Experimental pool boiling study on novel multistage cross-flow porous structure using FC-72 for high-heat-flux electronic applications

An experimental pool boiling study on multistage cross-flow porous structure was conducted under atmospheric pressure using saturated FC-72 as the working fluid. The novel design comprises two stages: each stage contains cross-flow rectangular channels of 1.5 mm in height and 1 mm wide connected by thorough circular pores 0.325 mm in diameter. The dimensions of the pores and channels in both stages were kept identical. Tests were conducted in three positions, enhanced surface horizontal position (EH), enhanced surface vertical position (EV-a), where upper stage channels were parallel to the buoyancy force, and enhanced surface vertical position (EV-b), where lower stage channels were parallel to the buoyancy force. The critical heat flux was significantly enhanced with an improved boiling heat transfer rate in all three positions using the multistage cross-flow porous structure. The highest heat flux of 107 W/cm2 was recorded in EV-b, followed by EV-a and EH with a wall superheat of 42 °C. The heat transfer coefficient was improved by 300% to 700%, and the critical heat flux was enhanced by 6.5 times compared to the plain surface. Improved performance is attributed to the cross-flow of multistage porous structure and orientation of the test section, which induced a breathing phenomenon and prevented the drying out of the test section by resisting the merging of large vapor bubbles for a longer period and decreasing its lingering time over the surface. Bubble behaviors were investigated utilizing a high-speed camera. Two separate modes of the boiling mechanism were observed. At low heat fluxes below 50 W/cm2, bubble discharge and liquid intake co-occurred at random pores and channels, whereas at high heat fluxes, volatile bubbles exhibited a breathing phenomenon.