Flow Boiling in Microchannels and Microgravity

A critical review of the state of the art of research on internal forced convection boiling in microchannels and in microgravity conditions is the main object of the present paper. In many industrial applications, two-phase flows are used for heavy-duty and reliable cooling and heating processes. The boiling phenomena are essential for evaporator heat exchangers, even in a very small scales, such as for PC cooling, refrigerators, HVAC systems. Even if the study of boiling is a standard research since a century, there are many aspects which are still under discussion, especially for forced convection boiling in small tubes. As the present review is pointing out, some literature results are still incongruous, giving critical uncertainties to the design engineers. The use of non-dimensional parameters is rather useful, but, especially in case of boiling, may provide an erroneous picture of the phenomena in quantitative and qualitative meaning. 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 Eo = g(ρL−ρV)L2/σ and its analogs, which are at the same time linked to the tube diameter and the gravity forces. In fact the Eötvös number 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. Since the global picture of such physical process in flow boiling remains unclear, we claim the necessity to define in the most complete way the status-of-the-art of such an important research field and critically investigate the successes and the weaknesses of the current scientific literature. Noteworthy, the distinction between a macroscale and a microscale regime is misleading, since it could bring to consider a drastical variation of the physical phenomena, which is in fact not occurring until extremely low values of the channel dimension. Instead there is a typical flow pattern, the confined bubble flow, which is the dominant flow mechanism in small channels and in microgravity. Furthermore the vapor quality is a very important parameter, whose role is not well described in the present pattern classification. The values and combinations of the dimensionless numbers at which such pattern appears is the main issue of the present researches. Noteworthy, the meaning of “micro” is here used, as in the present literature, in a broad meaning, not strictly linked to the actual size of the channel, but to a change of patterns (and other physical characteristics) linked to a given dimensionless scale.