Experimental study on condensation regimes and interfacial dynamics of steam direct contact condensation in biomimetic parallel microchannels

To reveal the condensation regime transition, vapor–liquid interface oscillation, and cross-branch bubble coalescence mechanisms during steam direct contact condensation (DCC) in parallel microchannels for high-heat-flux thermal management devices, a biomimetic parallel microchannel test section was designed and experimentally investigated using high-speed visualization. The experiments were conducted under steam mass flow rates of 0.3–1.0 g/min, subcooled water mass flow rates of 5–60 g/min, and subcooled water temperatures of 30–80 °C. The results show that three typical condensation regimes, different from those previously observed in macroscale and microscale single-channel, were discovered, namely coalescence–chugging regime, compound condensation regime, and internal chugging regime. In addition, asynchronous steam plume injection from multiple branches was observed, which induced cross-branch coalescence of small vapor bubbles. The bubble coalescence time exhibited a non-monotonic trend with increasing steam mass flow rate, first increasing and then decreasing, and reached a peak value of 127.2 ms at 0.5 g/min. The temporal evolution of the vapor–liquid interface height was quantified. It was found that the interface height first increased and then decreased with increasing steam mass flow rate, with a fluctuation range of 0.88–21.15 mm. Based on approximate three-dimensional reconstruction, the bubble surface area during the compound condensation regime was found to exhibit a three-stage evolution pattern of “oscillatory slow growth–rapid expansion–sharp shrinkage”. Specifically, the bubble surface area first increased from 8.35 mm 2 to 13.54 mm 2 , then rapidly increased to 32.94 mm 2 , and finally decreased to 11.84 mm 2 .