Fluid simulation of two-temperature non-LTE plasmas of C ₄ F ₇ N and C ₅ F ₁₀ O mixed with CO ₂ , N ₂ and O ₂ as eco-friendly SF ₆ replacements

Abstract C 4 F 7 N and C 5 F 10 O have been considered as promising candidates to replace SF 6 as arc quenching medium. To broaden their operational temperature range, these gases are often mixed with buffer gases such as CO 2 , N 2 , and O 2 . However, in low-temperature regions particularly near electrodes and arc fringes, plasma conditions deviate markedly from local thermodynamic equilibrium (LTE). To capture these non-LTE (NLTE) effects, we develop a two-temperature (2 T) arc fluid model for C 4 F 7 N, C 5 F 10 O and their various mixtures. Furthermore, a novel methodology is proposed to explicitly distinguish between electron and heavy-particle radiation contributions within the 2 T framework. Using this NLTE model, we systematically investigate the influence of different radiation treatments and buffer gas compositions on arc temperature evolution. The results show that C 4 F 7 N-CO 2 and C 5 F 10 O-CO 2 plasmas approach LTE conditions near the arc core, transition from NLTE to LTE at the arc boundary, and remain strongly non-equilibrium in electrode-adjacent regions. Although both mixtures exhibit LTE behavior in the core and NLTE characteristics in the outer regions, the degree of non-equilibrium is more pronounced in C 4 F 7 N-CO 2 due to weaker electron-heavy particle energy exchange. Regarding radiation modeling, the proposed dual-radiation approach accurately captures both global and local thermal features, avoiding the over-expansion of temperature fields observed in the heavy-particle-only method and the loss of spatial detail in the electron-only method. Buffer gases substantially alter the arc temperature distribution through combined effects of thermal conduction, chemical reactions, and electron attachment. Moreover, comparison with experimental arc voltage measurements confirms that the NLTE model achieves significantly higher predictive accuracy than conventional LTE models. These findings underscore the necessity of incorporating NLTE effects and separately treating electron and heavy-particle radiation in arc fluid modeling of SF 6 replacement gases.