A study of local heat transfer mechanisms along the entire boiling curve by means of microsensors

An array of 36 microthermocouples (38 μm diameter) embedded in a horizontal copper heater (distance to the surface 3.6 μm), a micro optical probe (tip diameter ∼1.5 μm) and a microthermocouple probe (tip diameter ∼16 μm), both moveable above the heater surface, are applied to study heat transfer mechanisms along the entire boiling curve under steady-state conditions. Test fluids are isopropanol and FC-3284. In nucleate boiling, very localized and rapid temperature drops are observed indicating high heat fluxes at the bottom of the bubbles. Already before reaching CHF, hot spots occur the size of which increases towards the Leidenfrost point. In the entire transition boiling regime wetting events are observed, but no ones in film boiling. In low heat flux nucleate boiling small vapor superheats exist in the bubbles and strong superheats in the surrounding liquid. This characteristic changes continuously with increasing wall superheat: the liquid surrounding the vapor approaches saturation whereas the vapor becomes more and more superheated. In film boiling the bubbles leaving the vapor film can reach superheats of 40 K near the surface. The optical probes confirm a liquid rich layer near the surface between nucleate boiling and high heat flux transition boiling. The void fraction in this layer increases continuously with the distance to the surface until a maximum value which seems to be linked to the bubble departure diameter.