【作者】 彭小飞；

【导师】 俞小莉；

【作者基本信息】 浙江大学 ， 车辆工程， 2007， 博士

【摘要】 随着现代发动机功率密度的不断提高，传统冷却液已经逐渐不能满足冷却系统高负荷的散热要求，有必要开发新型高效传热的冷却液。纳米流体技术的出现，为车辆冷却系统和车用散热器的发展提供了新的思路，为工程强化传热领域带来了新的研究方向。本文研制了300余种纳米流体的配方，对其悬浮稳定性、导热系数、比热容、粘度等重要特性参数分别做了较为详尽的试验研究，并分析推导了相应的预测模型。从众多配方中筛选了具有高沸点、高导热系数的有机型纳米流体，并与常规冷却液进行了基于车用散热器的高温小温差传热对比试验，验证了纳米流体在车用散热器中高温小温差传热的可行性和有效性。通过静置观察和TEM电镜观察研究了纳米流体的悬浮稳定性，试验表明，纳米粒子、基础液体、分散剂特性及超声振动等都关系着纳米流体的悬浮稳定性，普遍规律是粒子浓度低、粒径小、密度小、基础液体粘度大的纳米流体悬浮稳定性相对较好。采用自行设计的瞬态热线法装置测试了纳米流体的导热系数，结果显示纳米流体相对基础液体导热系数的增加率随着粒子体积份额的增加、粒径的减小和悬浮稳定性的优化而增加；分析了纳米流体导热增强机理，考虑了纳米粒子的小尺寸效应、纳米粒子的聚集、固液微界面和微对流，在H-C模型的基础上提出了低浓度纳米流体的导热系数修正预测模型。采用自行设计的比较量热法装置测试了纳米流体的比热容，结果显示纳米流体比热容比基础液体小，纳米粒子的体积份额越大则纳米流体比热容越小；分析了纳米流体比热容的增量来源于纳米粒子的小尺寸效应，推导了适合低浓度纳米流体比热容的预测模型。采用旋转粘度计测试了纳米流体的动力粘度，揭示了粘度与温度、粒子体积份额及悬浮稳定性之间的关系；在Einstein混合物粘度公式的基础上考虑了粒子聚集的影响，推导了低浓度纳米流体粘度预测公式。纳米流体的传热强化机制有静态机制和动态机制两个方面。静态机制来自于纳米流体导热性能的优化，动态机制来自于纳米流体系统中粒子的布朗运动引起的热扩散和碰撞作用，粒子之间、粒子与壁面之间在碰撞中产生能量损失，引起能量的迁移，是纳米流体传热能力增强的重要因素。将纳米流体应用到某型号的车用机油冷却器中，基于标准k-ε模型并采用以上理论分析，采用Fluent软件的数值模拟结果发现，在散热器传热中纳米流体较其基础液体换热量更大，内部湍动更强，温度分布更均匀。开发了具有高精度的纳米流体传热性能测试系统，以典型板翅式车用机油冷却器为传热对象，在水和防冻液在低温大温差（冷却液90℃，机油120℃）和Al₂O₃-PG90纳米流体及其基础液体在高温小温差（冷却液120℃、机油135℃）的工况下，测试了各种冷却液的传热性能及阻力性能。结果表明体积份额在5.0％以上的纳米流体能满足机油冷却器的传热需要，纳米流体具有比水、防冻液及基础液体更高的换热系数，且体积份额越大其传热系数增幅越大；并拥有比水和防冻液有更高的沸点温度和工作温度；纳米流体在散热器中的流动在Re＜1000时就已经达到紊流状态，纳米流体具有基础液体显示出更强的传热性能。纳米流体的传热系数相对基液的增幅比导热系数相对基液的增幅大，说明关于纳米流体传热机制的分析是正确的。纳米流体的传热性能较常规冷却介质有大幅提高，本文所使用的高沸点、高导热系数的有机型纳米流体，在车辆冷却系统中能形成较大的冷热流体（冷流体为环境空气）温差，有利于提高整个冷却系统的散热效率；并且由于基础液体是有机型的，预计具有较好的防腐、防冻和环保的功能，对车辆冷却系统具有重要意义，有利于开发新型的车用冷却液产品。本文的研究工作对新型高紧凑、高效率的散热器及高效、高沸点冷却介质的设计有指导性的作用，可以预计纳米流体在工程传热领域将有广阔的应用前景。更多还原

