【作者】 薛怀生；

【导师】 樊建人； 胡亚才； 岑可法；

【作者基本信息】 浙江大学 ， 工程热物理， 2007， 博士

【摘要】 纳米流体的沸腾换热效果及原因分析存在很大争议，或将核态沸腾恶化归结为沸腾表面粗糙度的变化，不然则将其并临界热流密度的强化归结为导热系数及液体对沸腾表面浸润能力的提高，作者则认为影响纳米流体沸腾换热效果的主要因素为其基本热物理性质及纳米颗粒在沸腾表面沉积和结垢，沸腾换热的效果随过程的延长及垢层微结构的变化而变化；先前，纳米流体的沸腾换热的研究主要集中于核态化沸腾区域，过临界沸腾区域的研究尚未开展。据此实际，本文开展相关工作。用瞬态热线法和哈克粘度计测定通过分散剂稳定的多壁碳纳米管水溶液的导热系数和粘度，对比分析预混酸处理多壁碳纳米管和通过添加分散剂两种方法制备的悬浮体系的稳定性及稳定机理，同时探讨纳米颗粒强化导热系数及纳米颗粒改变工质粘度的机理。多壁碳纳米管的添加没有根本性强化工质的导热系数，工质的粘度也没有数量级的增加。纳米颗粒的添加而引起的对流、颗粒的聚集结构及悬浮体系的化学性质等因素综合作用影响工质的导热系数，纳米颗粒的聚集结构是悬浮体系呈现非牛顿流体特征的根本原因。通过在直径为12mm的沸腾表面进行的多壁碳纳米管阿拉伯树胶水溶液的大容积沸腾实验研究纳米流体的沸腾换热特性，澄清莫衷一是的文献结果。纳米流体的沸腾换热效果弱于阿拉伯树胶水溶液，烧毁点的过热度增加而临界热流密度减小，同时，阿拉伯树胶水溶液的换热效果劣于水的换热效果。纳米颗粒在沸腾加热表面富集、结垢引起沸腾表面附近液体密度、沸腾表面上活化核心数目的改变；随沸腾时间的延长，垢层结构，包括毛细孔直径、空隙率、垢层厚度，不断发生变化，进而引起蒸汽在毛细孔中的流动阻力不断增加、加热表面和垢层间热阻增加，沸腾表面的活化核心数目减小；拉伯树胶在蒸发表面附近的局部富集、液体粘度大大增加，最终导致沸阿拉伯树胶水溶液腾换热恶化。用镀镍铜球作为瞬态量热器，通过铜球淬火得到淬火曲线，计算出纳米流体及其作为对比液的水及阿拉伯树胶水溶液的特征沸腾曲线，以了解纳米流体的沸腾换热特性，特别是在烧毁点及以后的沸腾换热特性。对比于水，阿拉伯树胶水溶液的临界热流密度、过渡态沸腾及Leidenfrost点换热性能得到强化；纳米流体的临界热流密度高于阿拉伯树胶水溶液的临界热流密度。阿拉伯树胶水溶液对镀镍铜板表面的浸润能力得到提高；镀镍铜球表面在每次实验后都有少量纳米颗粒沉积。分散剂的存在改善了液体的浸润特性，改变了汽泡在沸腾换热表面的成长机制，使液体在烧毁点及Leidenfrost点之间的沸腾换热性能得到强化；纳米颗粒在沸腾表面的沉积也通过改变汽泡成长机制强化液体的临界热流密度。为研究纳米流体的沸腾传热效果，进行了以碳纳米管悬浮液充装的重力热管的沸腾特性实验。以碳纳米管悬浮液为工质的重力热管的起沸温度、温度漂移及蒸发段管壁温度比水工质热管的高，热管的整体热阻增大，换热性能恶化。通过测定，水中添加碳纳米管颗粒后悬浮液的表面张力增大，使得加热壁面的活化成核点密度、数量、脱离频率、气泡体积及气泡间聚合能力发生改变；气—液界面处的温度梯度和浓度梯度引发Marangoni流动；易蒸发组分水在界面处的蒸发产生质扩散效应；同时，纳米颗粒使液体对沸腾表面的浸润能力增加，从而使沸腾表面的汽化核心受到湮没，沸腾表面的活化核心点数目减少。几种因素的共同作用使热管蒸发段管壁和热管的起动温度上升，热阻增大，热管的传热性能恶化。纳米流体的导热系数受到内部颗粒聚集结构的强烈作用，而沸腾换热特性则主要受纳米颗粒在沸腾表面沉积、结垢的影响。随过程的继续，两者必将受到削弱。更多还原

