Hydrodynamic analysis of liquid microlayer formation in nucleate boiling of water

Abstract

This paper presents analyses of the formation of "microlayers" i.e., very thin (∼ 1μm) liquid films often found beneath steam bubbles growing on a solid surface in low-pressure boiling. The hydrodynamics of bubble growth on a surface in pool boiling of water at atmospheric pressure have been modelled with interface-capturing Computational Fluid Dynamics (CFD) in a range of conditions that include a recent set of experiments, and transcend the remit of extant computational studies. Numerical predictions of bubble and microlayer behaviour were found in broad agreement with experimental measurements, and consistent with the current theoretical understanding of the microlayer hydrodynamic formation process. From recently reported direct measurements of microlayer thickness, a range of values of the likely thickness of the liquid film as formed on the solid surface have been inferred, and used to validate the current simulations, which clearly indicated the formation, beneath a growing steam bubble, of a liquid film of thickness comparable to values inferred from measurements. Notably, CFD simulations revealed the formation of a characteristic "dewetting ridge" i.e., an area where the film surface is markedly convex, in a microlayer region close to the triple contact line not accessible to any present-day optical measurement technique, and indicate that, in an otherwise flat microlayer, a noticeably curved region should be expected to form near the contact line. This paper provides enhanced insight into micro-scale features, such as the shape and thickness of microlayers, that typically evolve over extremely small time scales, and are therefore challenging to measure, and at present not amenable to any observation technique. A parametric study, covering a range of values of the capillary number between 7.86 × 10−4 and 7.86 × 10−2, and surface contact angles between 15˚ and 90˚, typical of wetting or partially wetting fluids on common engineering surfaces, confirmed the trends in microlayer behaviour predicted for the case of atmospheric-pressure water-boiling in laboratory conditions.