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。     

基于热舒适性纳米喷泉的流动与传热特性

目前关于纳米喷泉流动与传热特性及微区域热舒适度方面的研究较少,为了研究其流动与传热机理,本文基于气-液两相流模型和金刚石-乙二醇/水纳米流体高沸点、低冰点等优良特性,在有限柱体空间内建立了一种基于热舒适性的新型高效换热喷泉-“纳米喷泉”,对比研究了金刚石-乙二醇/水纳米流体、水射流工质分别对喷泉流动与传热的影响,同时讨论了纳米颗粒体积分数v和表征流体流动情况的雷诺数（Reynolds,Re）对喷泉流动与传热的影响,分析了局部温度分布、流线分布、平均温度和流体换热量在空间内的变化,并依据ASHRAE 55-1992标准和ISO7730标准对微区域的热舒适度进行了评价。结果表明：随着Re数和v增大,换热强度均得以不同程度地提升,热舒适程度也逐渐增加。Re=1.0×10⁵和Re=1.2×10⁵时,纳米颗粒体积分数从0增加至1%的换热强度提升较为明显,前后两种Re数可分别强化1.5%和2.8%;Re=1.4×10⁵时,体积分数从3%增至5%的换热强度可提升11.5%,强化效果最为明显。故较小Re数下,较低组分的纳米流体强化换热效果较好;而在较大Re数下,较高组分纳米流体的强化换热效果更好。     

水-铜纳米流体强化小型毛细泵回路换热特性

在稳定的低压条件下,对以水-Cu纳米流体为工质的小型平板式毛细泵回路（CPL）的换热特性进行了实验研究。实验中纳米颗粒的平均粒径为20 nm,纳米流体质量分数为0.2%～2.0%。工作压力为5.62、9.58、15.74 kPa。研究了纳米颗粒质量分数和运行压力对CPL换热性能、最大热通量和热阻的影响。实验结果表明,水-Cu纳米流体替代纯水能够显著提高CPL的换热性能,蒸发器的传热系数最大可提高40％,最大热通量提高18%。存在着一个对应于最大强化换热能力的最佳质量分数,在实验压力范围内最佳纳米颗粒质量分数为1.0%。水-Cu纳米流体是一种适合在CPL中使用的强化传热工质。运行压力对CPL换热特性也有明显影响,压力越高,CPL换热的强化效果越显著。     

水基SiO2纳米流体沸腾换热特性

纳米流体作为新型换热介质可广泛应用于多个领域。现有研究结果表明导致纳米流体沸腾换热性能变化的因素主要在于纳米颗粒在换热表面的沉积、加热表面粗糙度、表面张力、内部能量传递、气泡形成条件等。对水基SiO₂纳米流体进行池沸腾实验研究,得到SiO₂/水纳米流体与纯基液-去离子水核态沸腾换热特性的区别,比较不同颗粒粒径对纳米流体换热特性影响。结果表明：对于低浓度纳米流体,添加纳米颗粒后流体的换热特性与纯基液在相同条件下进行核态沸腾时的换热特性有较大差异,不同粒径之间换热特性变化明显,随着粒径的增加呈非线性增长趋势,随着热通量增大纳米颗粒粒径对换热特性的影响趋势增大。     

水基SiO_2纳米流体沸腾换热特性

纳米流体作为新型换热介质可广泛应用于多个领域。现有研究结果表明导致纳米流体沸腾换热性能变化的因素主要在于纳米颗粒在换热表面的沉积、加热表面粗糙度、表面张力、内部能量传递、气泡形成条件等。对水基SiO_2纳米流体进行池沸腾实验研究,得到SiO_2/水纳米流体与纯基液-去离子水核态沸腾换热特性的区别,比较不同颗粒粒径对纳米流体换热特性影响。结果表明:对于低浓度纳米流体,添加纳米颗粒后流体的换热特性与纯基液在相同条件下进行核态沸腾时的换热特性有较大差异,不同粒径之间换热特性变化明显,随着粒径的增加呈非线性增长趋势,随着热通量增大纳米颗粒粒径对换热特性的影响趋势增大。     

ZrO_2-水纳米流体管内层流的传热特性

制备了粒径为50 nm的ZrO2-水纳米流体,并通过添加分散剂NH4PAA改善纳米流体的稳定性。测量了4种不同质量分数(0.2%,0.4%,0.8%,1.2%)的ZrO2-水纳米流体在层流状态下的对流换热系数。实验结果表明:在相同雷诺数下,纳米流体的换热系数要比纯水的有所提高,并随着ZrO2纳米颗粒质量分数的增加而增大。当纳米流体的质量分数为0.2%,0.4%,0.8%,1.2%时,其平均换热系数比纯水分别提高了1.9%,2.4%,5.2%和8.8%。实验管道内的不同位置也影响纳米流体换热系数的提高,入口段的换热系数要比充分发展段提高得更明显,其主要原因是纳米颗粒对流体边界层的干扰。     

圆管内纳米流体层流流动及强化传热的数值研究

采用Al2O3-水纳米流体作为工质,用数值模拟方法对其圆管内层流流体的流动及换热过程进行了可视化研究,在雷诺数600~1200的范围内,分析了不同雷诺数下的流动换热效果及浓度对纳米流体层流传热性能的影响。结果表明：纳米流体中纳米颗粒的加入对流动通道内的速度和温度分布影响较大。随着纳米流体浓度的增加,流体的传热系数增大,...     

