Nucleate boiling: Microlayer formation, depletion, and contribution to bubble growth

Abstract

The growth of a vapour bubble at a heated surface involves various fluid mechanics, heat transfer and phase change phenomena. An understanding of the fundamentals of these phenomena on the scale of single bubbles is interesting in its own right from a scientific point of view, and is helpful for the further improvement of macroscopic boiling models. Vapour generation during nucleate boiling at atmospheric pressure conditions is known to occur not only from the curved surface of the bubble, but also from a very thin film of liquid forming between the heated wall and the underside of the bubble, the so-called ‘microlayer’. Microlayers are widely observed in experiments, but theoretical understanding of their formation, behaviour and significance to bubble growth is limited. This thesis presents various studies of the formation, depletion, and contribution to bubble growth of these microlayers. Mechanistic hydrodynamic-only CFD simulations of the formation of such microlayers during the early bubble growth stages, tracking the vapour-liquid interface, are performed. These computational models demonstrate that the balance between inertial and surface tension forces, and the resulting bubble shape, determine the presence and overall extent of microlayers underneath steam bubbles. Their thickness is strongly influenced by viscous effects in the near-wall region. Having identified the underlying physical mechanism behind the formation of microlayers, the thesis then presents CFD simulations of their formation and simultaneous, physically self- consistent evaporative extinction, including the conjugate heat transfer into the solid substrate. Identification of the classes of conditions under which microlayers are likely to be formed is presented, along with an assessment of their relative contribution of vapour to the overall bubble growth.