| Flow boiling in microchannels is one of the most potent convective heat transfer methods and has been a topic of numerous studies in the last several years. Better controllability and reproducibility of the boiling process in micro systems are essential for improved system performance. Furthermore, developing means to reduce boiling incipient superheat and to enhance heat transfer capacity in microchannels will enable the successful realization of numerous novel micro machines. This report outlines a detailed study to extend current knowledge for flow boiling in microchannels and to examine the effect of structured reentrant cavities for flow boiling. Microchannel devices were designed and fabricated, experimental setup was developed, and experiments were conducted for flow boiling in micro devices.;Local surface temperature was measured to obtain the local heat transfer coefficient for flow boiling. Flow visualization was conducted to observe flow morphologies. Flow patterns similar to conventional scale was discovered. The observed flow patterns were mapped and compared with flow maps from conventional scale channels. The local heat transfer coefficient was used as well as flow morphologies to determine boiling heat transfer mechanism governing the flow.;Boiling inception, onset of flow instability, and critical heat flux were also obtained to study the effect of structured reentrant cavities to enhance the boiling initiation and to suppress boiling instability or premature critical heat flux condition. Parametric studies were performed and comparison with existing macro- or microchannel correlations was made. It was found that reentrant cavities can reduced the superheat of boiling inception, increase the heat transfer coefficient during nucleate boiling, mitigate boiling instability, and increase critical heat flux value. |
