Mechanisms of Boiling in Micro-Channels: Critical Assessment
Numerous characteristic trends and effects have been observed in published studies on two-phase micro-channel boiling heat transfer. While macro-scale flow boiling heat transfer may be decomposed into nucleate and convective boiling contributions, at the micro-scale the extent of these two important mechanisms remains unclear. Although many experimental studies have proposed nucleate boiling as the dominant micro-scale mechanism, based on the strong dependence of the heat transfer coefficient on the heat flux similar to nucleate pool boiling, they fall short when it comes to actual physical proof. A strong presence of nucleate boiling is reasonably associated to a flow of bubbles with sizes ranging from the microscopic scale to the magnitude of the channel diameter. The bubbly flow pattern, which adapts well to this description, is observed, however, only over an extremely limited range of low vapor qualities (typically for quality less than 0.01-0.05). Furthermore, at intermediate and high vapor qualities, when the flow assumes the annular configuration and a convective behavior is expected to dominate the heat transfer process, the experimental evidence yields entirely counterintuitive results, with heat transfer coefficients often decreasing with increasing vapor quality rather than increasing as in macro-scale channels, and with a much diminished heat flux dependency compared with would be expected. In summary, convective boiling in micro-channels has been revealed to be much more complex than originally thought. The present review aims at describing and analyzing the boiling mechanisms that have been proposed for two-phase micro-channel flows and confronting them with the available experimental heat transfer results, while highlighting those questions that, to date, remain unanswered.