Flow boiling in parallel microchannels in a pumped two-phase loop: Flow visualization and thermal characteristics

Pumped two-phase loop (P2PL) capable of handling high heat fluxes at low thermal resistance while consuming minimal pumping power offers a promising cooling solution for emerging high-power electronics. However, existing research primarily focuses on standalone evaporators with fixed boundary conditions, neglecting the interactions between the evaporator and other components within the P2PL. To address this gap, in this study, high-speed visualization with simultaneous temperature and pressure measurements is used to investigate the flow boiling regimes and their impact on the thermal characteristics of R134a in parallel microchannels. The ranges of parameters are: mass flux from 135.7 to 339.3 kg/m2-s, heat flux from 0 to 33.7 W/cm2, a fixed inlet temperature of 10 degrees C, inlet subcooling from 0.6 to 3.5 degrees C, and vapor quality from subcooled to dryout. Flow separation at the microchannel inlet induced both pressure-driven flashing of the subcooled liquid and thermaldriven nucleation on the heated walls, leading to rapid bubble formation. At low heat fluxes, bubbly flow dominates the inlet, swiftly transitioning to slug flow. Increased heat fluxes induce a unique jet flow regime featuring a wavy vapor jet. The local heat transfer coefficient was used to track the influence of flow regime transitions on microchannel thermal characteristics.