二元体系雾化剂在多孔介质雾化器中的“非选择性蒸发”机理及实验验证

Non-selective evaporation mechanism by binary aerosol forming agents fromporous media atomizers and its experimental verification

为解释电子烟气溶胶中不同成分的比例与电子烟液中基本一致而与成分的沸点相对无关的特殊雾化现象,应用相图和毛细管蒸发理论,研究推导出二元体系雾化剂的“非选择性蒸发”机理,并通过检测蒸发温度、设计气流加热蒸发实验和毛细管“非选择性蒸发”实验观测平台验证了非选择性雾化的形成条件和形成过程。结果表明:(1)由于表面加热和传质受阻,在毛细管蒸发过程中形成了与液相主体组分比例不一致的蒸发层,使从蒸发层蒸发的气相组分的比例与传输进蒸发层的液相组分的比例一致,达成蒸发-传输平衡,从而形成持续的“非选择性蒸发”。(2)丙三醇-丙二醇蒸发温度(T)与二元体系液相主体组成下的泡点温度(T_b)相差30~49 K;而与同样组成比例的气相的露点温度(T_c)很接近,仅相差4~10 K,说明表面蒸发层液相组分构成与液相主体组分构成显著不同。(3)在气流表面加热条件下,水-甲醇、水-丙二醇和丙二醇-丙三醇3种二元体系的蒸发均显著偏离了理想选择性蒸发,表现出明显的“非选择性蒸发”特征,在液面覆盖多孔介质后,蒸发进一步偏离了选择性蒸发,该现象表明“表面加热”和“传质阻力”是形成“非选择性蒸发”的关键条件。(4)双段式单毛细管蒸发实验平台观察结果显示,水-甲醇二元体系蒸发速率具有由快到慢的明显拐点,说明蒸发层组成发生了从起始阶段与液相主体一致到稳定“非选择性蒸发”阶段与液相主体不一致的变化。(5)在颗粒示踪实验中观测到毛细管内的整体液流为单向,不存在大范围的对流传质,弯月面附近存在微区域对流,进一步验证了“蒸发层”的存在。实验验证结果多方面支持了所提出的“非选择性蒸发”机理。

To explain the peculiar atomization phenomenon that the proportion of different components in the aerosol of e-cigarettes is basically the same as those in the e-liquid,although the boiling points of the main carrier components are quite different,a“non-selective”evaporation mechanism by the binary aerosol forming agents was studied based on their binary phase diagram and capillary evaporation theory.The formation conditions and the process of“non-selective evaporation”were verified by measuring their evaporation temperatures,airflow during heating,using a bespoke capillary experimental platform.The results showed that:1)Because the surface heating in the porous media,the mass transfer inside the porous capillary were severely hindered,an evaporation layer was formed during the process of capillary evaporation,which was inconsistent with the proportion of the main components in the e-liquid phase.The proportion of the gas-phase components evaporated from the evaporating layer matched the proportion of the liquid-phase components transferred into the evaporating layer to achieve the evaporation-transfer equilibrium.This created a continuous and stable“non-selective evaporation”.2)The difference between the evaporation temperature(T)of glycerol and propylene glycol,and the bubble point temperature(Tb)of the liquid phase of the binary system was 30-49 K.T was very close to the dew point temperature(Tc)of the gas phase with the same component proportion,and the difference was only 4-10 K,indicating that the proportion of the liquid phase components of the surface evaporation layer was significantly different from the proportion of the main components in the liquid phase.3)Under the surface heating conditions,the evaporation of water-methanol,water-propylene glycol,and propylene glycol-glycerol binary systems deviated significantly from the thermodynamically predicted selective evaporation,thus termed as the“non-selective evaporation”.When the porous material was covered with the liquid surface,the evaporation of binary systems further deviated from the selective evaporation.This phenomenon showed that surface heating and mass transfer resistance were the key conditions for the formation of this“non-selective evaporation”.4)The experimental results of the two-stage single capillary evaporation platform showed that the evaporation rate of the water-methanol binary system had an obvious inflection point from fast to slow,indicating that the proportion of components in the evaporation layer changed from being consistent with the e-liquid at the initial stage to being different in the stable“non-selective evaporation”stage.5)The overall unidirectional liquid flow in the capillary tube was observed by the particle tracing experiment,and no large-scale convective mass transfer and microregional convection existed near the capillary meniscus,which further verified the existence of the“evaporation layer”.The experimentally results supported the proposed mechanism of“non-selective evaporation”in its main thermophysical characteristics.