Experimental Study on Heat Transfer Characteristics of PFO-50 in Immersion Ribbed-Array Channels Under Natural Circulation Subcooled Boiling Conditions

In this study, two brazed ribbed-array channels with surface roughnesses of 0.28 μm and 10 μm were fabricated to investigate natural-circulation phase-change cooling under various subcooling conditions. Increasing subcooling significantly enhanced boiling heat transfer, and for the 10 μm surface, raising subcooling from 5 K to 20 K increased the maximum average wall heat-transfer coefficient by 68.8%. A heat-transfer coefficient per unit mass flux was introduced to compare the two ribbed-array channels. At low heat fluxes (qh < 12 W/cm²), the 10 μm surface achieved a 38.2% enhancement, whereas at higher heat flux (qh ≈ 19 W/cm²), the enhancement decreased to 9.5%, indicating limited effectiveness of roughness at high heat loads. Finally, a heat transfer correlation for natural circulation pool boiling of PFO-50 was developed, yielding relative errors of 0.29–24.2% and achieving high confidence within ±20%.