纳米流体的聚集结构和导热系数模拟

本文根据布朗运动理论模拟纳米粒子在流体中的聚集过程，运用分形理论描述纳米粒子团的结构．考虑纳米粒子的运动传热，建立纳米流体的导热系数模型，理论预测值与实验结果显现了良好的一致性。     

Monodisperse copper‐ and silver‐nanocolloids suitable for heat‐conductive fluids

Copper colloid was prepared via reductive stabilization. The suspension of the trioctylaluminum‐stabilized copper colloid was peptized using Korantin SH and cashew nut shell liquid (CNSL). Fluids with particle sizes <10 nm were obtained with Korantin and 7–15 nm in the case of CNSL. However, the copper colloid is air sensitive. A very straightforward one‐step method leads to air‐stable silver nanofluids. Thermal decomposition of silver lactate in the presence of Korantin SH and mineral oil as the medium gave a silver nanofluid. Silver particle formation and air stability were monitored using UV–VIS spectroscopy. The presence of monodispersed spherical silver nanoparticles was confirmed. Transmission electron microscopy showed a two‐dimensional assembly of the silver particles with a size distribution of 9.5 ± 0.7 nm. FTIR has revealed information about the interaction between the surfactant and the silver surface. Copyright © 2005 John Wiley & Sons, Ltd.     

Preparation and characterization of monodisperse solvent-free silica nanofluids

A series of solvent-free ionic silica (SiO₂) nanofluids of 12.3–17.3 nm in diameter were synthesized by surface functionalizing nanoscale SiO₂ with a charged corona and ionically tethering with oligomeric chains as canopy. The structure and properties of the nanofluids were systematically characterized by Fourier transform infrared (FTIR), differential scanning calorimeter (DSC), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and rheology tests. The resultant nanofluids with low-molecular-weight oligomeric as canopy are homogeneous, stable yellow-like fluids with no evidence of phase separation at room temperature, while other nanofluids containing high-molecular-weight as canopy behave like a soft glassy, and they exhibit fluidity with still high modulus and viscosity above 60°C. For deeper understanding of the nature of SiO₂ nanofluids, the rheological behavior, thermal stability, as well as morphology of SiO₂ nanofluids were investigated in details. The flow properties of nanofluids could be easily regulated from soft glassy to free flowing liquids by varying the molecule weight of canopy. Most importantly, the thermal stability, rheological behavior, as well as morphology can be also regulated through varying molecule weight and thickness of canopy, which will guide our future work on synthesis of nanofluids with controllable physical properties.     

Core–Shell Engineering to Enhance the Spectral Stability of Heterogeneous Luminescent Nanofluids

The tendency to the miniaturization of devices and the peculiar properties of the nanoparticles have raised the interest of the scientific community in nanoscience. In particular, those systems consisting of nanoparticles dispersed in fluids, known as nanofluids, have made it possible to overcome many technological and scientific challenges, as they show extraordinary properties. In this work, the loss of the spectral stability in heterogeneous luminescent nanofluids is studied revealing the critical role played by the exchange of ions between different nanoparticles. Such ion exchange is favored by changes in the molecular properties of the solvent, making heterogeneous luminescent nanofluids highly unstable against temperature changes. This work demonstrates how both temporal and thermal stabilities of heterogeneous luminescent nanofluids can be substantially improved by core–shell engineering. This simultaneously avoids the leakage of luminescent ions and the effects of the solvent molecular changes.     

Investigation into the antibacterial behaviour of suspensions of ZnO nanoparticles (ZnO nanofluids)

The antibacterial behaviour of suspensions of zinc oxide nanoparticles (ZnO nanofluids) against E. Coli has been investigated. ZnO nanoparticles from two sources are used to formulate nanofluids. The effects of particle size, concentration and the use of dispersants on the antibacterial behaviour are examined. The results show that the ZnO nanofluids have bacteriostatic activity against E. coli. The antibacterial activity increases with increasing nanoparticle concentration and increases with decreasing particle size. Particle concentration is observed to be more important than particle size under the conditions of this work. The results also show that the use of two types of dispersants (Polyethylene Glycol (PEG) and Polyvinylpyrolidone (PVP)) does not affect much the antibacterial activity of ZnO nanofluids but enhances the stability of the suspensions. SEM analyses of the bacteria before and after treatment with ZnO nanofluids show that the presence of ZnO nanoparticles damages the membrane wall of the bacteria. Electrochemical measurements using a model DOPC monolayer suggest some direct interaction between ZnO nanoparticles and the bacteria membrane at high ZnO concentrations. On visiting from the Tianjin University of Science & Technology, Tianjin, P.R. China.     

Coupled flow and heat transfer of power-law nanofluids on non-isothermal rough rotary disk subjected to magnetic field

We study the coupled flow and heat transfer of power-law nanofluids on a non-isothermal rough rotating disk subjected to a magnetic field. The problem is formulated in terms of specified curvilinear orthogonal coordinate system. An improved BVP4C algorithm is proposed, and numerical solutions are obtained. The influence of volume fraction, types and shapes of nanoparticles, magnetic field and power-law index on the flow, and heat transfer behavior are discussed. The obtained results show that the power-law exponents (PLE), nanoparticle volume fraction (NVF), and magnetic field inclination angle (MFIA) have almost no effects on velocities in the wave surface direction, but have small or significant effects on the azimuth direction. The NVF has remarkable influences on local Nusselt number (LNN) and friction coefficients (FC) in the radial direction and the azimuth direction (AD). The LNN increases with NVF increasing while FC in AD decreases. The types of nanoparticles, magnetic field strength, and inclination have small effects on LNN, but they have remarkable influences on the friction coefficients with positively correlated heat transfer rate, while the inclination is negatively correlated with heat transfer rate. The size of the nanoparticle shape factor is positively correlated with LNN.     

碳纳米流体强化传热研究

在去离子水中分别添加了单壁、多壁碳纳米管材料,通过配比一定质量的亲水性分散剂,经超声波振荡试制了水基碳纳米流体。测试分析了不同碳纳米管材料质量分数下的纳米流体热导率和动力粘度等重要热物性参数。测试结果表明:碳纳米管粒子能够强化基液工质导热性能,随着其质量分数的增加,水基单壁、多壁碳纳米流体热导率均明显提高;单壁碳纳米流体粘度显著增加,多壁碳纳米流体粘度无明显变化,多壁碳管更适用于纳米流体强化传热。     

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.