纳米孔内液体蒸发建模及不凝气影响研究

Modeling liquid evaporation in nanopore and the effect of noncondensable gas

纳米多孔介质内的液体蒸发过程及不凝气的影响机制是能源化工等工程领域中的关键科学问题,基于气体动理论分析了蒸气在纳米孔道内的流动阻力特征,结合Boltzmann方程矩方法建立了单组分蒸发的理论模型,采用直接蒙特卡罗方法模拟了含不凝气条件下的纳米孔蒸发过程,基于模拟数据构建了蒸发速率的预测关联式.结果表明:纳米孔的蒸发过程可分为孔内蒸气流动、孔外动力学运动和对流-扩散流动3个部分,孔隙率和蒸气所占孔道长径比对蒸发速率具有显著影响,孔隙率越大则不凝气对蒸发速率的影响程度越大,当气体段孔道长径比为2.5时液体蒸发速率可降低约50%.基于单组分模型和无量纲关联式的蒸发速率预测结果与直接蒙特卡罗模拟结果的相对误差基本为±10%.

The liquid evaporation process in a nanoporous medium under the influence of noncondensable gasses is a fundamental problem in energy and chemical engineering.Based on the kinetic theory of gasses and the moment method for the Boltzmann equation,the flow resistance of vapor in nanoscale pores or channels was analyzed,and a theoretical model for the single-component evaporation problem was proposed.Further,direct simulation Monte Carlo was performed to study evaporation in the presence of noncondensable gasses.An empirical correlation was established to predict the liquid evaporation rate.The results showed that nanoporous evaporation can be divided into three parts:vapor flow within the nanopore or channel,kinetic expansion out of the nanopore,and convection-diffusion flow of vapor downstream.The porosity and aspect ratio of the vapor-occupied pore section have a profound influence on the evaporation rate.Moreover,the effect of noncondensable gas is more outstanding at larger porosities than at smaller porosities.The liquid evaporation rate can be reduced by 50%when the vapor-occupied pore section has an aspect ratio of 2.5.The model of single-component evaporation combined with the empirical correlation for dimensionless evaporation rate can predict direct simulation Monte Carlo results within 10%.