【作者】 阳倦成；

【导师】 李凤臣；

【作者基本信息】 哈尔滨工业大学 ， 工程热物理， 2013， 博士

【摘要】 在水中添加表面活性剂或者高分子聚合物能使其所形成的粘弹性流体在湍流状态下的流动摩擦阻力大幅降低，然而将其应用到具有换热单元的系统中会带来十分严重的传热恶化现象。在水中添加纳米尺度的高导热系数纳米颗粒并辅以一定的稳定剂所形成的纳米流体能增强换热系统中的对流换热特性，然而，由于固体颗粒的加入会带来额外的流动阻力。粘弹性流体的传热恶化问题以及纳米流体的流动阻力问题的解决是实现这两种流体在工程实际中应用的一个十分重要的课题。本文将从这一角度出发，结合粘弹性流体和纳米流体各自的优势，以期成功配制以粘弹性流体为基液的纳米流体并获得该种流体流动减阻和传热相对强化的效果。本文首先对实验所需的测量仪器以及自行搭建的圆管对流换热与流动阻力实验台进行了详细的介绍。而后，通过考虑纳米粒子的热扩散效应，推导了适用于纳米流体传热直接数值模拟的热扩散模型以及能量方程，并给出了直接数值模拟采用的方法。在实验方面，成功配制了较为稳定的以十六烷基三甲基氯化铵（CTAC）和水杨酸钠（NaSal）（1:1质量比）水溶液所形成的粘弹性流体为基液，以高导热系数的铜（Cu）纳米颗粒作为纳米粒子的纳米流体，称之为粘弹性流体基纳米流体（VFBN），并详细介绍了该种流体的制备方法。对VFBN的悬浮稳定性、导热系数、流变学特性以及表面张力进行了详细的实验研究，并通过对Li-Qu-Feng导热系数模型的改进获得了适用于球形铜纳米粒子和长棒状碳纳米管所形成的VFBN的导热系数预测理论模型。VFBN的导热系数测量结果表明该种流体导热系数明显高于水的导热系数，且导热系数随着铜纳米粒子体积分数和流体温度的增加而增大；VFBN的流变学性质测量结果表明该种流体与粘弹性流体基液相似，出现了粘度的剪切稀变特性，表现出获得流动减阻效果的可能性。在前期导热系数和流变学性质的测量基础上，采用圆管对流换热与流动阻力测试实验台，对水、粘弹性流体、水基纳米流体和VFBN的流动阻力和换热特性进行了测量。实验结果表明水基铜纳米流体流动相比水流动而言有明显的传热强化效果且传热效果随粒子体积分数和佩克莱特数（Pe）的增加而增大，而流动阻力增加并不明显。对于粘弹性流体流动而言，其流动具有传热恶化和流动减阻的特性，且传热恶化率大于流动减阻率，然而对添加Cu纳米粒子所形成的VFBN流动而言，Cu纳米粒子的加入对粘弹性流体基液的粘弹性现象有一定的削弱，但增强了其换热性能。通过对各种流体流动综合性能象限分布图分析可以发现，VFBN流动在具有湍流减阻效应的同时，还具有相比于粘弹性流体流动传热强化的效果，达到了本研究的预期目的。在理论研究方面，通过将计算结果与实验结果对比，确定了实验中铜纳米流体的扩散导热系数常数值。采用该种方法对纳米流体的流动和传热特性进行了直接数值模拟（DNS）。DNS计算得到的流体流动综合性能象限分布图趋势与实验基本一致。同时，通过对计算所得的各种流体的湍流速度场、温度场和变形场结果进行分析，并从动量方程和能量方程出发推导并计算了流体的流动阻力总贡献中的粘性贡献、湍流贡献和粘弹性贡献以及传热总贡献中的导热贡献、湍流贡献以及纳米粒子热扩散贡献，深入研究了VFBN流动减阻和传热强化的机理。最后，采用比拟理论，对各种流体的传热契尔顿-柯尔本因子和流动阻力系数的比值进行计算，结果再次表明以水和粘弹性流体为基液的纳米流体表现出了非常优异的传热性能。更多还原

