanofluid suspensions in various configurations of coiled square tubes, e.g., conical spiral, in-plane spiral, and helical spiral, are investigated and compared with those for water flowing in a straight tube. Laminar flow of a Newtonian nanofluid in coils made of square cross section tubes is simulated using computational fluid dynamics (CFD)approach, where the nanofluid properties are treated as functions of particle volumetric concentration and temperature. The results indicate that addition of small amounts of nanoparticles up to 1% improves significantly the heat transfer performance; however, further addition tends to deteriorate heat transfer performance.     

Nanofluids for heat transfer: an engineering approach

An overview of systematic studies that address the complexity of nanofluid systems and advance the understanding of nanoscale contributions to viscosity, thermal conductivity, and cooling efficiency of nanofluids is presented. A nanoparticle suspension is considered as a three-phase system including the solid phase (nanoparticles), the liquid phase (fluid media), and the interfacial phase, which contributes significantly to the system properties because of its extremely high surface-to-volume ratio in nanofluids. The systems engineering approach was applied to nanofluid design resulting in a detailed assessment of various parameters in the multivariable nanofluid systems. The relative importance of nanofluid parameters for heat transfer evaluated in this article allows engineering nanofluids with desired set of properties.     

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.     

Numerical investigation of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/water nanofluid laminar convective heat transfer through triangular ducts

In this article, laminar flow-forced convective heat transfer of Al₂O₃/water nanofluid in a triangular duct under constant wall temperature condition is investigated numerically. In this investigation, the effects of parameters, such as nanoparticles diameter, concentration, and Reynolds number on the enhancement of nanofluids heat transfer is studied. Besides, the comparison between nanofluid and pure fluid heat transfer is achieved in this article. Sometimes, because of pressure drop limitations, the need for non-circular ducts arises in many heat transfer applications. The low heat transfer rate of non-circular ducts is one the limitations of these systems, and utilization of nanofluid instead of pure fluid because of its potential to increase heat transfer of system can compensate this problem. In this article, for considering the presence of nanoparticl: es, the dispersion model is used. Numerical results represent an enhancement of heat transfer of fluid associated with changing to the suspension of nanometer-sized particles in the triangular duct. The results of the present model indicate that the nanofluid Nusselt number increases with increasing concentration of nanoparticles and decreasing diameter. Also, the enhancement of the fluid heat transfer becomes better at high Re in laminar flow with the addition of nanoparticles.     

Intensification of convective heat transfer in water/ethylene glycol based nanofluids containing TiO2 nanoparticles

This paper presents a study of heat transfer performance of water, ethylene glycol (EG) and their mixtures of varying compositions and comparison thereof. The present work demonstrates the enhancement in convective heat transfer in nanofluids. The nanofluids were prepared by adding TiO₂ nanoparticles (having a particle size below 100 nm) in a base fluid. A binary mixture of EG (40%) and water (60%) was used as a base fluid. Nanofluids with varied volume fraction between 0 and 0.5 (volume fraction of TiO₂ nanoparticles) were considered in the present study. The experimental setup used was consisting of a test section that includes 750 mm long copper pipe with 8 mm inner diameter and a heater. The test section was covered with an insulation layer to minimize the heat losses. Temperature measurement was done with thermocouples. The experiments were conducted to study the effects of solid volume fraction, nanofluid flow rate and the inlet temperature on the heat transfer performance of the nanofluids. The results show an enhancement in heat transfer coefficient with increased volume fraction of TiO₂ nanoparticles. The maximum enhancement of 105% in heat transfer coefficient was observed for the nanofluid with solid volume fraction of 0.5.     

Critical heat flux of nanofluids inside a single microchannel: Experiments and correlations

This study investigated experimentally the CHF phenomena of aqueous-based alumina nanofluids in single microchannels, and assessed the validity of a number of microchannel based CHF correlations using experimental nanofluids data. While usual approaches for CHF enhancement are through the modification of different tube surfaces or employing different inserts, this work showed that CHF in microchannels could be enhanced significantly by the inclusion of small concentrations of nanoparticles. The CHF value was found to increase with increase of mass flux, initial subcooling and alumina nanoparticle concentrations. The maximum subcooled CHF enhancement occurred at the lowest mass flux and highest alumina concentration within the experimental range. In addition, the Lee and Mudawar correlation was modified to predict the critical heat flux of water and nanofluids. The new model was examined by experimental data and 24% and 30% mean absolute error were observed for water and alumina nanofluid respectively.     

