Effects of Gravity Orientation on Boiling Hysteresis and Intermittent Vapor Blockage During R245fa Boiling in an Embedded Distributed Jet Array

Variable-orientation thermal management is increasingly required in embodied-intelligence robots and aerospace electronics, where changes in gravity orientation can alter vapor–liquid distribution and compromise two-phase cooling stability. Although the distributed jet array shortens vapor-discharge paths and enhances high-heat-flux cooling, the effects of gravity orientation on boiling hysteresis, vapor blockage, and critical heat-transfer performance remain unclear. This study constructed a rotatable R245fa mechanically pumped two-phase loop integrating an embedded distributed jet array boiling module and a miniature test chip with multipoint in situ thermometry. Complete boiling curves were obtained for both the increasing and decreasing heat flux branches at different gravity inclination angles (0.2°–180.0°) and jet Reynolds numbers (4728–7880), using synchronized thermal–hydraulic measurements and high-speed visualization. Boiling hysteresis was governed mainly by the orientation-sensitive onset of nucleate boiling, whereas the boiling extinction point was comparatively insensitive to orientation, producing a non-monotonic response. By contrast, critical heat flux decreased monotonically with increasing inclination angle. As the angle increased from 0.2° to 180.0°, critical heat flux decreased by 48.0%, 39.1%, and 25.5% at Reynolds numbers of 4728, 6304, and 7880, respectively, suggesting that higher jet inertia may reduce orientation sensitivity. Orientations unfavorable for vapor removal also narrowed the stable nucleate boiling window and promoted gravity-biased vapor accumulation, intermittent vapor blockage, and pressure fluctuations. Correlations were developed for the two-phase heat transfer coefficient and critical heat flux, with a gravity-orientation factor incorporated to account for orientation-dependent vapor removal and liquid rewetting. The corresponding mean absolute percentage errors were 2.89% and 3.76%, respectively. These results reveal stage-dependent orientation effects and provide experimental data and empirical tools for evaluating the operating limits and orientation tolerance of R245fa mechanically pumped two-phase cooling devices.