Revealing Microstructured Surface Critical Heat Flux Degradation Mechanisms and Synergistic Pool Boiling Enhancement in Fluorinated Fluids

Fluorinated dielectric fluids are widely utilized as heat transfer fluids for two-phase cooling of electronics, capitalizing on the fluids' large latent heat release during boiling for efficient heat flux removal. Recent studies have optimized surface micro/nanostructures on aluminum alloy through chemical etching, achieving more than 2× enhancements in boiling heat transfer coefficients (HTCs) of these fluids compared to plain surfaces. However, these microengineered surfaces suffer from critical heat flux (CHF) reduction of nearly 40%, with the mechanisms driving this CHF reduction remaining unclear. Here, we investigate the mechanism resulting in the poor CHF of microstructured surfaces and develop a guideline to synergistically enhance the HTC and CHF of these surfaces. Immersion boiling tests in fluorinated and nonfluorinated fluids, coupled with wickability and elemental analysis, revealed that surface degeneration─caused by fluorine deposition forming C-F bonds with adventitious carbon─has minimal impact on CHF in fluorinated fluids. To further verify that surface degeneration is not responsible for CHF reduction, pool boiling experiments with cavity sizes from 1 to 5 μm identified the 5 μm cavity surface, AM-H(400)E(5), as achieving the highest HTC in both HFE-7100 and ethanol. However, CHF reductions of 30-50% were consistently observed, regardless of whether the surface transitioned to hydrophobicity or retained superhydrophilicity. Arising from this investigation, it is concluded that the increased nucleation site density on AM-H(400)E(5), which leads to the overcrowding of bubbles, is the primary cause of CHF reduction. To overcome these limitations, we devise a method of hierarchical addition of microstructures on macro-fins to simultaneously enhance HTC and CHF, creating a single-process two-tier hierarchical structure by leveraging on AM to fabricate the macrostructures. The two-tier macro/microstructure design has successfully enhanced HTC and CHF by 99 and 202.2%, respectively, compared to the best single-tier microstructured surface. This approach not only effectively delay undesirable vapor layer formation but also provides a robust guideline for enhancing boiling performance in other fluorinated fluids, including refrigerants R134a and R1234ze(E).