Experimental study of flow boiling performance of open-ring pin fin microchannels

A type of open-ring pin fin microchannels (ORPFM) was proposed and developed for advanced microchannel heat sinks. They consist of an internal cavity for bubble nucleation and two inner small and outside large rings with separated and converged flow passages. Two arrangements of open-ring pin fins, inline and staggered ones, were prepared and compared for optimization. Flow boiling experiments were performed to assess the two-phase boiling performance of both inline and staggered ORPFM. Tests were conducted using subcooled deionized water with an inlet subcooling of 10°C and mass fluxes of 200 kg/m²·s and 300 kg/m²·s. Two-phase boiling heat transfer performance, pressure drop and two-phase flow instabilities of the ORPFM were systematically explored. Experimental results indicated that both inline and staggered ORPFM samples presented a small wall superheat of 1-2°C for onset of nucleate boiling (ONB). Nucleate boiling, both nucleate boiling and thin film evaporation, and convective boiling in turn dominated with increasing heat flux and vapor quality for ORPFM. The inline ORPFM generally presented a 10% to 80% augmentation in boiling heat transfer at moderate and high heat fluxes, a 2%-20% smaller pressure drop, and more stable flow boiling process with a mitigation of two-phase flow instabilities than the staggered one. Therefore, the inline ORPFM seemed to be more favorable for high heat flux dissipation of advanced microchannel heat sinks.