Experiments on pool boiling regimes and bubble departure characteristics of single vapor bubble under subcooled bulk conditions

摘要

An experimental study for understanding the effects of subcooling of bulk fluid on the phenomenon of nucleate boiling heat transfer was carried out. The whole field experimental data was recorded in a purely non-intrusive manner using rainbow schlieren deflectometry as the optical diagnostic tool. Boiling experiments were conducted over a range of heat flux conditions (1 <= q' <= 40 kW/m(2)) and for varying levels of subcooling (0 K <= Delta(subcooling) <= 10 K). The recorded images were first been interpreted to understand the effect of subcooling on various regimes of boiling phenomenon, which include convection, isolated nucleate boiling and vertical coalescence of the departing bubbles. Observations on convection regime showed an increasing influence of natural convection heat transfer. Based on the formation of vapor bubble and its departure characteristics, the isolated nucleate boiling regime of the classical boiling curve was further segmented into three sub-regimes namely, stable vapor bubbles, oscillating vapor bubbles and oscillating-departing vapor bubbles. A mechanism explaining the oscillations of vapor bubbles under subcooled condition was proposed that takes into account the relative dominance of condensing apex region and evaporating microlayer region. Various bubble dynamic parameters such as equivalent bubble diameter, departure frequency, aspect ratio and oscillation frequency were determined quantitatively to understand the effect of subcooling on the bubble departure characteristics. Based on the experimental results, an empirical correlation for predicting the bubble departure diameter and evaporative heat flux was developed and proposed.