Enhanced flow boiling in microchannels using auxiliary channels and multiple micronozzles (Ⅰ): Characterizations of flow boiling heat transfer

摘要

Flow boiling in parallel microchannels can be dramatically enhanced through inducing self-excited and self-sustained high frequency two-phase oscillations as demonstrated in our previous studies using a two-nozzle microchannel configuration. Two-phase mixing induced by the rapid bubble collapse is shown to be the major enhancement mechanism. However, in the two-nozzle configuration microchannels, the mixing effect is limited to the downstream of the microchannels, meaning that only half length of the entire microchannel is functionalized as designated. In this study, a four-nozzle microchannel configuration is developed with an aim at extending the highly desirable mixing effect to the entire channel. Flow boiling in the four-nozzle configuration microchannels is experimentally studied with deionized water and the mass flux ranging from 120 kg/m~2 s to 600 kg/m~2 s. The onset of nucleate boiling temperature is considerably reduced by ～14% because of more nucleation sites created by the multiple nozzles. Equally important, the improved microchannel configuration successfully extends the mixing to the entire channel as validated by the enhanced heat transfer rate and visualization study. Compared to the previous two-nozzle configuration, the overall heat transfer coefficient (HTC) is significantly improved primarily owing to the enhanced nucleate boiling. For example, the peak overall HTC of 262 kW/m~2 K is achieved at a mass flux of 150 kg/m~2 s, accounting for ～83.7% enhancement. Additionally, the peak effective HTC reaches 97.6 kW/m~2 K, accounting for ～67% enhancement.