Study on the heat transfer mechanism in nucleate boiling of water by measuring local heat flux and temperature

Abstract

In this experimental work, the local heat flux and temperature on the boiling surface are measured to determine the heat transfer mechanism taking place during nucleate boiling. Several representative types of heat transfer mechanisms have already been proposed by many researchers. These are evaporation on a microlayer, evaporation of a thin liquid meniscus at the three-phase contact line, convection in a liquid area and quenching heat transfer on an advancing area. An important aspect to note is that the heat flux during each of these mechanisms can differ drastically according to the dynamically different areas characterized by the temporal bubble morphology (e.g., the microlayer, contact line, liquid area, and advancing area). So if the heat flux is measured, the specific dynamic surface areas can be classified and this results in the identification of the specific surface heat transfer mechanisms involved. Using infrared thermometry, the temperature distribution of boiling surface can be measured when mid-wave optical access to the surface is possible. Then, the heat flux is calculated by the heat equation with this temperature information as the boundary conditions. The contact line length density(CLD) and the time-averaged wetted area fraction(WF) are also measured for an infrared opaque indium-tin-oxide surface over an electrically heated silicon substrate which is transparent to infrared. With this information, the dominant heat transfer mechanism during the nucleate boiling of water is evaluated. In conclusion, the convection heat transfer on the liquid area is the predominant heat transfer mechanisms during the nucleate boiling of water. Its contribution to overall heat transfer is about 80% in the low input heat flux region, but gradually decreased as the input heat flux increases. It appears that the increased quenching and evaporation on the contact line is the main cause of this trend.