Microlayer dynamics during the growth process of a single vapour bubble under subcooled flow boiling conditions

Abstract

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.

Microlayer dynamics of hydrodynamically interacting vapour bubbles in flow boiling

“…The methodology is also discussed by many authors and, in the recent past, by Sinha et al. (2022).

Figure 3.( a ) Representative raw image and split images; ( b ) pictorial flow of methodology for calculating bubble equivalent diameter; ( c ) bubble base radius calculation methodology; ( d ) and ( e ) methodologies adopted to determine bubble's aspect ratio and centroid, respectively.

…”

“…Unlike pool boiling, the microlayer associated with a single vapour bubble in flow boiling is characterised by an asymmetric profile (Sinha et al. 2022). Therefore, one can expect complicated microlayer(s) dynamics when multiple bubbles are involved in flow boiling.…”

“…In the last decade, quantification of microlayers has been done using thin-film interferometry (TFI) (Gao et al 2012;Suryanarayan & Srivastava 2021) and infrared (IR) thermal imaging (Jung & Kim 2015;Bucci, Buongiorno & Bucci 2021) in pool boiling. Sinha, Narayan & Srivastava (2022) conducted experiments on a vertical flow boiling channel and analysed microlayer thickness evolution beneath a single vapour bubble using TFI. During the bubble growth cycle, the microlayer evaporates and substantially contributes to the bubble's growth while cooling the heater substrate.…”

“…To the best knowledge of the present authors, there is no availability of any experimental study involving multiple nucleations related to microlayers in the context of the flow boiling regime. Unlike pool boiling, the microlayer associated with a single vapour bubble in flow boiling is characterised by an asymmetric profile (Sinha et al 2022). Therefore, one can expect complicated microlayer(s) dynamics when multiple bubbles are involved in flow boiling.…”

On the Coupled Thermal and Hydrodynamic Interaction of Adjacently Located Vapor Bubbles on Highly Wetting Surfaces

“…The respective spatial resolutions of the data were 8 and 27 μm. The details of the thin-film interferometry technique and its implementation for mapping microlayer dynamics under the nucleate pool boiling regime may be found in some of our published articles. , For completeness, the working principle of thin-film interferometry to characterize thin liquid layers is shown schematically in Figure . In brief, the technique works on the principle of interference of two coherent beams.…”