Influence of wettability on boiling heat transfer and critical heat flux 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. In nuclear power generation, the initiation of a CHF event may cause fuel damage through fuel melting or through zircaloy oxidation. CHF marks the upper limit for safe reactor operation in light-water reactor systems. The understanding of the physical triggering mechanisms for CHF and the accurate prediction of CHF values is important not only for safe reactor operation, but also for efficient plant design and effective fuel utilization. Gamma radiation present in commercial nuclear reactors may affect the surface wettability of fuel rods through the Radiation Induced Surface Activation (RISA) effect, and present power limits on nuclear reactors may be underestimated. 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 thesis, the surface wettability influence is studied on the boiling curves and specifically the point of DNB. 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 and are compared with existing 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. Few models accurately predicted the initiation of CHF, and a more conservative model is needed that more accurately incorporates the effect of surface wettability