【作者】 朱蓓蓓；

【导师】 高洪涛；

【作者基本信息】 大连海事大学 ， 轮机工程， 2010， 博士

【摘要】 溴化锂吸收式制冷机具有广阔的市场前景。在吸收式制冷系统中,发生在吸收器中的溴化锂浓溶液吸收水蒸气的过程是系统功能实现的关键阶段,而提高吸收速率一直被认为是提高系统效率和减少交换面积的重点。在溴化锂水溶液中加入少量的异辛醇等表面活性剂后,可以极大地提高吸收过程的传热传质能力。表面活性剂具有显著的强化吸收作用,国内外的学者也在这方面做了许多研究,但尚未得到清晰的和公认的解释。本文采用分子动力学模拟与实验研究相结合的方法,从微观角度研究表面活性剂的吸收促进机理,以求丰富和改进现有的表面活性剂强化吸收的理论。主要研究成果归纳如下：采用分子动力学方法研究了温度对水的气液界面微观结构的影响。发现：液相密度是温度的减函数,界面厚度是温度的增函数；界面的存在对水分子局部结构几乎没有影响,但对水分子的取向有影响：界面处水分子的偶极方向平行于界面,但随着温度的升高,界面对水分子取向的影响越来越小；温度几乎不改变水分子的局部结构；液相处,平均每个水分子形成氢键的数目几乎稳定于某一数值,并且是温度的减函数,随着水分子到达界面处,氢键的数目单调减少。采用分子动力学方法研究了温度和浓度对溴化锂水溶液气液界面微观结构的影响。发现：溴化锂水溶液的液相密度是浓度的增函数,界面厚度是浓度的减函数,而温度对液相密度和界面厚度的影响与浓度对其的影响相反；界面的存在并未影响离子周围水分子的局部结构和取向有序性；随着温度的升高,离子周围水分子取向的有序性不再那么明显；溴化锂水溶液浓度的改变对离子周围水分子取向分布的影响不大；温度和浓度的改变对离子周围水分子局部结构的影响不大。采用分子动力学方法研究了醇类添加剂的数目对醇-水的混合物气液界面微观结构的影响。模拟结果表明：醇分子吸附在气液界面处,并在界面处优势取向；短链醇具有较强的溶解性；当正辛醇、异辛醇或正癸醇的数目较少时,醇分子在气液界面处形成单层,随着醇分子数目的增加,在气液界面处形成双层；界面厚度是醇分子数目的增函数。采用分子动力学方法研究了醇类添加剂的种类、数目及溴化锂水溶液的质量分数对醇-溴化锂水溶液混合物气液界面微观结构的影响。模拟结果表明：正烷醇分子吸附在气液界面处,并在界面处优势取向,并且这种优势取向随着正烷醇数目的增加而更加显著；界面厚度是正烷醇数目和烃基链长度的增函数；随着正烷醇数目的增加,烃基链垂直于界面的方向有序性更加明显；离子与正烷醇中羟基存在着相互作用；正丁醇或异辛醇关于溴化锂水溶液的溶解性是溴化锂水溶液质量分数的减函数,符合盐析效应理论。采用Daiguji等提出的模型,将醇分子置于吸收端溴化锂水溶液的两个气液界面处,采用分子动力学方法研究非平衡条件下,醇类添加剂对溴化锂水溶液吸收水蒸气的影响。模拟结果表明：正丁醇、正已醇、正辛醇、异辛醇、正癸醇均在溴化锂水溶液吸收水分子的过程中表现出强化作用,与实验趋势是吻合的。对表面张力进行了实验和分子动力学研究。采用表面张力仪测量了常温下水、质量分数为60%的溴化锂水溶液的表面张力,与文献中的测量值接近。测量了单独添加正已醇、正辛醇、异辛醇、仲辛醇以及两两混合前述物质后的溴化锂水溶液的表面张力,发现添加两种添加剂后溶液的表面张力与单独添加时较低的张力值接近。采用分子动力学方法研究了温度、浓度对水及溴化锂水溶液表面张力的影响。计算结果表明：水和溴化锂水溶液表面张力的计算值小于实验值,但随着温度、浓度的变化趋势与实验趋势是一致的。采用分子动力学方法计算了分别加有正丁醇、正已醇、异辛醇的质量分数为60%的溴化锂水溶液的表面张力,发现添加剂都能一定程度地降低溶液的表面张力；同时计算了加有不同数目正辛醇的溴化锂水溶液的表面张力,发现溴化锂水溶液的表面张力是正辛醇数目的减函数,与实验趋势一致。更多还原

