摘要
相变储能技术是利用相变储能物质相变时吸热和放热来储存和释放能量的技术。相变储能技术在太阳能、建筑节能、工业余热和纺织业等领域都有广泛应用。石蜡是常用的有机相变储能物质,主要由直链烷烃混合而成,可用通式CnH2n+2表示。在实际应用中,总是希望相变储能物质具有与相应工况匹配的相变温度,通过各种有机物的混合可以调节相变温度,因此,对有机相变物质混合比例的研究,从而获取具有特定相变温度的混合物,有助于相变储能技术的进一步应用。与传统研究有机混合物相变温度的差示扫描量热法(DSC)和动态热机械分析法(DMA)不同,本文采用分子动力学模拟(MD)方法进行研究。
模拟势函数为包含了分子间作用力L-J力场和分子内作用力OPLS力场的ORGANIC势函数;求解运动方程采用校正预测法,校正次数为5;温度控制采用速度标定法;压力控制为Parrinello-Rahman方法;应用周期性边界条件。
本文研究共包括两个部分。第一部分模拟计算采用等温等压(NTP)系综对十七烷、十五烷及十七烷/十五烷混合体系(共6个体系, 530个碳氢粒子)的相变行为进行模拟研究,温度范围为310K~270K,温度间隔为5K。根据各体系自扩散系数和比体积随温度的突变,确定了各混合体系的相变温度点。纯物质相变温度与实验值吻合。与此同时,通过统计十七烷体系粒子运动的内坐标,获得了十七烷分子键长、扭曲键角以及二面角随温度变化趋势,对纯物质在熔化过程中的微观结构变化进行了讨论。
第二部分模拟计算采用等温等体积(NTV)系综对十七烷、二十烷及十七烷/二十烷混合体系(共11个体系, 3100个碳氢粒子)的相变行为进行模拟,温度范围为340K~270K,温度间隔为10K。根据各体系自扩散系数随温度的突变,确定了各混合体系的相变温度点。纯物质相变温度与实验值吻合。
模拟研究发现,对于二元烷烃混合物可以通过调节组分比例,得到具有一定相变温度的混合物,实现一定温度下相变储能的应用要求。
Phase change energy storage technology, which is a storing and releasing energy technology by phase change material’s reception and release of heat, is used widely in solar energy, building energy conservation, industrial exhaust heat and textile industry. Paraffin made up of straight chain hydrocarbon is one of the organic phase change materials. Its general expression is CnH2n+2. In the practical application, phase change temperature must consistent with different working conditions and can be changed by adjusting constituent of organic mixture. Therefore, it is important to study how to gain the special phase change temperature by adjusting constituent of mixture. Comparing with the tradition method differential scanning caborimetry and dynamic thermomechametry, molecular dynamics simulation is used as the main study method in this article.
The potential function is ORGANIC which includes inter-molecular force L-J and intra-molecular force OPLS. Predictor-corrector method is undertaken to compute equation of motion. Velocity scaling method is used to control temperature. Parrinello-Rahman is used to control pressure and periodic boundary condition is undertaken in every simulation system.
There are two parts in this paper.
In the first part, molecular dynamics simulation (NTP ensemble) is adopted to study the melt behavior of Heptadecane, Pentadecane and Heptadecane-Pentadecane mixed system (six systems in all, 530 particles). Self-diffusion coefficient and specific volume changing along with temperature are drafted by collecting the movement locus of particles to determine the phase change temperature of every system. At the same time, Bond length, Bond angle and Dihedral angle changing along with temperature are drafted by collecting the internal coordinate of particles to study material microscopic structure of Heptadecane in melting process.
In the second part, molecular dynamics simulation (NTV ensemble) is adopted to study the melt behavior of Heptadecane, Eicosane and Heptadecane-Eicosane mixed system (eleven systems in all, 3100 particles). Phase temperatures of these systems are gained by self-diffusion coefficient changing along with temperature.
The results show that different phase transition temperatures can be gained to fit different working conditions by adjusting constituent of mixture.
引文
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