纳米流体及纳米表面的管内对流强化传热

发布时间：2018-02-21 05:07

本文关键词： 纳米流体混合对流一步湿化学法表面活性剂微细通道流动沸腾纳米工程表面局部干涸　出处：《浙江大学》2015年博士论文　论文类型：学位论文

【摘要】：宏观尺度强化传热技术的发展已达到一定高度,并接近饱和；微米／纳米尺度的新技术、新材料、新方法能够提供宏观尺度下难以实现的优势,已经成为强化传热学科纵深发展的新动力。本文的研究目的是,通过采用微米／纳米尺度的强化传热技术,对管内单相对流换热及流动沸腾换热的基本物理现象及其强化机理有更深入的理解,以为新型微纳结构强化技术的开发提供数据支持与理论指导。本文的基本内容是以纳米流体的单相对流换热为出发点,探究纳米流体与传热界面的相互作用,基于纳米流体对传热表面的改造机理,将研究扩展到纳米表面的微细通道流动沸腾换热。本文首先对纳米流体和微纳结构工程表面的研究现状作了详尽的综述和分析,从中发现若干主题亟待研究：纳米流体的混合对流、一步法纳米流体对流换热、活性剂对换热的影响、纳米工程表面运用于微细通道沸腾传热。对于纳米流体在大管径横管内的混合对流,本文系统地研究了二氧化硅纳米颗粒原生粒径、颗粒体积浓度、基液粘度和普朗特数等参数对于管内层流混合对流换热的影响,纳米流体混合对流换热相对于基液出现了恶化,采用混合对流判别数和均相模型统一解释了纳米流体的混合对流传热特性。采用一步湿化学法制备了较大量稳定的二氧化硅纳米流体,在细通道内分别就其层流、湍流、混合对流进行了对流换热系数的测试,考虑热物性的改变后,其对流换热系数可以用传统的关联式预测,并不存在奇异的强化效应。一步法纳米流体无需使用表面活性剂,然而两步法必须采用活性剂才能稳定,这将对实验产生干扰。本文通过实验论证了表面活性剂SDBS对于细通道内流动沸腾换热的影响,活性剂溶液的表面张力大大降低,可以减小沸腾换热中汽泡的尺寸,减轻细通道中汽泡的融合以及局部干涸,减缓换热系数随干度的增加而急剧恶化的趋势。纳米工程表面技术可视为对纳米流体的扬弃,特别适用于微细通道内流动沸腾换热的强化。本文搭建了适用于测试可控参数微纳结构表面的微细窄通道流动沸腾测试系统,结合高速摄像系统,对过冷沸腾和饱和沸腾中的局部干涸和重新润湿现象进行了可视化流型观察,研究了流速、热流和干度等参数对换热机理的影响。采用PECVD制备了二氧化硅表面,发现沉积层的厚度对于表面的亲疏水特性有重要影响,当沉积厚度为100nm时,表面呈现超亲水特性。采用超亲水纳米二氧化硅表面用于微细窄通道内饱和沸腾换热,可改变环形流的液膜分布,在干度提高时可避免微细通道中常出现的传热恶化现象。
[Abstract]:The development of macro-scale heat transfer enhancement technology has reached a certain height and is close to saturation. New technologies, new materials and new methods at micron / nano-scale can provide advantages that are difficult to achieve at macro scale. Has become a new driving force for the in-depth development of the subject of enhanced heat transfer. The purpose of this paper is to use micron / nanoscale enhanced heat transfer technology. The basic physical phenomena and strengthening mechanism of single-phase convective heat transfer and flow boiling heat transfer in the tube are further understood. The basic content of this paper is to explore the interaction between nano-fluid and heat transfer interface based on the single-phase convection heat transfer of nano-fluid. Based on the modification mechanism of nano-fluid on heat transfer surface, the flow boiling heat transfer of micro-channel is extended to nanoscale surface. Firstly, the research status of nano-fluid and micro-nano structure engineering surface is reviewed and analyzed in detail in this paper. It is found that a number of topics need to be studied: mixed convection of nanoscale fluids, convection heat transfer of one-step nanoscale fluids and effect of active agents on heat transfer. Nano-engineering surface is applied to micro channel boiling heat transfer. The primary particle size and particle volume concentration of silica nanoparticles are systematically studied for the mixed convection of nano-fluids in large diameter transverse tubes. The effects of viscosity of base fluid and Plantt number on the convection heat transfer of laminar mixed flow in the tube, the mixing convection heat transfer of nano-fluid is worse than that of the base liquid. Mixed convection discriminant number and homogeneous phase model were used to explain the heat transfer characteristics of nanofluids. A large number of stable silica nanofluids were prepared by one-step wet chemical method. The convection heat transfer coefficient of mixed convection is tested. Considering the change of thermal properties, the convection heat transfer coefficient can be predicted by traditional correlation formula, and there is no singular enhancement effect. However, in order to be stable in the two-step method, it will interfere with the experiment. In this paper, the effect of surfactant SDBS on the boiling heat transfer in a thin channel is demonstrated, and the surface tension of the surfactant solution is greatly reduced. The bubble size in boiling heat transfer can be reduced, the bubble fusion and local drying up in fine channel can be alleviated, and the heat transfer coefficient will deteriorate sharply with the increase of dryness. Nanoengineering surface technology can be regarded as the sublation of nanoscale fluid. It is especially suitable for the enhancement of flow boiling heat transfer in micro-channels. In this paper, a micro-narrow channel flow boiling measurement system, which is suitable for testing the surfaces of micro-nano structures with controllable parameters, is built, and a high-speed camera system is developed. The local drying and rewetting phenomena in subcooled boiling and saturated boiling were observed, and the effects of flow rate, heat flux and dryness on the heat transfer mechanism were studied. Silica surface was prepared by PECVD. It is found that the thickness of the deposited layer has an important effect on the hydrophobic properties of the surface. When the deposition thickness is 100 nm, the surface exhibits superhydrophilic properties. The surface of superhydrophilic nano-silica is used for saturated boiling heat transfer in the narrow channel. The liquid film distribution of annular flow can be changed, and the deterioration of heat transfer in microchannels can be avoided when dryness is increased.
【学位授予单位】：浙江大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TK124

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