Effects of bubble coalescence on pool boiling heat transfer and critical heat flux – A parametric study based on artificial cavity geometry and surface wettability

•Pool boiling experiments on surfaces with artificial cavities were performed.•The effect of surface morphology on bubble coalescence was investigated.•Circular cavities with various diameters and pitch sizes were fabricated on silicon surfaces.•Different bubble coalescence types were observed on tested samples.•There exists a critical pitch size/diameter ratio for the horizontal bubble coalescence. Controlling the onset of boiling is highly desirable for enhancing boiling heat transfer. In this study, a systematic set of pool boiling experiments on surfaces with artificial cavities were performed to investigate the effect of surface morphology on bubble coalescence and resultant boiling heat transfer performance. Circular cavities with various diameters and pitch sizes were fabricated on silicon surfaces. The effect of surface wettability on the performance of the structured surfaces were examined with the use of 50 nm thick Teflon film. Using a high speed camera to examine the bubble dynamics, the results reveal that there exists a critical hole pitch size/hole diameter ratio (P/D = 10), below which horizontal bubble coalescence occurs at the lower wall superheats. Furthermore, the visual results indicated that surface wettability alters the critical heat flux (CHF) mechanism. In contrast to the hydrophobic surfaces, hydrodynamic instability is the main reason for CHF occurrence on the hydrophilic surfaces. The results indicate that although increasing the hole diameter enhances the CHF for all the fabricated samples, the effect of pitch size depends on the surface wettability with the CHF increasing with pitch size on the hydrophobic surface and decreasing with pitch size on the hydrophilic surface. A maximum heat transfer coefficient enhancement of 100% was achieved on the hydrophilic structured surface relative to the hydrophobic structured surface. The maximum CHF increase was 100% on the hydrophilic surface and 48% on the hydrophobic surface.