Modelling Boiling Flows in Microchannels with a Multiphase CFD Code

Over the past two decades, numerical modelling of boiling flows in microchannels has gained significant importance across diverse domains encompassing electronics and aerospace devices, where it plays a pivotal role in fostering the creation of compact cooling systems. Furthermore, this research is of critical importance in optimizing the drying process of porous media, especially within the field of nuclear industry applications, particularly in the context of addressing failed fuel rods. Boiling flows in microchannels exhibit marked distinctions from their counterparts in conventional channels. Typically, the bubble diameters in microchannels exceed or closely match the channel dimensions, thereby introducing notable confinement effects as bubbles expand against the channel walls. Therefore, surface tension and wettability effects assume a pronounced role in this scenario and need to be considered. Finally, microchannels typically witness limited occurrences of boiling at their channel walls, with nucleate boiling being less prevalent, as the primary boiling takes place at the interface between bubbles and the liquid. Numerical simulations of boiling flows in microchannels were conducted using the industrial software neptune_cfd. Using the pre-existing Large Interface Model, these simulations employed an interface tracking approach that considers surface tension and wettability effects when modelling vapour bubbles. This paper introduces a novel analytical mass transfer model based on a cut-cell method. Notably, this new model exhibits rapid mesh convergence, with validation studies demonstrating its ability to reach convergence with cells three times larger than the previous model. The validation of this method encompassed a range of 3D scenarios, including the growth of a vapour bubble in an unbounded superheated liquid (the Scriven case) and the buoyant ascent of a bubble within a superheated liquid. Furthermore, this study delves into more intricate scenarios, specifically those characterized by confinement and wettability effects within realistic microchannels.