On a visible-transparent boiling surface, the detailed geometry of a microlayer can be detected using a total reflection technique combined with laser interferometry. On an infrared-opaque boiling surface, the surface temperature and heat flux distribution can be obtained using a highspeed infrared thermometry technique. In the present study, an experimental technique to study heat transfer in the microlayer is described which permits the simultaneous use of the total reflection combined with laser interferometry and the infrared thermometry. Boiling takes place from an infrared-opaque and visible-transparent indium-tin-oxide thin-film heater deposited on an infrared-and visible-transparent calcium fluoride substrate. Details of microlayer geometry and the associated surface temperature and heat flux distribution are obtained using an integrated experimental technique. Heat transfer mechanisms in the microlayer are quantitatively discussed.
Simulations of the formation of small steam bubbles indicate that the rate of growth of bubbles is very sensitive to the rate of evaporation of the micro-layer of liquid beneath the bubble. Such evaporation is rapid, and is modelled as being driven by the large heat flux through the thin liquid layer caused by the difference in temperature between the solid-liquid interface, and the saturation temperature in the interior of the bubble. However, application of this approach to recent experimental measurements of Jung and Kim generated anomalous results. In this paper we demonstrate that a model of the micro-layer heat flux that includes an allowance for the finite evaporative thermal resistance is able to eliminate these anomalies. This evaporative thermal resistance is a consequence of near-interface molecular dynamics, characterised by a quantity termed 'evaporation coefficient'. Whilst in most engineering applications evaporative thermal resistance is small compared to conductive resistance, here, with the micro-layer thickness ranging from a few microns down to zero, it becomes of considerable importance. Selection of a molecular 'evaporation coefficient' to restore consistency to the anomalous measurements allows a plausible numerical value to be inferred. For the several times and multiple locations studied, a fairly consistent value of between 0.02 and 0.1 is indicated, (for saturated water in laboratory conditions), which itself is consistent with earlier literature values of this rather difficult quantity. It is shown that the evaporative resistance always represents a large fraction of the conductive resistance, and for important phases of the process dominates it. The need for inclusion of this phenomenon in the microlayer models used in bubble analysis is clear.
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