Flame Propagation of refrigerant R-1234yf (CF3CFCH2) in humid air: A DNS study

Abstract

The next-generation refrigerant R-1234yf (${\text{CF}}_3{\text{CFCH}}_2$) is expected to be widely used but is mildly flammable, requiring new fire-safety considerations. Water vapor can significantly facilitate the combustion of R-1234yf, increasing the flame speed by a factor of up to three. This study employs direct numerical simulations (DNS) to investigate the flame dynamics and assess the flame propagation behavior of humid R-1234yf-air mixtures. Effects of gravity, radiation, differential diffusion, and air humidity are taken into account in the DNS for a comprehensive assessment. It is found that although air humidity significantly increases the unstretched flame speed for lean mixtures at ambient conditions, radiation and strong Markstein effects inhibit combustion, ultimately resulting in complete extinction. This underscores the influence of the Markstein effects and highlights a potentially underestimated hazard under rich conditions when relying solely on the unstretched flame speed of refrigerants. In addition, this work provides a holistic analysis of buoyant R-1234yf flames in humid air, focusing on flame evolution, flame structures, and the Markstein effects. In particular, the Markstein numbers are separately determined for positive and negative components of curvature and strain rate. In this study, positive curvature indicates a flame front convex toward the reactants. It is found that Markstein effects due to positive curvature are the dominant factor, leading to inhibited flame propagation, particularly in lean conditions, while Markstein effects due to strain rate have a minor influence. The Markstein numbers in lean and rich flames in response to air humidity vary notably. In rich conditions, higher humidity reduces the Markstein number for positive curvatures, which promotes flame propagation. Conversely, under lean conditions, no significant effects of humidity levels on the Markstein numbers can be observed.