Critical heat flux on laser-textured surface in vertical flow boiling

The critical heat flux (CHF) marks the upper limit of safe operation of heat transfer systems that utilize two-phase boiling heat transfer. In a heat flux controlled system, exceeding the CHF results in rapid temperature excursions which can be catastrophic for system components. Recent studies have focused on the influence of surface wettability on the departure from nucleate boiling (DNB) through surface modifications and coatings, though many of these studies are limited to pool boiling systems. In this study, the surface wettability influence is studied on the boiling curves and specifically the DNB points. A femtosecond laser is used to texture the surface to change the wettability from hydrophilic to hydrophobic. A parametric study is performed with mass flux, pressure, and inlet subcooling in a vertical rectangular channel that is heated from one side. CHF excursions are triggered under various system conditions in the turbulent flow regime and are compared with existing DNB models. For the experimental conditions considered, the hydrophobic surface showed delayed onset of nucleate boiling compared to the hydrophilic surface, shifting the boiling curves to higher wall superheat. The hydrophobic surface also showed significantly lower CHF for the same system conditions and less sensitivity to changes in subcooling. The surface wettability shows a substantial influence on the DNB point and should be incorporated in future flow boiling models for CHF.