【作者】 苏风民；

【导师】 马学虎； 陈嘉宾；

【作者基本信息】 大连理工大学 ， 化学工程， 2009， 博士

【摘要】 利用纳米流体强化基础液体的导热性能和扩散性能的作用来提高NH₃/H₂O泡状吸收过程的传热传质特性是一种新型的吸收强化方法,其研究刚刚起步。现有文献对适合吸收过程的纳米流体制备、纳米流体的热物理性质、以及对吸收强化机理等方面的研究还很不完善,本文尝试在这些方面进行一些探索性的工作,为这种强化方法的进一步研究和工业化应用奠定基础。1)本文选择化学性质稳定的碳纳米管（CNTs）作为纳米颗粒,在氨水中制备了稳定的CNTs-NH₃双组分纳米流体,并设计一系列的实验,研究了CNTs-NH₃双组分纳米流体的物理性质,考察了颗粒质量百分比、氨的浓度和温度等因素对双组分纳米流体物理性质的影响。实验结果表明,碳纳米管的加入确实提高了氨水的导热系数,并且强化效果随着颗粒质量百分比的增加和温度的升高而增加。但是,由于氨分子的存在影响了碳纳米管的分散性,因此导热系数的强化效果低于以纯水为基础液体的纳米流体;双组分纳米流体的运动粘度和表面张力与纯氨水相比变化很小,只是随着碳纳米管质量百分比的增加而略有增加,并且这些性质随氨的浓度和温度的变化趋势与纯氨水是一致的。2)本文利用可视化实验手段考察了荧光素在SiO₂-H₂O和CNTs-H₂O两种纳米流体的扩散情况。实验结果发现,荧光素在纳米流体中的扩散速率要大于其在纯水中的扩散速率,并且对于纳米流体中荧光素的扩散速率的强化效果来说,纳米颗粒的加入量存在一个最佳值。3)本文结合对双组分纳米流体热物性的研究结果和相关文献,从NH₃/H₂O泡状吸收过程中的热量传递过程、质量传递过程、气泡生成过程三个角度,探讨了纳米颗粒的加入对吸收过程的强化作用。纳米颗粒的加入提高了吸收工质-氨水导热性能和扩散性能,引入了传输效应,增加了氨水中氨气的气含率,进而改善了吸收过程的热质传递特性。在此基础上,设计了NH₃/H₂O泡状吸收实验装置,考察了碳纳米管的质量百分比、氨的初始浓度、氨气流速三个因素对CNTs-NH₃双组分纳米流体吸收强化特性的影响,并对其强化机理进行了分析。实验结果发现,与前人的研究结果不同,双组分纳米流体中NH₃/H₂O泡状吸收强化比并没有随着碳纳米管加入量的增加而一直增加,而是呈现一种先增加,后下降的趋势。泡状吸收强化比随着基础液体中氨的初始浓度的增加而增加。氨的初始浓度越大,氨水的吸收潜能越小,纳米流体对吸收的强化作用越明显。氨气流量的变化在本实验变化的范围内,对双组分纳米流体中NH₃/H₂O泡状吸收的强化效果影响很小。4)利用高速摄像技术观察了气泡在SiO₂-H₂O和SiO₂-C₂H₅OH两种透明纳米流体中的形成过程,并结合气泡动力学,分析了纳米流体中气泡尺度变化的原因。结果发现,纳米颗粒的存在降低了气泡的脱离半径,提高了气泡的脱离频率。在相同的气体流量下,气泡的半径越小,液相中的气含率越大。这样的实验结果证明了纳米流体中气泡气含率增加除了文献报道的—颗粒能够阻止气泡在运动过程中的合并这个原因外,气泡脱离半径的减少也是一个重要原因。同时该实验结果也说明纳米颗粒对气泡尺度的影响是双组分纳米流体强化NH₃/H₂O泡状吸收过程的一个重要因素。更多还原

