Observation of heat transfer mechanisms in saturated pool boiling of water by high-speed infrared thermometry

We investigated experimentally the heat transfer mechanisms in saturated pool boiling of water. In the experiment, the temperature of a sapphire heated wall with a titanium thin-film heater was visualized using a high-speed infrared camera with a spatial resolution of 82 μm/pixel and a framing rate of 3,000 fps. Local heat transfer characteristics of the fundamental heat transfer processes, including microlayer evaporation, dry-out, transient heat conduction immediately after rewetting, and convective heat transfer, were investigated based on the surface heat flux distribution obtained by three-dimensional transient heat conduction analysis of the heated wall. The contribution of microlayer evaporation, which shows a high heat flux far exceeding the applied heat flux, to the bubble growth was found to be about 50%, and the heat transfer within the microlayer was dominated by one-dimensional heat conduction in the thickness direction. It was confirmed that the local heat removal immediately after rewetting of the dry patch can be reproduced by the transient heat conduction model. The enhancement of convection by the isolated bubble motion was small, while the interaction between bubbles agitated the liquid strongly and enhanced the convective heat transfer. Via partitioning the heat flux distribution by image analysis, the convective heat transfer was found to be the dominant wall heat transfer mode, and the contribution of the microlayer with an area coverage ratio with respect to the total heat transfer area of less than 10% was small, around 25%.