Molecular insights into the role of tetrafluoromethane in mitigating the boiling crisis suffered by liquid nitrogen in superconducting apparatus.

Nitrogen/tetrafluoromethane (N 2 /CF 4) has emerged as an effective refrigerant mixture, which plays a significant role in various high- T c superconducting (HTS) power devices and energy systems. Several studies have documented the superior performance of such zeotropic binary mixtures in heat transfer and energy conservation compared to pure liquids. However, boiling heat transfer characteristics of liquid mixtures on the molecular scale are not fully understood, especially the mechanisms associated with the role of the additive in regulating thermal properties. Here, we performed the molecular dynamics simulations for the pure N 2 and the mixtures containing 20 and 40 mol% of CF 4 to probe into the boiling heat transfer process on an ideal copper substrate. Analyses suggest that the boiling process of the N 2 /CF 4 mixture shares similar features with the pure N 2 , but the binary refrigerant manifests advantages at higher substrate temperatures. Specifically, the additive CF 4 delays the onset of film boiling, and the operational temperature range could be enlarged by almost 40% compared to pure N 2 in terms of the 40 mol% mixture. Moreover, the mixture with CF 4 additive maintains a small interfacial resistance even if the substrate temperature exceeds the critical value of film boiling for pure N 2 , highlighting the potential to use such mixtures for devices that may suffer from high heat flux levels. Finally, analyses regarding the mixture composition and the solid-liquid interactions confirm the essential role of the additive CF 4 in mitigating the mismatch of the vibrational density of states at a high substrate temperature, which may reveal the mechanisms concerning the CF 4 improving the heat transfer performance of the original N 2. These findings provide a better understanding of the advantage of N 2 /CF 4 at high substrate temperatures, and they lay a foundation for designing the cooling systems of HTS apparatuses. • The boiling heat transfer of N 2 /CF 4 liquid mixture was investigated via molecular dynamics simulations. • The CF 4 delays the onset of film boiling and enlarges the operational temperature range. • The mixtures containing CF 4 maintain efficient heat transfer at high substrate temperatures. • The additive CF 4 mitigates the VDOS mismatch, playing a vital role in promoting heat transfer performance. [ABSTRACT FROM AUTHOR]