【Abstract】 Following the specific power’s enhancement of modern engines, conventionalengine coolants gradually can not meet the high heat loads of vehicle cooling systems.The appearance of nanofluids offers a new available approach for cooling systems andheat exchangers, and is becoming a new direction for engineering heat transfer.In this paper, over 300 varieties of nanofluids formulation have developed, andtheir suspension stability, thermal conductivity (k), specific heat capacity (C),dynamic viscosity (η) are all experimentally and theoretically studied in detail.Models of the k, C andηare respectively developed based on the micro-mechanism.These models are significant for designing nanofluids for special intents. The highboiling point & high thermal conductivity nanofluids have been selected from theformulations. Applying these nanofluids, the high temperature-low temperaturedifference (HTLTD) heat transfer tests have been done on certain vehicle oil cooler toverify the feasibility and validity of the nanofluids compared with water andconventional coolant.The suspension stability tests are based on the still layered and TEM figure, and theresults show that the suspension stability of nanofluids are seriously related to theproperties of nanoparticle, basefluid, dispersant and supersonic vibration. The generallaw is that low particle concentration, little particle dimension, high basefluidviscosity help to suspension stability.The transient-hot-wire technique systems were carefully self-designed to measurethe thermal conductivity of nanofluids. The results show that the thermal conductivityof nanofluids increases with the particle volume fraction and suspension situation,while decreases with the particle dimension. Based four micro-mechanisms: themicro-dimension effect, congregating of particles, micro-interface of solid-liquid andmicro-convective in the mixture system, the thermal conductivity model wasdiscovered for the low concentration nanofluids.The specific heat capacity of nanofluids has been investigated applying thecomparative heat capacity systems self-designed. The results show that nanofluidsheat capacity is lower than that of basefluid, and decrease with the particle volumefraction; meanwhile, the source of the decrease is concluded to develop the specificheat model for low concentration nanofluids.Applying the rotary viscometer, the viscosity of nanofluids is tested. The resultsshow that the viscosity is affected by the temperature, particle volume fraction andsuspension stability. Based on the Einstein solid-liquid mixture’s viscosity equation,the nanofluids viscosity model was deduced from the test values.The heat conduction enhancement can be owing to two factors: static and dynamicmechanisms. The static one is from the optimization of thermal conductivity and thedynamic one is from the thermal dispersion and particle collision because of theparticle’s Brownian motion in the nanofluids system. The collision between theparticles and the particle and the wall will lead to energy loss and conversion, whichis a fundamental factor to improve the thermal performance of nanofluids. Apply theFLUENT software to numerically simulate a vehicle oil cooler and the results show that nanofluids have a bigger heat flow, higher heat transfer coefficient than theirbasefluids.The high temperature, low temperature difference, low fluid flow, high precisionthermal performance test rig for nanofluids as the coolant is developed. Based on atypical oil cooler, the heat transfer and pressure performances of the water andanti-freeze fluid at low T-big T difference (max. T of 90 degree for coolant, while max.T of 120 degree for oil) and nanofluids and their basefluid (max. T of 120 degree forcoolant, while max. T of 135 degree for oil) are tested respectively. The results showthat the nanofluids with above 5% volume fraction could meet the heat transfer needsof the heat exchanger; the nanofluids have higher heat transfer coefficient than water,anti-freeze fluid and basefluid; and the heat transfer efficient is improved with thevolume fraction of nanofluids; nanofluids have higher boiling point and worktemperature that water and coolant, which makes it can work at a relative lowerpressure or none pressure circumstance.Nanofluids have excellent thermal performance compared with the conventionalcoolants. The organic high boiling point-high thermal conductivity nanofluids in thisstudy will cause bigger temperature differences to the cool fluid in the radiator—thesurrounding air, which is profitable to the whole vehicular cooling system’s efficiency.The basefluid which is organic is good to prevent corrosion and frost and protectenvironment. These properties are significant to vehicular cooling system. The studyin this paper could direct to design the high compact and efficient heat exchangers.The nanofluids could be envisioned to have a wide application in the engineering heattransfer field.更多还原