Bubble dynamics during capillary-fed nucleate boiling in porous media

Boiling from structured surfaces offers an effective heat transfer enhancement strategy. While there have been attempts to develop models to predict the boiling heat transfer coefficient from sintered screen meshes or sintered particles, less is known about the detailed mechanisms of bubble growth and departure in a porous medium. In this work, we study the growth of a vapor bubble in a micro-scale porous medium in which bubble growth is impeded by the drag offered by the porous medium. We use a volume-of-fluid model to track the liquid-vapor interface during bubble growth. Isotropic and anisotropic arrangements of sintered particles are modeled as uniform particles aligned in hexagonal and square-packed arrangements. A porosity range of 25%-75% is considered, while the particle diameter and wick thickness are held at 200 μm and 1 mm, respectively. The growth of a water vapor bubble nucleating at the interface between the porous medium and the substrate at a wall superheat of 5 K is investigated. Uniform bubble departure, present during the pool boiling of water, is not observed; instead, vapor columns are formed in the porous medium. A moderate wick porosity (50-70%) is found to assist the formation of vertical vapor columns in the wick pores. In contrast, laterally distributed vapor structures form at small porosities (< 50%). Based on the observed vapor column structures in the wick pores, an approximate mathematical model is proposed to optimize the wick thickness for maximum boiling performance. A wick thickness-to-particle diameter ratio in the range of 4 to 5 is found to optimize the heat transfer performance during boiling.