Review of Kelvin's Equation and Its Modification in Characterization of Mesoporous Materials

Physical and mathematical models as well as calculation methods of nitrogen bed on porous media have been introduced to evaluate the structural parameters of mesoporous materials. Kelvin＇s equation is a link between the relative adsorbate pressure, the mean pore radius, and pore capillarity on the basis of macroscopic capillary condensation. However, Kelvin＇s equation has been identified that it underestimates the calculated pore size of a material especially in the boundary of pore size which is between 2 and 4 nm.Various modifications on Kelvin＇s equation were mentioned in order to develop a new model to improve the accuracy of pore size calculation. The problems on conventional mathematical models were analyzed and discussed. A number of calculation methods on physisorption and pore size, especially fundamental theories of physisorption, basis of models and their deficiencies are reviewed. It can provide guidance on developing a modified Kelvin＇s equation for pore size calculation.

周期性热作用下相变材料内部相变传热特征实验研究

为了揭示周期性热作用下相变材料内部相变传热特征,为建筑外表相变隔热设计和计算提供支撑,本文设计制作了能模拟太阳运行、提供周期性加热的装置,并将正18烷封装于塑料圆桶容器中,使容器底部和侧面绝热,制作了测试试件。利用热电偶和巡检仪自动记录了周期性热作用下相变材料内部各层温度的变化。测试结果表明,在稳定周期性热作用下,材料内部相变传热特征主要可表述为:1)材料在非稳态周期性相变传热阶段,各层温度在达到相变温度点之前,温度变化明显;相界面会随着周期性加热次数的增加间断性地向内部扩展;温度平均值和振幅值向稳态周期性相变传热时的平均值和振幅值靠近。2)材料在稳态周期性相变传热阶段,内部各点温度也表现出与外界热作用等周期性变化,温度振幅从外到内依次减小。这与发生在固体中的周期性传热特性类似。但材料内温度变化呈现折转现象,在温度明显升高前出现等温滞后,在温度降低时会出现等温冷却。这与发生在固体中的传热现象有很大的区别。3)材料在稳定周期性热作用下,内部不会出现多个相界面共存现象。4)空气流速对相变材料中温度振幅和相界面移动速率都有较大影响,提高空气流速可以降低温度振幅和减慢相变进程。这意味着当相变材料用于建筑外表进行相变隔热时,可以通过组织自然通风减少其用量。