Experimental research on stabilities,thermophysical properties and heat transfer enhancement of nanofluids in heat exchanger systems

Stable TiO₂-water nanofluids are prepared by a two-step method, stabilities of nanofluids are investigated by precipitation method and transmittance method respectively, and thermal conductivities and viscosities are also measured. An experimental system for studying the heat transfer enhancement of nanofluids is established, and heat transfer and flow characteristics of TiO₂-water nanofluids in heat exchanger systems with a triangular tube and circular tube are experimentally studied. The effects of nanoparticle mass fractions (ω=0.1 wt%-0.5 wt%) and Reynolds numbers (Re=800-10000) on the heat transfer and flow performances of nanofluids are analyzed. Fitting formulas for Nusselt number and resistance coefficient of nanofluids in a triangular tube are put forward based on the experimental data. The comprehensive performances of nanofluids in a triangular tube are investigated. It is found that nanofluids in a triangular tube can significantly improve the heat transfer performance at the cost of a small increase in resistance coefficient compared with that in a circular tube, especially the resistance coefficients are almost the same between different nanoparticle mass fractions at turbulent flow. It is also found that the comprehensive evaluation index η decreases with Reynolds number at laminar flow but a critical maximum value appears at turbulent flow.     

Application of different CFD multiphase models to investigate effects of baffles and nanoparticles on heat transfer enhancement

In this work, the effect of baffles in a pipe on heat transfer enhancement was studied using computational fluid dynamics (CFD) in the presence of Al₂O₃ nanoparticles which are dispersed into water. Fluid flow through the horizontal tube with uniform heat flux was simulated numerically and three dimensional governing partial differential equations were solved. To find an accurate model for CFD simulations, the results obtained by the single phase were compared with those obtained by three different multiphase models including Eulerian, mixture and volume of fluid (VOF) at Reynolds numbers in range of 600 to 3000, and two different nanoparticle concentrations (1% and 1.6%). It was found that multiphase models could better predict the heat transfer in nanofluids. The effect of baffles on heat transfer of nanofluid flow was also investigated through a baffled geometry. The numerical results show that at Reynolds numbers in the range of 600 to 2100, the heat transfer of nanofluid flowing in the geometry without baffle is greater than that of water flowing through a tube with baffle, whereas the difference between these effects (nanofluid and baffle) decreases with increasing the Reynolds number. At higher Reynolds numbers (2100–3000) the baffle has a greater effect on heat transfer enhancement than the nanofluid.     

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%。     

Thermophysical property evaluation of β-cyclodextrin modified ZrO2 nanofluids for microchannel heat exchange

Microchannel technology is an effective method to solve the heat transfer of microelectronics. Nanofluids are considered to have great application potential in microchannel heat exchangers. In this experiment, β-cyclodextrin (β-CD) was used to modify ZrO₂ nanoparticles. The morphology, functional group, and crystal structure of the nanoparticles before and after modification were studied. Ethylene glycol aqueous solution-based nanofluids were prepared using a two-step method. Its physical properties were studied. The modified nanofluid has better stability. The thermal conductivity of the nanofluid was measured and the mathematical model was analyzed. The results showed that the nanofluid with a concentration of 0.10 vol% was 37.82% higher than the base fluid at 60 °C. The results of the mathematical analysis indicate that the fabrication of nanofluids using β-cyclodextrin-modified ZrO₂ has great potential for application in heat transfer in microelectronic microchannels.     

Particle shape effect on heat transfer performance in an oscillating heat pipe

The effect of alumina nanoparticles on the heat transfer performance of an oscillating heat pipe (OHP) was investigated experimentally. A binary mixture of ethylene glycol (EG) and deionized water (50/50 by volume) was used as the base fluid for the OHP. Four types of nanoparticles with shapes of platelet, blade, cylinder, and brick were studied, respectively. Experimental results show that the alumina nanoparticles added in the OHP significantly affect the heat transfer performance and it depends on the particle shape and volume fraction. When the OHP was charged with EG and cylinder-like alumina nanoparticles, the OHP can achieve the best heat transfer performance among four types of particles investigated herein. In addition, even though previous research found that these alumina nanofluids were not beneficial in laminar or turbulent flow mode, they can enhance the heat transfer performance of an OHP.     

Optimization of natural convection heat transfer of Newtonian nanofluids in a cylindrical enclosure

This study characterizes and optimizes natural convection heat transfer of two Newtonian Al2O3 and TiO2/water nanofluids in a cylindrical enclosure. Nusselt number (Nu) of nanofluids in relation to Rayleigh number (Ra) for different concentrations of nanofluids is investigated at different configurations and orientations of the enclosure. Results show that adding nanoparticles to water has a negligible or even adverse influence upon natural convec-tion heat transfer of water:only a slight increase in natural convection heat transfer of Al2O3/water is observed, while natural convection heat transfer for TiO2/water nanofluid is inferior to that for the base fluid. Results also reveal that at low Ra, the likelihood of enhancement in natural convection heat transfer is more than at high Ra:at low Ra, inclination angle, aspect ratio of the enclosure and nanoparticle concentration influence natural convec-tion heat transfer more pronouncedly than that in high Ra.     

