The phenomena of microlayer formation and its dynamic characteristics during the nucleate pool boiling regime have been widely investigated in the past. However, experimental works on real-time microlayer dynamics during nucleate flow boiling conditions are highly scarce. The present work is an attempt to address this lacuna and is concerned with developing a fundamental understanding of microlayer dynamics during the growth process of a single vapour bubble under nucleate flow boiling conditions. Boiling experiments have been conducted under subcooled conditions in a vertical rectangular channel with water as the working fluid. Thin-film interferometry combined with high-speed cinematography have been adopted to simultaneously capture the dynamic behaviour of the microlayer along with the bubble growth process. Transients associated with the microlayer have been recorded in the form of interferometric fringe patterns, which clearly reveal the evolution of the microlayer beneath the growing vapour bubble, the movement of the triple contact line and the growth of the dryspot region during the bubble growth process. While symmetric growth of the microlayer was confirmed in the early growth phase, the bulk flow-induced bubble deformation rendered asymmetry to its profile during the later stages of the bubble growth process. The recorded fringe patterns have been quantitatively analysed to obtain microlayer thickness profiles at different stages of the bubble growth process. For Re = 3600, the maximum thickness of the almost wedge-shaped microlayer was obtained as δ ~ 3.5 μm for a vapour bubble of diameter 1.6 mm. Similarly, for Re = 6000, a maximum microlayer thickness of δ ~ 2.5 μm was obtained for a bubble of diameter 1.1 mm.

No abstract

The bubble growth and its corresponding microlayer dynamics are strongly coupled from the point of bubble inception to its eventual liftoff. This paper discusses the complex and interesting interaction between a bubble and a microlayer through high-speed photography and thin-film interferometry in vertical flow boiling conditions. We analyzed existing force balance models and bubble growth rate models using experimental data. Our analysis revealed that the existing force balance models show severe limitations in predicting bubble dynamics, and the success of models reported by researchers is mainly due to over-parametrization and over-fitting. We show through our experimental results that the movement of the bubble in the flow direction and depletion of the upstream microlayer are strongly correlated with bubble diameter and growth rate. We discuss a non-dimensional approach based on forces acting on the bubble to predict the bubble movement in the flow direction. Furthermore, we report an interesting stage of the bubble ebullition cycle, where the bubble does neither liftoff nor contact the heater surface.

No abstract

No abstract