Heat transfer and pressure drops in micro-tubes for ground and space applications

The aim of the research is to compare and understand the bubble dynamics, heat transfer mechanism and pressure drops in subcooled flow boiling, at both, microgravity and normal gravity conditions. Nowadays, there are few studies on this topic, nevertheless the possible applications are many, from the spatial usage to microelectronics cooling. Micro-exchangers are the next generation of cooling systems useful for both terrestrial and space applications where weight and dimensions are important. Flow boiling is the best way to reach high heat fluxes. Micro-tubes, thanks to their dimension, can be used in a wide range of microgravity systems i.e. satellites for communications, thermal management for the International Space Station, cooling of hi-power electronic devices, nuclear space reactors, etc. To develop and design thermal systems for small applications, it is necessary to achieve a detailed understanding of all flow boiling aspects, also under low gravity conditions, at low pressure and at high thermal fluxes. A deepen analysis of available correlations in the literature has been performed and a comprehensive comparison with experimental data provided by two ENEA facilities (MICROBO and BOEMIA) has been carried out. The analysis considers 17 micro-channel correlations, 5 macro-channels and 2 models to calculate heat transfer. Instead, 5 correlations were chosen to calculate saturated boiling pressure drops and 3 for subcooled boiling. As most of data are in subcooled boiling, its effect on prediction is discussed and a model is proposed to calculate vapor quality in the channel. Moreover, a new methodology to calculate pressure drops has been developed and discussed.