【Abstract】 Absorption chillers using the working pair water-lithium bromide (LiBr) have obtained broad market prospects.In absorption refrigeration system, the water vapor absorption into LiBr aqueous solution in absorption vessel is the key stage of systematic function. Improving the absorption speed is thought to be important in advancing system efficiency and reducing the exchange area. Adding a little amount of surfactant such as 2-ethyl-l-hexanol into LiBr aqueous solution can enhance the heat and mass transfer greatly. Due to its great application value, this effect has been the subject of numerous investigations in recent years, while the working mechanism is not fully understood. The present dissertation mainly focuses on molecular dynamics simulation methods as well as experimental research to study the absorption enhancement mechanism of surfactant in microscopic view, willing to enrich and ameliorate existing theories.Molecular dynamics simulation methods are carried out to investigate the temperature effect on the microstructure of liquid-vapor interface of water. Simulation results show that the density of bulk liquid decreases with the increase of temperature; however, interface thickness increases with the increase of temperature. The local structure of water molecules is not affected by the presence of interface. Water molecules are observed to show preferred orientational order at the liquid-vapor interface:water permanent dipoles prefer to lie parallel to the interface; however, with the increase of temperature the interface has little influence on the orientationl order of water molecules. In bulk liquid region, the number of hydrogen bonds per water molecule is roughly constant, and decreases when temperature increasing. As water molecules approach the interface, the number of hydrogen bonds per water molecule decreases monotonically.Molecular dynamics simulation methods are used to study the microstructure of the liquid-vapor interface of lithium bromide aqueous solution with various concentration at different temperature. The simulation results demonstrate that liquid density increases with the increasing concentration of the electrolyte solution, meanwhile, interface thickness decreases gradually; however, the temperature has contradict effect on liquid density and interface thickness. The presence of interface cannot affect the orientation order and local structure of water surrounding by ions. The orientation order of water molecules neighbored by ions becomes not clear with the increase of temperature. The concentration of electrolyte solution has little effect on the orientation profile of water surrounding by ions. The variation of temperature or concentration has little influence on the local structure of water molecules around ions.Molecular dynamics simulation methods are adopted to study the effect of the amounts of alcohols on the microstructure of the liquid-vapor interface of the mixture of water and alcohols. The simulation results indicate that alcohol molecules tend to adsorb at the interface and show their dominant orientation with the hydrophobic hydrocarbyl pointing into the vapor phase and hydrophilic hydroxyl pointing into the liquid phase, while the hydroxyl groups forming a hydrogen bonding network with water which makes the alcohol molecules seek more hydrophilic interactions with water molecules. Alcohols with short hydrocarbon chains have intensive solubility. When the concentration of n-octanol,2-ethyl-l-hexanol or n-hexanol is slightly lower, the alcohols can form monolayer at the interface, and graduate into bilayers with the increase of the amounts of alcohols. The interface thickness increases with the increase of the amounts of alcohols.Molecular dynamics simulation is introduced to study the impact of the types, the amounts of alcohols and the concentration of LiBr aqueous solution on microscopic structure of the liquid-vapor interface of the mixture of alcohols and electrolyte solution. The computed results reveal that n-alcohol molecules tend to adsorb at the interface with preferred orientation, meanwhile, the tendency of this kind of preferred orientation becomes distinct with the increase of the amounts of n-alcohol molecules. The interface thickness increases with the increase of the amounts of n-alcohol molecules or the length of hydrocarbon chains. The hydrocarbon chains of n-alcohol molecules are inclined to be close to stay upright near the interface while the amounts of n-alcohol molecules is more, meanwhile, this orientational ordering becomes significant with the increase of the amounts of n-alcohols. The direct interactions between hydroxyl hydrogen of n-alcohols and anion exist, and moreover, there are much stronger electrostatic interactions between oxygen of n-alcohols and cation. The dissolvability of n-butanol or 2-ethyl-l-hexanol related to LiBr aqueous solution decreases with the increase of the concentration of LiBr aqueous solution, which is consistent with the salting-out effect theory.The dynamic process of water vapor absorption into electrolyte solution with or without alcohol surfactants is explored by molecular dynamics simulation under non equilibrium conditions. Taking the model proposed by Daiguji et al., this dissertation puts alcohols on the two interfaces of LiBr aqueous solution in absorption side. The simulation results suggest that in comparison to lithium bromide aqueous solution without surfactants, the electrolyte solution with surfactants can absorb more water molecules distinctly for 100 ps, which conforms to the experimental tendency.The experimental and molecular dynamics methods are used to study the surface tension. Tensiometer is used to measure the surface tension of water and LiBr aqueous solution at room temperature. The experimental value of surface tension is close to the value from literatures. N-hexanol, n-octanol,2-ethyl-l-hexanol and 2-octanol are added into LiBr aqueous solution by two ways:alone or two of them. The experimental results signify that the surface tension of LiBr aqueous solution with compound surfactants is near to the one which makes surface tension smaller. The computed value of surface tension of water and LiBr aqueous solution is smaller than the experimental value. But the variation tendency with the temperature or concentration of LiBr aqueous solution accords with the experimental results. The simulation results show that n-butanol, n-hexanol or 2-ethyl-l-hexanol can reduce the surface tension of LiBr aqueous solution, meanwhile, the surface tension decreases with the increase of the amounts of n-octanol, which meets the experimental results.更多还原