Non-intrusive diagnostics of single bubble nucleate pool boiling heat transfer phenomena: Boiling curve under subcooled conditions*

Experimental investigation of single vapor bubble formation during nucleate pool boiling of water under sub-cooled conditions has been reported. Measurements have been performed for different values of sub-cooling levels (∆T_sub = 0 to 10^oC) in a non-intrusive manner using rainbow schlieren deflectometry technique. The recorded schlieren images are qualitatively as well as quantitatively analyzed to understand the relative influence of various phenomena such as natural convection, bubble growth, evaporative and superheat layers on the overall heat transfer rates during a single bubble cycle. Based on the spatial extent of thermal boundary layer near the heated substrate and the two-dimensional temperature distributions, individual contributions of various heat transfer sub-processes have been estimated. Of all the sub-processes, the contribution of natural convection towards the overall heat transfer rates was found to be relatively dominant at any given subcooled level. Based on the non-intrusive measurements, the classical boiling curve has been extended to subcooled conditions to further classify the nucleate boiling regime into three meta-stable regimes (oscillating, departing and stable vapor bubbles).