Dynamic interaction of growing bubble and microlayer: Need for reconciliation of experiments and theory in flow boiling

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.

Microlayer dynamics of hydrodynamically interacting vapour bubbles in flow boiling

“…Our past experiments (Vadlamudi et al 2022) on single vapour bubbles in flow boiling indicated that at lower flow rates (for instance, at Re = 2400), after the bubble growth-dominated phase (We gr < 1), Eo is generally greater than 1, implying that buoyancy dominates the depleting mechanism, while We remains below 1. However, higher flow rate conditions (Re = 6000) result into Eo < 1 (close to zero) and We > 1 indicating the negligible impact the buoyancy has over capillary and inertial forces on the depletion of the microlayer after the growth-dominated phase (We gr < 1).…”

“…Moreover, one of the recent works has emphasised the need for an accurate description of the microlayer's shape and thickness profile for a reasonable estimate of bubble growth, even in the case of single vapour bubble nucleation in flow boiling (Vadlamudi et al. 2022). As in practical boiling applications, multiple bubbles form on the heater substrate and, due to the possible interaction of these bubbles (depending on their relative spacings), microlayer characteristics will not be the same as in single nucleation cases.…”

Ignored/Intriguing Processes Involved During Bubble Lift-Off Stage in Flow Boiling

“…For instance, prediction of bubble dynamics parameters-bubble departure diameter and frequency, which are essential for component scale modeling involving heat flux partitioning, is premised upon force balance approach. However, recent experiments in both pool [1] and flow boiling [2] conditions revealed that force balance approach suffers from severe limitations and it is inapplicable to accurately predict either bubble departure or bubble lift-off. Moreover, microlayer (a thin trapped liquid layer underneath the bubble) dynamics -spread and thickness -are important in calculation of bubble growth rate.…”

“…In the last few years, such detailed studies are being carried out both in pool and flow boiling conditions [3]- [5]. In our previous works, we have reported bubble, microlayer and dry patch dynamics involved in flow boiling for single [2], [3], [6], [7] and multiple nucleations [8].…”

“…During the initial stage of bubble growth, microlayer expands symmetrically in all directions even in flow boiling. However, after the reduction in growth rate, the bubble base starts to recede, as does the microlayer [2], [3], [6]. Usually, the microlayer depletes from upstream direction due to the combined effect of flow inertia and capillary force and from the centre due to the expansion of dry patch, which is driven by evaporation on hydrophilic surface.…”