【Abstract】 The viscoelastic fluid such as aqueous solution of surfactant or polymer caninduce a great reduction of flow resistance at turbulent flow state. However, avery serious heat transfer deterioration always occurs simultaneously in a systemcontains heat transfer units. The nanofluids prepared by suspending somenanoscaled particles with high thermal conductivity in water and adding auxiliarystabilizer can enhance the heat transfer performance of a heat transfer system.Nevertheless, the extra flow resistance will be introduced caused by thesuspended soild particles. To solve the issues of heat transfer deterioration inviscoelastic fluid turbulent flow and increase of resistance in nanofluid flow is animportant subject in practical engineering application for these two kinds offluids. From this viewpoint, the present thesis will combine the advantage ofviscoelastic fluid with that of nanofluid, hoping to prepare a kind of stablenanofluid using viscoelastic fluid as base fluid successfully and realize the effectsof fluid resistance reduction and heat transfer enhancement.Firstly, the required experiment equipments and self-made pipe flowconvective heat transfer and flow resistant test experiment system are introduced.Then, by considering the thermal dispersion effect of nanoparticles, the thermaldispersion model and energy equation which used to simulate heat transfer effectof nanofluid is deduced. And then, the direct numerical simulation method isintroduced at last.In the experiment study aspects, the nanofluid which named viscoelasticfluid based nanofluid (VFBN) has been successfully prepared by using aqueoussolution of cetyltrimethylammonium chloride (CTAC) and sodium salicylate(NaSal) with1:1in mass ratio as viscoelastic base fluid and copper (Cu) particleswith high thermal conductivity as nanopaticles. The detailed preparation methodof VFBN is introduced. Subsequently, the studies of suspension stability, thermalconductivity, rheology and surface tension for VFBN are carried out in thepresent thesis. A theoretical prediction model for thermal conductivity of VFBNwith spherical copper nanoperticles or long cylindrical carbon nano-tubes is proposed by modifying Li-Qu-Feng’s thermal conductivity model. Themeasurement results of thermal conductivity of VFBN show that this kind offluid has a larger thermal conductivity than water, and its thermal conducticity isincreased with the increase of volume fraction of copper nanoparticle and fluidtemperature. The rheology study results show that the VFBN retains the shearthinning behavior in its viscosity, which implies a potential flow drag reducingability.Based on the studies of thermal conductivity and rheology of tested fluid, theflow resistance and heat transfer performance of water, viscoelastic base fluid,water based nanofluid and VFBN are measured by using pipe flow convectiveheat transfer and flow resistance test experiment system. The experiment resultsshow that the water based nanofluid flows own a singnificant heat transferenhancement than water flows and its heat transfer enhancement is increased withthe increase of nanoparticle volume fraction and Peclet (Pe) number, however,the increase of flow resistant is not obvious. As for viscoelastic fluid flows, theheat transfer is deteriorated and flow resistance is reduced, besides, the heattransfer reduction rate is always larger than flow drag reduction rate. Whenadding copper nanoparticles in viscoelastic fluid and forms a VFBN, for this kindof fluid flows, the added nanoparticles weaken the viscoelasticity of base fluid,while enhance the heat transfer performance. By ploting and analyzing the flowcomprehensive performance phase diagram of different fluid flows, theviscoelastic fluid based nanofluid flows show a flow drag reduction and heattransfer enhancement than viscolelastic fluid flows, and the intended purposes ofthe present study are achived.In the theoretical study aspects, the constant thermal dispersion coefficientof copper nanofluid in experiment is got by comparisons of the simulated resultsand experiment results. Direct numerical simulations (DNS) of nanofluid flowsare carried out by using this method. From the fluid flow comprehensiveperformance phase diagram got by DNS results, it shows the similar trend withexperiment results. Meanwhile, through analyzing the simulated velocity field,temperature field and conformation filed of different fluid cases, deducing andcalculating the flow resistance with viscous, turbulent and viscoelasticcontributions and heat transfer rate with conductive, turbulent and thermal dispersion contributions of nanoparitcles from momentum equation and energyequation respectively, the mechanisms of heat transfer enhancement and flowdrag reduction of VFBN is studied further. Finaly, based on analogy theory, theratio of Chilton-Colburn factor to flow friction coefficient of different fluid flowcases is calculated. The results show that nanofluid which use water andviscoelastic fluid as base fluids show significant enhancement of heat transferperformance again.更多还原