Flow boiling of refrigerants inside a glass minichannel

This thesis consists of two parts; the first presents a critical review of the state of the art of research on internal forced convection boiling in microchannels and in microgravity while the second focuses on the experimental investigation of the onset of nucleate boiling. The critical review wants to answer to the necessity to define in the most complete way the status-of-the-art of this important research field and to critically investigate the successes and the weaknesses of the current scientific literature. The idea to consider the channel microsize together with the microgravity effects in a single review is due to the fact that the transition between confined and unconfined bubble flows may be defined using dimensionless numbers, such as the Eötvös number. This is linked to the tube diameter and the gravity forces and it tends to zero either when the gravity tends to zero or when the tube diameter tends to zero, but physical phenomena appear different considering separately either only the tube size or only the microgravity condition. It is pointed out that some literature results are still incongruous and that the global picture of boiling process remains still unclear. Part II contains the experimental investigation of boiling and in particular of onset of nucleate boiling in a glass minichannel using two different refrigerants R-134a and R-245fa. During the experimental activity some new interesting trends appeared, linked to the nucleation that always starts from the upper side of the heater and only later from the lower side of the heater. The onset of nucleate boiling was investigated thanks to the transparent ITO heater that allow the visualization of the boiling process. The heat transfer coefficients are evaluated as function of the vapour quality, heat flux and wall superheat and they increase increasing the mass flux values.