【Abstract】 Nanofluids,which can enhance the thermal conductivity and mass diffusivity of the base fluid,is a novel approche to improve the heat and mass transfer in the NH₃/H₂O bubble absorption process,.The previous investigations are limited because of many encountered problems,such as the preparation of the specific nanofluids for the absorption process,the transportation properties of the binary nanofluids,the factors and the mechanism affecting the absorption enhancement.This dissertation mainly focuses on the following aspects for the NH₃/H₂O bubble absorption process using binary nanofluids.1)The carbon nanotubes（CNTs） are used as the nanoparticles,and the CNTs-NH₃ binary nanofluid is prepared without any surfactants addition.The physical properties,such as thermal conductivity,surface tension and kinematic viscosity of binary nanofluids,are measured.The effects of the mass fraction of carbon nanotubes,the concentration of ammonia and the temperature are systematically studied with a series of experiments.The results show that the CNTs-NH₃ binary nanofluids have remarkably higher effective thermal conductivities than that of the aqueous ammonia,and the thermal conductivity ratio between the binary nanofluid and the base fluid increases with the mass fraction of CNTs and the temperature increasing. However,the thermal conductivity ratios of CNTs-NH₃ nanofluids are lower than that of the CNTs-H₂O ones due to the influence of ammonia ions on the dispersion of CNTs in the base fluid.The addition of CNTs has only a slight effect on the surface tension and kinetic viscosity of the CNTs-NH₃ binary nanofluid,and the variation trend of these properties with the concentration of ammonia and the temperature is same with that of the aqueous ammonia.2) The effects of the nanoparticles on the mass diffusion are investigated experimentally by visualizing the diffusion process of a fluorescein in the SiO₂-H₂O nanofluid and the CNTs-H₂O nanofluid.The results show that the diffusion rates of the fluorescein in the two kinds of nanofluids are higher than those in water,and an optimum mass fraction of nanoparticles exists for the enhancement of the diffusion rate in the nanofluids.3) The experiments are conducted for the enhanced performance of the binary nanofluid for the NH₃/H₂O bubble absorption process.The effects of the mass fraction of CNTs,the initial concentration of ammonia and the flow rate of ammonia vapor on NH₃/H₂O bubble absorption characteristics are studied.The results show that the effective absorption ratio doesn’t increase linearly with the mass fraction of nanoparticles,but increases firstly,and then decrease.Namely, there is an optimum mass fraction of nanoparticles for the enhancement of the bubble absorption.The effective absorption ratio increases with the initial concentration of ammonia increasing.The absorption potential of aqueous ammonia generally decreases with increasing the ammonia concentration,i.e.the lower the absorption potential is,the more significant the enhancement.The flow rate of ammonia vapor has little effect on the enhancement of bubble absorption in the binary nanofluid within the experimental range.Based on the previous study about the thermal physical properties of the binary nanofluid and the relevant reports in literature,the mechanism of the binary nanofluid for enhancing bubble absorption process is analyzed with respect to the effects of the nanoparticles on the heat transfer process,mass transfer process and the forming process of the bubbles in the NH₃/H₂O bubble absorption process.The presence of nanoparticles can improve the thermal conductivity and mass diffusivity of aqueous ammonia,stimulating the grazing effect,and increase the gas holdup of ammonia vapor in aqueous ammonia.And hence it can enhance the heat and mass transfer in the bubble absorption.4) The forming processes of the bubbles in two transparent nanofluids of SiO₂-H₂O and SiO₂-C₂H₅OH are observed by the high-speed CCD camera,and the changes of the bubble detaching size in the nanofluids are analyzed according to the dynamics of the bubble.The results show the presence of the SiO₂ nanoparticles decreases the radius of the bubble and increases the detaching frequency of the bubbles in the two nanofluids.Moreover,the radius of the bubble decreases with the mass fraction of the nanoparticles increasing.The smaller bubbles are contributed to the higher gas holdup at the same flow rate of the gas.Therefore,it can be proved that the nanoparticles affect assuredly the forming process of the bubbles and are one of the most important factors for the increase of the gas holdup in the nanofluid and for enhancing bubble absorption process.更多还原