High spatiotemporal resolution measurement of water flow boiling heat transfer in a horizontal square minichannel using infrared thermography

In this study, the heat transfer fluctuations of water flow boiling in a horizontal square minichannel with a side length of 2 mm was investigated using infrared thermography with a high spatiotemporal resolution (4000 fps, 0.025 mm/pixel). Simultaneously, two high-speed cameras were used to visualize the behavior of the gas-liquid interface. The mass flux was 100 or 200 kg/(m²·s), and the vapor quality ranged from slug to annular flow. The wall heat flux was varied in the range of 10–220 kW/m², focusing on the case of 220 kW/m², where fast and complex fluctuations in the flow-boiling heat transfer were clearly visualized. In addition, image analysis was performed to partition the instantaneous heat transfer coefficient distribution into the fundamental processes of flow boiling (liquid convection, microlayer evaporation, dryout, three-phase contact line, and rewetting), and the contribution of each process to the heat transfer was investigated. The results showed that liquid convection was dominant under the present experimental conditions, accounting for approximately 85–95 % of the total heat transfer. Here, liquid convection includes the effects of turbulence in the liquid phase caused by boiling nucleation and acceleration associated with the two-phase gas-liquid flow. The contribution of microlayer evaporation due to boiling nucleation was approximately 5–8 % of the total heat transfer when the heat flux was 220 kW/m², which decreased significantly as the heat flux decreased. It was also found that dryout occurred even under low vapor quality, and that when the dryout was partial, the decrease in heat transfer was limited by the contribution of the three-phase contact line formed at the outer edge of the dryout.