Thermal evaluation of flow boiling in a non-uniformly distributed pin-fin heat sink under transiently varying inlet temperatures

In real-world cooling applications, systems experience unsteady startup periods where evolving inlet temperatures significantly impact thermal management. Addressing this gap, this study is the first to experimentally investigate saturated flow boiling within a novel double-pool micro pin-finned heat sink (DP-HS) under transiently varying inlet temperatures. Time-dependent data and high-speed visualization (2000 fps) were recorded for inlet temperatures rising approximately from 37.5 °C to 77 °C, tested across heating powers of 180–270 W and mass fluxes of 189–265 kg m −2 s −1 . Results reveal that lower inlet temperatures enhance two-phase heat transfer coefficient, which drops by a maximum of 57.2% (at 180 W, 265 kg m −2 s −1 ) upon reaching steady-state. Under present conditions, a marked transition in the influence of mass flux was noted when mass flux increased from 189 to 227 kg m −2 s −1 , after which further improvements diminished. Moreover, forward-acting inertial forces and unbalanced surface tension force (unique to DP-HS) overcome resistive evaporation momentum, ensuring continuous surface rewetting. Furthermore, the DP-HS geometry efficiently manages high thermal loads, limiting the average wall superheat increase to 2.3 °C when heating power rises from 180 W to 270 W. Optimizing inlet temperature and exceeding this mass flux threshold are essential for maximizing flow boiling performance.