Minimum heat flux and minimum film-boiling temperature on a completely wettable surface: Effect of the Bond number

Abstract

We investigated the effect of Bond number of sphere Bo(s) and surface super-hydrophilicity at minimum film-boiling temperature T-MFB and minimum heat flux q ''(min) using quenching experiment at atmospheric pressure and the saturation temperature of water. In particular, we focused on the vapor-releasing dynamics in film boiling and evaluated the main parameters such as vapor-bubble releasing frequency f(b) and vapor-bubble departure diameter D-b. We selected two sizes of quench sphere (sphere diameter D-s = 15 mm and 25 mm) based on critical Bond number Bo(C) to evaluate the vapor-releasing dynamics depending on the Bo(s). The super-hydrophilic surface was prepared by the anodic oxidation on zirconium (Zr-702) sphere. High speed visualization and inverse heat transfer calculation facilitate a qualitative and quantitative analysis of film boiling heat transfer. The surface super-hydrophilicity of the quench sphere increases T-MFB and q ''(min): 12% and 366% increase for D-s = 15 mm and 20% and 305% increase for D-s = 25 mm, respectively. D-b strongly depends on D-s and exhibits a relatively weak dependency to the surface super-hydrophilicity. f(b) is affected by the D-s and the surface super-hydrophilicity. The increase in T-MFB is explained by the liquid-solid contact in film boiling. The D25-CWS exhibits the large area fraction of liquid-solid contact versus total heat transfer surface compared to the D15-CWS. The increase in q ''(min), is related to minimum frequency of vapor-bubble releasing to sustain the stable liquid-vapor interface f(b,min) because the large f(b,min) indicates the fast destabilization of the liquid-vapor interface in film boiling during quenching. (C) 2017 Elsevier Ltd. All rights reserved.