Pool Boiling Versus Surface Wettability Characteristics

Abstract

-The present paper reports results of pool boiling experiments on surfaces with different wettabilities. The main aim is to identify and quantify the difference in pool boiling characteristics (Onset of Nucleate Boiling-ONB, bubble detachment diameter and time) between Hydrophilic Surfaces (HPS) and Super-Hydrophobic Surfaces (SHS). A combined sample (SHS_D) with an inner SHS part and an outer HPS part is also tested, which clearly shows that the Super-Hydrophobic surface is a preferential area for the ONB, and allows to control the bubbling position. It is found that not only the ONB, but also the bubble growth and detachment characteristics are highly influenced by the wettability of the heated surface. The analysis of the experimental results reveals that SHSs require a lower wall-superheat for ONB to occur. Moreover, in SHSs higher bubble departure times as well as higher equivalent bubble detachment diameters are encountered. The vapour phase tends to stick stronger on a SHS than on a HPS, showing a tendency to create a vapour film. Furthermore, using a post processing method developed in MATLAB, the apparent contact angle at various successive time instances is extracted from the experimental snapshots for the cases of the HPS and the SHS_D. Finally, the capability of a previously verified, enhanced, CFD-based, VOF numerical model [1-3] to capture adequately the apparent triple line contact angle, from the nucleation up to the departure of an isolated bubble from the heated surface, is checked for one of the considered superheats for a HPS case.