Experimental investigation of gas-phase crossflow between slanted subchannels

Void drift between neighboring subchannels is a crucial phenomenon for two phase transport because it directly alters heat transfer characteristics, yet a comprehensive understanding spanning channel angulation and operating conditions is still lacking. This study presents experimental data on the gas phase migration across the gap in five channel orientations (horizontal, vertical, and 45 degrees tilted) under a wide range of mass fluxes and flow conditions. A total of 60 experiments (12 per orientation) were conducted (air mass flux G_g=0.16-2.20 kg/m^2 s, water mass flux G_l=120-1200 kg/m^2 s), covering both bubbly flow, and slug flow. Using high speed shadowgraphy, we captured the operating conditions under which bubbles cross the inter subchannel gap. We identify three governing dimensionless groups: the Reynolds number (Re), Bond number (Bo), and the bubble diameter ratio λ (Db/Dh; Db: bubble diameter and Dh: channel's hydraulic diameter). These parameters exhibit threshold behavior, beyond which bubbles traverse the gap and crossflow is triggered. To unify inertial, interfacial, and geometric (channel angulation) effects, we propose a new physics‑based bubble migration model. The model consolidates Re, Bo, and λ into a power‑law expression that can predict the onset boundary of crossflow across all tested conditions. These findings provide a foundation for a universal crossflow prediction framework for inclined and reoriented subchannels, which can be further refined through additional experiments and validations across broader operating conditions and geometries.