纳米流体强化传热机理及数学模型的研究

发布时间：2018-07-17 07:58

【摘要】：纳米流体是将纳米颗粒添加到基础液体中形成稳定悬浮的多相系统。由于纳米流体在粘度、比热容、导热率、换热能力等性能方面都显著区别于传统传热工质,可以作为更高效的传热和冷却介质。对于提高汽车发动机发动机传热效率、改善排放及降低油耗,促进冷却系统向紧凑化、轻型化、高效化发展。本文围绕纳米MgO粉体的制备实验、纳米流体的制备与分散剂的选择实验、纳米MgO在丙二醇中分散于稳定性的研究、纳米流体运输参数的研究、纳米流体传热性能及纳米流体数学模型的研究几方面内容,研究MgO/丙二醇纳米流体悬浮液的稳定性,探讨流体强化传热机理。主要工作如下:1、纳米MgO粉体制备实验用不同的液相化学法制备MgO纳米粉体,熟悉制备流程,掌握制备技术。2、纳米流体的制备及分散剂选择实验通过直接将MgO纳米粉体与丙二醇直接混合的“两步法”制备MgO/丙二醇纳米流体;采用超声振动、机械搅拌等方法及沉降试验优化选择出能使MgO/丙二醇纳米流体稳定悬浮的分散剂。3、纳米MgO在丙二醇中稳定性研究通过球磨改性的方法,用偶联剂改性纳米MgO,增加悬浮液分散稳定性;并且实验分析了超声振动、机械搅拌、分散剂等因素对纳米流体的影响,纳米MgO在丙二醇中的最佳分散工艺。从颗粒沉降与扩散运动的角度对纳米流体稳定性机理进行理论分析,得出结论,要想使纳米颗粒在基液中长期稳定,就必须使纳米颗粒具有较小的表面能和较大的扩散速率并且纳米颗粒间位能的高低对纳米流体的稳定有重要影响,总位能越低,纳米流体越稳定。4、纳米流体运输参数研究研制了用电流量法测定流体比热容的装置,测量了不同温度下不同浓度的MgO/丙二醇纳米流体的比热容,发现流体比热容随温度变化的增益特性。采用NDJ-8S型数显粘度计测量了MgO/丙二醇纳米流体的粘度,实验结果表明,颗粒的份额、基液属性、温度和悬浮液的稳定性对纳米流体的粘度有重要影响。5、纳米流体传热特性研究测定不同丙二醇基纳米流体的自然对流换热系数,并设计组装可用于测量纳米流体导热系数的瞬态热丝法测量装置,测量不同纳米流体的导热系数,并结合实验结果探讨纳米流体强化传热的特异性及规律。6、纳米流体数学模型研究通过对纳米颗粒引入分形维数的概念,并分析纳米颗粒微观结构,对传统的H-C模型进行修正,推导出一种预测低浓度纳米流体导热系数的导热模型。通过研究温度和浓度对粘度的影响,推导出了关于温度和浓度因素的粘度计算模型。
[Abstract]:Nano-fluid is a multi-phase system in which nanoparticles are added to the base liquid to form a stable suspension. Because the properties of nano-fluid in viscosity, specific heat capacity, thermal conductivity and heat transfer ability are obviously different from the traditional heat transfer working medium, it can be used as a more efficient heat transfer and cooling medium. It can improve the heat transfer efficiency of automobile engine, improve the emission and reduce the fuel consumption, and promote the development of the cooling system towards compact, light and high efficiency. This paper focuses on the preparation of nano-MgO powders, the preparation of nano-fluids and the selection of dispersant, the stability of nano-MgO dispersion in propylene glycol, the transport parameters of nano-fluids. The heat transfer performance of nano-fluid and the mathematical model of nano-fluid are studied in this paper. The stability of MgO / propylene glycol nano-fluid suspension is studied and the mechanism of fluid heat transfer enhancement is discussed. The main work is as follows: 1. The preparation of nano-MgO powders was conducted by different liquid-phase chemical methods. They were familiar with the preparation process. Master preparation technology .2.The preparation and dispersant selection experiment of nano-fluids prepared by "two-step method" directly mixing MgO nano-powder with propylene glycol; Ultrasonic vibration, The dispersant. 3, which can make MgO / propylene glycol nano-fluid suspension stably, was optimized by mechanical agitation and sedimentation test. The stability of nano-MgO in propylene glycol was studied by ball milling method. The effects of ultrasonic vibration, mechanical stirring, dispersant and other factors on nano-MgO were analyzed, and the optimum dispersion process of nano-MgO in propylene glycol was analyzed. From the angle of particle sedimentation and diffusion movement, the stability mechanism of nano-fluid is theoretically analyzed, and the conclusion is drawn that in order to make the nanoparticles stable in the base solution for a long time, The surface energy and diffusion rate of nanoparticles must be small, and the potential energy between nanoparticles has an important effect on the stability of nano-fluids, and the total potential energy is lower. The more stable the nanoscale fluid is, the more stable the nanometer fluid transport parameter is. The specific heat capacity of the MgO / propylene glycol nanoscale fluid at different temperature is measured by electric flow method, and the specific heat capacity of MgO / propylene glycol nanoscale fluid at different temperature is measured. The gain characteristics of the specific heat capacity of the fluid with the change of temperature are found. The viscosity of MgO / propylene glycol nanofluids was measured by NDJ-8S digital display viscometer. The temperature and the stability of suspensions have an important influence on the viscosity of nano-fluids. The heat transfer characteristics of nano-fluids are studied and the natural convection heat transfer coefficients of different propylene glycol groups are measured. A transient hot-wire measuring device was designed and assembled to measure the thermal conductivity of nanoscale fluids, which can be used to measure the thermal conductivity of different nanoscale fluids. Combining with the experimental results, the specificity and regularity of heat transfer enhancement of nano-fluid are discussed. The mathematical model of nano-fluid is studied by introducing the concept of fractal dimension to nanoparticles, and analyzing the microstructure of nano-particles, and modifying the traditional H-C model. A thermal conductivity model for predicting the thermal conductivity of low concentration nanoscale fluids is derived. By studying the influence of temperature and concentration on viscosity, the viscosity calculation model of temperature and concentration factors is derived.
【学位授予单位】：青海大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TB383.1

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