Microlayer formation and depletion beneath growing steam bubbles

Abstract

Microlayers, the few-microns-thick layers of liquid that sometimes remain beneath bubbles growing on a heated substrate, are widely observed in experiments, but theoretical understanding of their formation, behaviour and role in bubble growth is limited. In this paper we present detailed interface-tracking simulations of the formation and depletion of such microlayers. Validation of our results is presented to the degree that available measurements of such a rapid and microscopic phenomenon allow. The work extends previous mechanistic hydrodynamic-only CFD simulations of early microlayer formation up to typical bubble departure times. These calculations confirm the understanding that the bubble growth rate and the resulting bubble shape determine the presence and overall extent of microlayers underneath steam bubbles. Their thickness is strongly influenced by viscous effects and surface tension. We then present coupled, physically self-consistent CFD simulations of the formation and evaporative depletion of such microlayers. This modelling suggests strongly that the evaporation process itself constitutes a significant fraction of the small resistance to heat and mass transfer presented by the very thin liquid layer. Inclusion of representations of evaporative thermal resistance, consistent with those suggested in the literature, is seen to promote the prediction of microlayer formation. Identification of the classes of conditions under which microlayers seem likely to be formed is presented, along with an assessment of their relative contributions to bubble growth. Comparisons of the predictions with recent detailed microlayer measurements indicate good agreement.

Modelling of Boiling Flows for Nuclear Thermal Hydraulics Applications—A Brief Review

“…Another important area of active research is focussed on the evaporation process as it happens near the solid surface. Two evaporation mechanisms have been so far identified, one taking place at the solid-vapour-liquid contact line [65] and involving the effect of molecular interactions between the solid substrate and the evaporating vapour-liquid interface, and one in which a liquid microlayer [66][67][68] of a few microns in thickness is observed to form and subsequently deplete due to evaporation on the heated surface. These are still poorly understood phenomena currently being investigated from a fundamental perspective.…”

“…Extension to the more challenging flow boiling conditions is at present outside the remit of current modelling techniques. Nevertheless, recently reported applications [67,69] enabled gaining unprecedented insight on the hydrodynamics and heat transfer of bubble behaviour in boiling.…”

“…A typical example is application to modelling the process of formation and depletion of a liquid microlayer beneath a growing bubble, as shown in Figure 10. Hänsch and Walker [67,73] developed a hydrodynamic, level set-based model of bubble behaviour and coupled it to a simplified thermal model of the microlayer beneath the bubble. The coupled simulation was used to track the depletion of the film and to model transient heat conduction in the solid and its thermal response to the latent heat transfer associated with film depletion, which indicated that an as yet unknown thermal resistance phenomenon associated with the evaporation process [66] is the main factor limiting the rate of microlayer depletion.…”

“…Microlayer formation and depletion beneath a bubble growing at a heated wall, a typical example of the importance of variables associated with the solid surface. From Hänsch et al[67,73].…”

Direct numerical simulation of nucleate boiling in zero gravity conditions

“…published works [45,46,19] where it is demonstrated that the microlayer formation and depletion are dynamic phenomena affecting the first part of the bubble growth only. Therefore, the existence of a micro-layer in the present configuration could maybe affect the tran-sitory behaviour of the bubble, but the micro-layer will be anyway depleted before a stationary equilibrium is reached.…”