Experiments on the effects of varying subcooled conditions on the dynamics of single vapor bubble and heat transfer rates in nucleate pool boiling regime

Abstract

Dependence of bubble dynamic parameters and the associated heat transfer mechanisms on varying levels of sub-cooled conditions has been experimentally studied. Isolated nucleate pool boiling experiments have been performed to generate a single vapor bubble on an ITO (Indium Tin Oxide) glass substrate for three degrees of sub-cooling (ΔTsub-cooled = 2 °C, 5 °C and 0 °C). Field measurements have been made using gradient-based rainbow schlieren deflectometry. Various sub-phenomena associated with a bubble cycle, for instance, bubble inception and its growth, bubble oscillation and its departure from the heated substrate, have been visualized in real time for the range of bulk conditions employed. The recorded rainbow schlieren images have been analyzed to determine the transient variations of bubble dynamic parameters such as equivalent bubble diameter, bubble base diameter and aspect ratio. Effects of sub-cooled level on bubble oscillations have been estimated based on the dynamic variations in the hue values in the vicinity of the oscillating bubble. Spectral analysis of the hue variations showed an increasing trend in the oscillation frequency with sub-cooling conditions (foscillation = 52.8 Hz and 47.2 Hz for ΔTsubcool = 5 °C and 2 °C respectively). Based on the whole-field temperature distribution retrieved through the quantitative analysis of the schlieren images, individual contributions of the various heat transfer mechanisms (natural convection, bubble growth and superheat layer) towards the overall heat transfer rates have been determined. Results revealed that the contribution of natural convection heat transfer increases from 54% under saturation conditions to up to 99% for ΔTsub-cooling = 5 °C.