Influence of water vapor on flame propagation in CH2F−CF3 (R-134a)/oxygen mixtures

The first laminar flame speed measurements of the hydrofluorocarbon refrigerant CH 2 F − CF 3 (R-134a) with oxygen are presented for both dry and humid conditions. Flame speeds were determined from pressure-rise measurements in a spherical combustion chamber using multi-zone data reduction models that account for radiation heat losses, and the results were compared with numerical simulations. For dry mixtures, the laminar flame speed exhibits a maximum near ϕ = 0.7 , which is significantly leaner than the peak typically observed for hydrocarbon flames. Analysis of flame structure and sensitivity coefficients shows that this behavior arises from changes in the radical pool. Lean mixtures are dominated by OH-driven oxidation chemistry, while richer mixtures lean towards fluorocarbon reaction pathways that are slightly less effective at sustaining flame propagation when the equivalence ratio is increased. The influence of water vapor was examined for oxidizer water mole fractions up to 4 %. Previous studies of halogenated refrigerant flames with F/H ratio > 1 have shown a flame speed increase with added water vapor. Unexpectedly, water vapor affects laminar flame speed only slightly for the presented conditions. Our experiments indicate a small reduction in flame speed within the measurement uncertainty, while simulations predict decreases by about 6 %. Although water addition raises the peak OH concentration by approximately 75 % at ϕ = 0.7 , the overall heat release rate decreases due to shifts in reaction pathways. In particular, enhanced OH consumption through CF 2 + OH = CFO + HF , together with reduced heat release from fuel decomposition and fluorocarbon oxidation reactions, leads to a minor inhibition effect. Numerical calculations at higher water vapor concentrations predict a transition to strong flame speed promotion under ultra-lean conditions, where increased OH production dominates the reaction network. These results highlight the complex role of water vapor in the combustion chemistry of hydrofluorocarbon refrigerants. Novelty and significance statement This study provides the first experimental flame speed measurements of R-134a/ O 2 mixtures and offers new insight into the influence of water vapor on fluorinated refrigerant combustion. While water vapor is often associated with combustion promotion in halogenated hydrocarbon systems through increased OH radical production, the present work shows that water vapor has only a very weak effect on laminar flame speed in R-134a/ O 2 flames, with simulations and experimental trends indicating slight inhibition rather than promotion. The results show that the additional OH formed in humid mixtures can be redirected into fluorocarbon radical pathways, particularly through reactions involving CF 2 , which limit the effectiveness of OH -driven chain propagation. These findings highlight the critical role of fluorocarbon radical chemistry in controlling flame propagation in hydrofluorocarbon refrigerants and demonstrate that increased OH concentrations do not necessarily lead to enhanced reactivity. The work provides improved mechanistic understanding of the coupling between OH and CF 2 pathways in humid R-134a flames, which is important for reliable kinetic modeling and for assessing combustion behavior of refrigerants in environments where humidity may influence flame propagation.