Single bubble dynamics during nucleate flow boiling on a vertical heater: Experimental and theoretical analysis of the effect of surface wettability, roughness and bulk liquid velocity

The present study reports the mutual effect of heater surface wettability, roughness and bulk liquid velocity on the bubble dynamics and departure in nucleate boiling. Boiling experiments were conducted at atmospheric pressure with degassed-deionized water at low subcooling (1.9 +/- 0.25 K) for vertically oriented stainless steel heaters. Self-assembled monolayer (SAM) coating and wet-etching technique were used to alter the heater surface wettability and roughness. Liquid contact angle hysteresis (theta(hys)) and root mean square roughness (Sq) of the heater surfaces were adjusted between 42.32A degrees <= theta(hys) <= 68.56A degrees and roughness 0.01 mu m <= Sq <= 0.549 mu m. High resolution optical shadowgraphy has been used to record the bubble life cycle. Experimental results show that higher bulk liquid velocity yields smaller bubble departure diameters for all heater surface characteristics. Bubble departure diameters are greater for low wetting surfaces. The bubble growth rate and departure diameter were found maximum for an intermediate surface roughness Sq between 0.108 and 0.218 mu m. The corresponding roughness height is referred to as the 'optimal roughness height' in this study. Eventually, a bubble departure criterion was derived from the expressions of forces which act on a nucleating bubble throughout its growth cycle. 90% of the departing bubbles satisfy the bubble departure criterion with +/- 25% deviation. (C) 2019 Elsevier Ltd. All rights reserved.