Liquid evaporation and the associated vapor transport in micro/nanopores are ubiquitous in nature and play an important role in industrial applications. Accurate modeling of the liquid evaporation process in nanopores is critical to achieving a better design of devices for enhanced evaporation. Although having high impact on evaporation rate, vapor transport resistance in micro/nanopores remains incompletely understood. In this study, we proposed a new model which, for the first time, considered vapor transport in finite-length pores under various Knudsen regimes and then coupled the transport resistance to liquid evaporation. Direct Simulation Monte Carlo and laboratory experiments were conducted to provide validation for our model. The model successfully predicts the variation of pore transmissivity with Knudsen number and nanopore size, which cannot be revealed by prior theories. The relative error of model-predicted evaporation rate was within 1% in L/r = 0 cases and within 3.5% in L/r > 0 cases. Our model is featured by its applicability under the entire range of Knudsen numbers. The evaporation of various types of liquids in arbitrarily sized pores can be modeled using a universal relation.

The liquid evaporation from nanoscale pores has attracted much attention from researchers due to its importance in water treatment and device cooling related applications. It is crucial to investigate the receded liquid case as the vapour flow resistance in a nanopore has high impacts on the evaporation rate. This paper proposed a semi-empirical analysis on nanoporous evaporation with a receded liquid surface under the influence of the Knudsen number. The vapour flow dynamics in a nanopore was examined considering the multiple reflections of vapour molecules. We calculated the value of pore transmissivity based on transitional gas flow correlation which incorporated the effect of the Knudsen number. Direct simulation Monte Carlo method was employed to provide validation for the present model. The vapour density jump near the liquid surface and the pressure ratio between the far field and saturation value were predicted by our model with good precision. It was shown that the vapour flow resistance in the nanopore accounted for more than 90 % of the total resistance in present cases. With increasing Knudsen number, the pressure ratio gradually drops and reaches an asymptotic level. This suggested a relatively higher evaporation resistance in free molecular regimes. The present work revealed the importance of the Knudsen number in nanoporous evaporation with receded liquids, providing insights into the governing factors under various Knudsen regimes.

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Liquid evaporation from micro/nano scale pores is widely encountered in cutting-edge technologies and applications. Due to its two- (or three-) dimensional feature, the nano-porous evaporation is less understood compared to the one-dimensional evaporation of planar liquid surface. This paper reported a novel study of the inter-pore interference effect in nano-porous evaporation, clarifying the variation of net evaporation rate from individual nanopore when the inter-pore distance, neighboring nanopore diameter or liquid temperature were respectively changed. Molecular simulation results showed that the reduction of inter-pore distance could enhance the evaporation rate from nanopores, by augmenting the vapor convection effect and suppressing the condensation flux. This interference effect was more pronounced at lower evaporation intensity, with the evaporation flux being different by up to 25% from the one-dimensional case. The inter-pore interference was equally observed for Knudsen number of 0.1 and 10. Additionally, the non-uniformity in nanopore size distribution had no influence on the evaporative mass flux within present parameter range. The non-uniformity in nanopore temperatures, however, could affect the net evaporation from individual nanopore, similarly by modulating the vapor convection magnitude in adjacent to the interface and the condensation flux. The effect of inter-pore interference is found to be essential at low evaporation intensity, which is highly relevant in industrial applications such as water evaporation under atmospheric pressure.