Validity of one-dimensional evaporation model for confined microchannels

Abstract

Evaporation lies at the heart of many industrial processes, ranging from power generation, thermal desalination, to advanced cooling systems. These technologies leverage the efficient mass and heat transfer that accompanies evaporation, making accurate evaluation of evaporative transport essential for optimal design. The kinetics of evaporative transport is governed by a thin non-equilibrium region near the liquid surface, known as the Knudsen layer. Within this layer, the flow deviates from the continuum, rendering computational fluid dynamics (CFD) inapplicable. To circumvent the complexity of multi-scale simulations, a one-dimensional (1D) evaporation model is conventionally employed to predict the flux from the Knudsen layer and applied to the CFD simulations as the boundary condition. This simplification minimally affects the overall mass flux when the system is much larger than the Knudsen layer. However, its accuracy becomes questionable in micro/nano devices. Here, we assess the accuracy of this approach by simulating evaporation into a microchannel using the direct simulation Monte Carlo method and comparing the results with CFD simulations coupled with the 1D model. Results confirm that the tangential velocity at the liquid surface has little impact on the accuracy of the 1D model, and that combining it with a reliable slip model allows effective prediction even under strong confinement. However, the presence of a large non-equilibrium pressure region may compromise predictions when the channel height is comparable to the mean free path. These findings provide guidance and define the applicability limits for the use of the 1D evaporation model in confined geometries.