纳米流体强化微尺度换热的研究进展

综述了纳米流体强化微尺度换热的研究进展。总结了近几年纳米流体应用于微尺度换热器中的实验研究。从两方面对纳米流体强化微尺度换热的机理进行了分析:纳米颗粒本身对换热特性的强化;纳米颗粒改变换热表面特性对换热的强化。介绍了纳米流体强化微尺度换热的数值模拟研究。指出了当前研究中存在的问题和不足,并对今后的研究工作进行了展望。     

振荡热管内不同形态纳米颗粒流动及传热特性

主要针对不同形态的纳米颗粒在振荡热管内的流动及热传输特性进行了实验研究。在相同的压力、相同的热管倾角、不同的充液率条件下,对振荡热管内工质分别为Cu-水纳米流体以及Cu-水纳米流体中Cu纳米颗粒沉积后溶液的流动以及热传输特性进行了实验研究,并与工质为蒸馏水时进行了对比实验分析,以此来研究振荡热管内气、液以及纳米颗粒多相流动存在时,对热管传输特性的影响。实验表明：当振荡热管内存在气、液以及不同形态的纳米颗粒多相流动时,对其传热特性会产生很大的影响,在一定条件下会起到强化传热的作用。     

纳米流体在芯片微通道中的流动与换热特性

对去离子水及体积分数分别为0.15%和0.26%的水基γ-Al₂O₃纳米流体在当量直径为194.5 μm的硅基梯形芯片微通道内的层流流动和换热特性进行了实验研究。考察了Reynolds数、Prandtl数以及体积分数对流动换热的影响。结果发现,使用纳米流体后,压降无明显增加,纳米流体的流动阻力特性与去离子水基本相同;对流换热Nusselt数较去离子水有明显提高,且随着体积分数的增加而增加;相同泵功下换热热阻显著下降。实验还发现纳米流体的强化传热效果在较高温度时更加明显。根据实验数据得到了梯形硅微通道内低浓度纳米流体的层流对流换热关联式。研究结果对于集成高效芯片散热系统设计具有重要意义。     

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.     

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.     

Investigating the efficacy of nanofluids as coolants in plate heat exchangers (PHE)

The efficacy of nanofluids as coolants is investigated in the present study. For the nanofluids tested, systematic measurements confirmed that the thermophysical properties of the base fluid are considerably affected by the nanoparticle addition. A typical nanofluid, namely a 4% CuO suspension in water, is selected next and its performance in a commercial herringbone-type PHE is experimentally studied. The new experimental data confirmed that besides the physical properties, the type of flow inside the heat exchanging equipment also affects the efficacy of a nanofluid as coolant. The fluid viscosity seems also to be a crucial factor for the heat exchanger performance. It is concluded that in industrial heat exchangers, where large volumes of nanofluids are necessary and turbulent flow is usually developed, the substitution of conventional fluids by nanofluids seems inauspicious.     

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。     

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.     

一种新型裂解炉炉管强化传热数值模拟

利用计算流体动力学(computational fluid dynamic,CFD)方法对含新型内插件强化传热辐射炉管(fortified induced turbulence,FIT)进行了流体流动与传热特性的研究,采用RNG双方程模型求解了动量方程和能量方程,给出了FIT炉管内的流体流动和传热特性,包括速度场、湍动强度和温度场的分布;计算了FIT炉管的强化传热因子和压降。研究结果表明,FIT炉管内插件迫使流体流动由活塞流转变为旋转流,增强了流动湍流程度,符合流动-能量场协同理论,同时流体边界层由于FIT炉管的特殊结构而减薄。FIT炉管具有增强辐射传热、减薄边界层、增加比表面积和旋流增强等强化传热特性。相比于普通当量圆炉管,FIT强化传热炉管的整体传热能力提高了20%左右,证明该新型炉管强化传热效果显著,可以在工程实际中应用。     

粗糙肋面上湍流热量传递中场协同关系的数值分析

用数值模拟的方法将湍流流动的传热分为传热层流底层和湍流层,来考察传热层流底层中温度梯度矢量与速度夹角对湍流流动换热的影响,用场协同理论分析了湍流条件下粗糙肋面的传热强化问题.     

单侧加热方形通道内超临界水传热研究

数值模拟了辅助冷却剂超临界水在单侧加热方形通道中的流动传热特性,从边界层厚度与近壁区湍动能两方面阐述传热恶化产生和恢复的机理。研究了不同工况（压力、入口温度、热通量、质量流量、流动方向和管径）下超临界流体常用传热关联式的适用性,发现Fan关联式预测精度较高。采用PEC因子对不同强化传热结构（双通道和凹槽）进行评价,发现上下双通道PEC因子普遍小于1,综合强化换热效果不佳,而偏下游的非对称倒角凹槽结构PEC因子为1.13~1.51,不同工况下均为最大值。场协同原理分析也证明偏下游的非对称倒角凹槽结构具有最佳的综合换热性能。     

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.