Mechanistic prediction of wickability and CHF enhancement on micro- and nano-engineered surfaces

Increasing pool boiling critical heat flux (CHF) is beneficial to next-generation thermal management schemes as well as existing equipment and systems. Significant enhancement in CHF has been reported for engineered surfaces. Several different CHF enhancement mechanisms and related models and correlations have been proposed including augmented surface area, contact line pinning, wettability, wickability, separated microlayer evaporation, controlled liquid and bubble flow field; yet no consensus on the fundamental enhancement has been achieved. Here, we investigate the effect of capillary wicking on pool boiling CHF enhancement. Six different silicon microstructured surfaces with two different square pillar sizes (10 µm and 15 µm), three different spacing (10 µm, 15 µm, and 20 µm), and a fixed height (20 µm) were fabricated and tested for wicking and pool boiling heat transfer performance. The geometries were varied to introduce a systematic variation in surface roughness, capillarity and permeability. First we measure surface capillary wicking using a custom-built test setup. The data are in good agreement with a mechanistic wickability predictive model for structured surfaces. Then, CHF is measured for these surfaces and compared to the predictions of the Drexel University model based on a non-dimensional wicking number; a linear dependence of CHF enhancement on the wicking number is confirmed. Additionally, the experimental CHF results were compared with two other CHF models, roughness based model and rewetting timescale of an irreversible dry spot model, from the Massachusetts Institute of Technology.