Flow boiling inside 2 mm and 4 mm channels: micro and macro scale correlation comparison

The aim is to compare and understand the bubble dynamics and heat transfer in flow boiling, at both, microgravity and normal gravity conditions. Actually, there are few studies on this topic, nevertheless the possible applications are many, from the spatial usage to microelectronics. In this phase of the research, a preliminary analysis of the available correlations in the literature has been performed and a comprehensive comparison with experimental data provided by ENEA has been carried out. A set of seventeen empirical correlations have been selected from the literature, five for macro-scale and twelve for microscale channels, to grant the wider possible completeness. The analysis has been performed by comparing the heat transfer coefficients obtained from the correlation against the experimental data obtained in the ENEA facility MICROBO with refrigerant FC-72 as working fluid and two tube diameters, 2 and 4 mm, close to the transition between the micro- and macro-scale. Heat transfer coefficient at normal gravity condition has been derived from the experimental data and compared with the predicted by the selected literature correlations. As most of data are in subcooled boiling, its effect on correlations’ prediction is discussed. Results evidence how close the results are to the micro to macro scale transition. However, it is important to highlight that the correlations have been developed neither for the refrigerant FC-72 nor for sub-cooled flow boiling. Further studies will be performed to improve the knowledge on the sub-cooling phenomena.