A Kind of Nanofluid Consisting of Surface-Functionalized Nanoparticles

A method of surface functionalization of silica nanoparticles was used to prepare a kind of stable nanofluid. The functionalization was achieved by grafting silanes directly to the surface of silica nanoparticles in silica solutions (both a commercial solution and a self-made silica solution were used). The functionalized nanoparticles were used to make nanofluids, in which well-dispersed nanoparticles can keep good stability. One of the unique characteristics of the nanofluids is that no deposition layer forms on the heated surface after a pool boiling process. The nanofluids have applicable prospect in thermal engineering fields with the phase-change heat transfer.     

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。     

Influence of heat flux and Reynolds number on the entropy generation for different types of nanofluids in a hexagon microchannel heat sink

Based on the Second Law of Thermodynamics, the entropy generation is studied for laminar forced convection flow of different nanoparticles(Al_2 O_3, CuO and SiO_2) mixed with water through a hexagon microchannel heat sink(HMCHS). The effects of different heat fluxes and Reynolds numbers on the entropy generation for different nanofluids, volume fractions and nanoparticles diameter are investigated. The heat flux is in the range of 125 to 500 kW·m~(-2) and the Reynolds numbers vary between 200 and 1500. The thermal, frictional and total entropy generations are calculated by integrating the volumetric rate components over the entire HMCHS. The results clearly show that the rise in the heat flux leads to an increase in the thermal entropy generation for nanofluids and pure water but they don't have any influence on the frictional entropy generation. Moreover, when the Reynolds number increases, the frictional entropy generation increases while the thermal entropy generation decreases. The results revealed that at low heat fluxes and high Reynolds numbers, pure water gives the lowest entropy generation, while at high heat flux the nanofluid has to be used in order to lower the overall irreversibility.     

Role of nanoparticles on nanofluid boiling phenomenon: Nanoparticle deposition

Suspended nanoparticles inside the nanofluids can modify the characteristics of heated surfaces and the physical properties of the base liquids, offering a great opportunity to optimize boiling heat transfer. This paper reviews the mechanisms of nanoparticle deposition and the effects induced by deposited nanoparticles on surface roughness, force balance at the triple line, surface wettability, active nucleation site density, receding and advancing contact angles, boiling heat transfer coefficient and critical heat flux. Both enhancement and deterioration effects on boiling heat transfer coefficient and critical heat flux have been discussed. Most of the existing experimental data confirms the enhancement of critical heat flux using alumina nanofluid, however there is no consistency about its boiling heat transfer coefficient.     

Anomalous heat transfer modes of nanofluids: a review based on statistical analysis

This paper contains the results of a concise statistical review analysis of a large amount of publications regarding the anomalous heat transfer modes of nanofluids. The application of nanofluids as coolants is a novel practise with no established physical foundations explaining the observed anomalous heat transfer. As a consequence, traditional methods of performing a literature review may not be adequate in presenting objectively the results representing the bulk of the available literature. The current literature review analysis aims to resolve the problems faced by researchers in the past by employing an unbiased statistical analysis to present and reveal the current trends and general belief of the scientific community regarding the anomalous heat transfer modes of nanofluids. The thermal performance analysis indicated that statistically there exists a variable enhancement for conduction, convection/mixed heat transfer, pool boiling heat transfer and critical heat flux modes. The most popular proposed mechanisms in the literature to explain heat transfer in nanofluids are revealed, as well as possible trends between nanofluid properties and thermal performance. The review also suggests future experimentation to provide more conclusive answers to the control mechanisms and influential parameters of heat transfer in nanofluids.     

熔盐储热材料比热容强化的研究进展

强化熔盐材料比热容可以有效增强熔盐材料的蓄热能力,减小蓄热系统面积及热损失,进而降低蓄热成本,是近年来中高温储能领域的研究热点。本文主要从熔盐储热材料比热容强化的必要性、强化方法和强化机理等方面综述了近年来熔盐传热蓄热材料比热容强化的研究进展。具体阐述了添加可溶性添加剂和掺杂异质纳米颗粒形成纳米流体两种强化熔盐比热容的方法及目前存在的问题,重点探讨了熔盐纳米流体的制备方法、异质纳米颗粒体系、强化效果及比热容强化机理等问题。此外,指出了当前利用纳米流体强化熔盐储热材料比热容方面存在的不足：研究体系单一、悬浮稳定性差和比热容强化机理不完善等,并对熔盐纳米流体的未来发展方向,即多体系熔盐纳米流体的开发,多手段比热容强化机理的揭示和多方法熔盐纳米流体物性的测量进行了展望。