【Abstract】 There are many discrepancies and contradictions about the nanofluid boiling heat transfer performance and the roots. Some got the results that nanoparticles deteriorate the nucleate boiling heat transfer and presumed that the surface roughness becomes smoother due to deposition of the nanoparticles and the concurrent reduction of nuclei of vapor bubble. Others concluded that nanofluid wets the boiling surface easier than the base fluid, and that dryout is delayed by continuous supply of liquid. The author conjectures that the thermo-physical properties of nanofluid, deposition and fouling of nanopaticle, and change of the foul microstrucsture with time would be the key factors to determine the nanofluid boiling heat transfer behavior. The literatures on nanofluid boiling heat transfer monotonously focous on the nucleate regime and on critical heat flux (CHF). The post-critical regime has not yet touched upon. Relevant works have been thus conducted.Thermal conductivity and viscosity of the multiwalled carbon nanotube (CNT) nanofluid stabilized by dispersant were measured by transient hot wire method and Haake viscometer, respectively. Stability of the nanofluids formulated by concentrated acid mixture and by addition of dispersant was compared. Mechanisms underlying the modified thermal conductivity and rheology of the nanofluid were provided. Thermal conductivity and viscosity of the nanofluid change marginally. Microconvection due to nanoparticle, aggregation, and chemistry of the suspension impacts both parameters methoned above. Non-Newtonian behavior of the nanofluid is dominated by the aggregation.To clarify controversies about the boiling heat transfer characteristics of nanofluid, nucleate pool boiling experiments were performed on a 12 mm diameter copper surface utilizing aqueous gum Arabic (GA) solution of CNT. As compared with aqueous GA solution, nucleate boiling heat transfer coefficient of nanofluid is lowered at the same superheat and a reduced CHF extended to higher temperature difference between the fluid and the heating wall. Boiling heat transfer performance of water is deteriorated by addition of GA powder. Density of the fluid in the vicinity of the heating wall is locally increased and nuclei on the surface are fluctuating with depositing and fouling of the nanoparticles. Microstructure of the foul, involving diameter and length of the capillary and porosity, changes while boiling proceeds. The depositing progress gradually increases the density and thickness of the deposit, deactivating the larger nucleation sites and closing the capillaries. The heat transfer coefficient is reduced and eventually leveled off by the additional resistance of the deposits. In the neighborhood of the heating wall, since viscosity is markedly enlarged due to evaporation of the volatile component and local enrichment of GA, boiling heat transfer of aqueous GA solution is diminished.Characteristic boiling curve, determined by transient calorimeter method, was employed to gain insight into the boiling heat transfer performance of CNT nanofluid. Special interests have been focused on in the post-critical regime. Comparatively, CHF, transitional boiling heat transfer rate, and heat flux at Leidenfrost of water are augamented by dispersion of GA powder. CHF of nanofluid is higher than aqueous GA solution. It is observed that GA solution wets the copper sheet much easier than water does and that there is a trace of deposits on the copper sphere surface. Improved wettability by the presence of the dispersant renovates the heat transfer mechanism. And thus, heat transfer performance in the transitional boiling regime incorporating CHF and Leidenfrost point gets an improvement. CHF of the nanofluid is intensified by way of the changed bubbling mechanism.To explore the boiling characteristics, experiments were carried out on a two-phase closed thermosyphon with carbon nano-tube suspensions as working media. In contrast to typical thermosyphon, the one filled with a nano-particle suspension has adversely high boiling incipience temperature, temperature excursion, and thermal resistance. The carbon nano-tube suspensions deteriorate the performance of the gravity-assisted heat pipe. Measurements showed that suspending carbon nano-tubes to bulk water renders it enlarged surface tension. Changes of the interfacial properties lead to reduced active nucleation sites, density and departure frequency, and to enlarged bubble volume and coalescence readiness. Marangoni flow is induced by temperature and concentration gradients. From preferential evaporation of volatile component at the local interface arises mass transfer. All factors function together and result in high temperature excursion, evaporator wall temperature, and thermal resistance. Heat transfer for carbon nano-tube suspension filled gravity-assisted heat pipe deteriorates.Thermal conductivity of the nanofluid is strongly influenced by the aggregation of the nanoparticles. Depositing and fouling of the nanoparticles on the heating surface govern the boiling heat transfer performance of nanofluid. Aging etches the merits of nanofluid away relentlessly.更多还原