Synergistic effects of engineered microstructures, manifold size and spatial orientation on flow boiling mechanics in minichannels

Flow boiling in minichannels is a highly effective thermal management approach. Open minichannels, characterized by an extra manifold above the flow channels, exhibit reduced pressure drop and two-phase flow instability. However, manifold designs are often selected without systematic evaluation, and their impact on flow boiling performance under different spatial orientations remain unclear. This study investigates the synergistic influences of surface morphology, manifold size, and spatial orientation on flow boiling in open minichannels. Experiments were conducted at the refrigerant mass flow rates ($\dot{m}$) of 0.005 kg/s and 0.009 kg/s (corresponding to mass fluxes G of 27 to 187 kg/m²·s), and effective heat fluxes (q_eff) of 2.9 kW/m² to 170 kW/m², by supplying 7 °C subcooled liquid to the minichannels inlet in three spatial orientations (horizontal, vertical upward, and vertical downward flow). Compared to conventional closed plain minichannels, our results show that increasing manifold size and the integration of surface microstructures not only significantly improves thermohydraulic performance, with enhancement factor (Φ) up to 25 in horizontal flow, but it also delays the occurrence of dryout in vertical upward flow to an outlet vapor quality (x_outlet) of 0.98. These enhancements are attributed to the suppression of flow instabilities even at a lower mass flux (G = 27 kg/m²·s) enabled by larger manifolds combining with increased nucleation site density on the cooling surface. In vertical downward flow, however, increasing manifold sizes exacerbates flow maldistribution and increases the possibility of liquid film rupture, resulting in about 50 % reduction in critical heat flux (CHF) as compared to the closed minichannels, where severe dryout was observed at q_eff = 149 kW/m² and x_outlet = 0.86. In contrast, closed minichannels exhibits relatively consistent boiling characteristics across different spatial orientations, due to the dominant bubble explosive growth effect. In all, this work not only successfully identifies the synergistic effect of surface morphology, manifold size, and spatial orientation on flow boiling characteristics of minichannels, but it also provides comprehensive guidelines for optimizing minichannel thermohydraulic performance in different orientations.