An experimental investigation on bubbles departure characteristics during sub-cooled flow boiling in a vertical U-shaped channel utilizing high-speed photography

Classified as a passive heat transfer enhancement technique, U-shaped channels are commonly encountered with applications associated with boiling heat transfer such as air conditioning systems, evaporators, and boilers. On the other hand, understanding heat transfer in flow boiling is heavily dependent upon bubbles dynamic behavior, which is influenced by flow and geometrical conditions. In this paper, bubbles departure characteristics are experimentally studied in upward sub-cooled flow boiling of distilled water in a vertical U-shaped channel. The influence of flow conditions, embracing heat flux, mass flux, and inlet sub-cooling on bubbles characteristics, encompassing departure diameter, growth and waiting times, and nucleation frequency, is investigated through conducting 68 experiments. All the experiments are carried out at atmospheric pressure over a Nichrome heating surface installed on the outer wall of the channel, with heat flux, mass flux, and inlet flow temperature in the ranges of 26.1-61.5 kW.m(-2), 114-255kg.m(-2)s(-1), and 1 to 8 degrees C. A high-speed camera is utilized to capture bubbles growth process and departure instance. The results reveal that as wall heat flux increases and mass flux and inlet sub-cooling decrease, bubbles departure diameter and frequency rise, while waiting time declines. Also, lower growth times are detected when wall heat flux and mass flux increase and inlet sub-cooling declines. Two new correlations are developed to predict present and previously published experimental data of departure diameter and nucleation frequency with mean standard deviations of 18.0% and 18.5%, correspondingly.