环境压力对双组分运动液滴蒸发特征影响的理论研究

Theoretical Study of Effects of Ambient Pressure on Evaporation Characteristics of Bicomponent Moving Droplet

关键词:  双组分  运动液滴  蒸发  传热传质

Key words:  Bicomponent  Moving droplet  Evaporation  Heat and mass transfer

摘要:

为获得亚临界条件下，环境压力对双组分运动液滴蒸发特性的影响规律，基于多组分液滴蒸发模型，对癸烷—乙醇二元混合液滴在高温氮气环境下的运动蒸发过程开展模型研究。通过模型计算，获得了液滴内部浓度分布、温度分布和液滴速度随时间的变化；分析了在环境压力0.1MPa和2MPa下，不同浓度液滴温度、尺寸及表面乙醇浓度的变化特征。结果表明:双组分液滴运动蒸发过程中，内部存在显著浓度和温度梯度，传热扩散快于传质扩散。常压环境下，液滴温度经历瞬态加热和平衡蒸发两个阶段，而高压环境下，液滴温度逐渐升高至接近环境温度。双组分液滴蒸发过程中由于组分浓度随时间变化，其无量纲尺寸随时间变化明显偏离“d 2理论”；环境压力越高，液滴运动的贯穿距离越短，蒸发速率越慢。乙醇浓度对液滴蒸发速率的影响有两方面:蒸发前期，乙醇浓度越高，液滴蒸发速率越快；而蒸发后期，乙醇浓度越高，液滴蒸发速率反而越慢。

Abstract:

In order to obtain the effects of ambient pressure on evaporation characteristics of bicomponent moving droplet under subcritical condition，a mathematic study based on multicomponent droplet evaporation model was performed to simulate the motion and evaporation process of a binary decane – ethanol droplet at high temperature nitrogen environment. By numerical calculations，the time evolutions of internal concentration distribution and temperature distribution within droplet and droplet velocity were obtained，and the variations of droplet temperature，size and surface ethanol concentration for different concentration droplets were analyzed at ambient pressure of 0.1MPa and 2MPa. The numerical results show that during the evaporation process of a bicomponent moving droplet，there exists significant concentration and temperature gradients within the droplet，and the heat diffusion is faster than mass diffusion. The droplet temperature variation includes transient heating and equilibrium evaporation at atmospheric pressure. While at high pressure environment，the droplet temperature increases gradually to reach the ambient temperature. The variation of dimensionless droplet size with time deviated from the ‘d2 law’ due to the concentration change with time. With a higher ambient pressure，the droplet penetration distance is shorter，and the evaporation rate is slower. The effect of ethanol concentration has two aspects on droplet evaporation: at earlier stage，the evaporation rate increases with a higher ethanol concentration; while at later stage，the evaporation rate decreases with a higher ethanol concentration.