Subcooled flow boiling in microchannels: Heat transfer enhancement via topology optimization and transient characteristics of microbubble emission

In recent years, subcooled flow boiling in microchannels has attracted extensive research attention due to its applications in high heat flux cooling scenarios. In this study, subcooled flow boiling in microchannels with a baseline configuration (BC) and two topologically optimized configurations (TOC-I and TOC-II) is investigated through flow visualization experiments. The effects of mass flux (330.1, 660.2, and 1320.3 kg/m²·s), inlet subcooling (40, 60, and 80 K), and heater heat flux (0-700 W/cm²) on two-phase flow patterns and thermo-hydraulic characteristics are examined. Particular focus is given to the bubble dynamics and transient responses of wall temperature and pressure drop associated with the microbubble emission boiling (MEB) phenomenon. The results show that all three configurations exhibit Type-I Bubbly Flow (TIBF), Slug Flow (SF), and Droplet Flow (DF). In contrast, Type-II Bubbly Flow (TIIBF) and Annular Flow (AF) are unique to BC, while Evaporated Flow (EF) appears exclusively in TOC-I and TOC-II. TOC-II exhibits the lowest wall superheat under identical heat flux conditions, owing to its large heat transfer area and the micro pin fins with topologically optimized configuration, which enhances fluid mixing and disrupts the formation of the thermal boundary layer. Compared to BC, the maximum reduction in wall superheat reaches 29.5 K. Under most operating conditions, TOC-I demonstrates a higher heat transfer coefficient than both BC and TOC-II, reaching a peak value of 80.4 kW/m²·K under the SF regime, which is 40.2 % higher than the peak value of 57.3 kW/m²·K observed in BC. The time-averaged pressure drop in TOC-I is comparable to that in BC and slightly lower than that in TOC-II, under most operating conditions. Notably, MEB with two distinct evolution modes is observed in BC, with a complete evolution cycle lasting approximately 30-70 ms. Furthermore, during intense MEB activity, a transient and anomalous drop in wall superheat is observed as heat flux increases, accompanied by temperature and pressure drop oscillations with amplitudes of up to 8 K and 6.4 kPa, respectively.