Capillary evaporation model of binary mixed solution of propylene glycol and glycerin in e-cigarette atomizer
For the current e-cigarette products on the market, the main method to generate e-cigarette aerosol is by heating the atomiser where the vaporization-condensation process occurs. E-liquid is mainly composed of a mixed solution of propylene glycol (PG), glycerin (VG) and additives (nicotine, flavours, etc.). By establishing an energy conservation equation, one can predict the vaporisation rate of the e-liquid, the temperature distribution and the evolution of the aerosol composition. PG and VG in the e-liquid were initially believed to follow the phase transition law of non-azeotropic mixtures, which means that the evaporation of PG is prior to that of VG. However, studies indicate that the ratio of PG to VG in aerosol is close to the ratio in raw e-liquid due to the working temperature of 200 °C in an e-cigarette atomiser. To understand the above phenomenon, a capillary evaporation model for the binary mixed solution of PG and VG at the microscopic scale was established by using Ansys Fluent software. According to the simulation, a fluctuation-stabilisation process of the aerosol composition was observed. During this process, PG preferentially evaporated near the capillary gas-liquid interface (meniscus). However, owing to the limitation of the mass transfer effect of capillaries, the PG liquid could not be replenished immediately to the meniscus, and consequently a local area with VG enrichment was formed. The above complex process promotes the evaporation of VG, and increases the proportion of VG in the aerosol, followed by a steady state. In addition, this model also predicts the effects of heating temperature on the wall, the ratio of PG to VG in the solution and the capillary size on the fluctuation duration, respectively. The results show that by increasing heating temperature on the wall and the proportion of VG as well as decreasing the capillary diameter, the steady state form can be reached more efficiently.