Experimental study of buoyancy-driven flame propagation in near-limit H2–O2 mixtures

Near-limit H 2 -O 2 flame propagation under normal gravity is strongly affected by buoyancy, but the morphology transition, its coupling with pressure evolution and flame-speed reduction, and the applicability of spherical flame evaluation methods remain insufficiently clarified. In this study, H 2 -O 2 flames near the lower flammability limit (LFL) were experimentally investigated in a constant-volume chamber using high-speed schlieren imaging. Image-derived geometric parameters were used to classify propagation regimes, evaluate buoyancy effects using the Richardson number (Ri) , analyze buoyant rising velocity ( v rise ), and assess spherical-flame stretch extrapolation. As the H 2 concentration decreased from 15 vol% to 5 vol%, the flame behavior changed from instability-controlled spherical propagation at 15–12 vol% to a transition regime at 10–7 vol%, and finally to buoyancy-induced near-limit propagation with jellyfish-like and split flame structures at 6.5–5 vol%. The maximum pressure ( P max ), decreased by more than 70% between 10 vol% and 8 vol%, coinciding with suppressed downward propagation and incomplete mixture consumption. Under these conditions, conventional spherical flame stretch extrapolation produced deviations exceeding 50% due to flame deformation and upward drift. Ri analysis showed that buoyancy effects intensified with flame growth and decreasing H 2 concentration, while the equivalent radius ( R eq. ), best matched the morphology-based regime classification. v rise analysis further indicated that a constant drag coefficient ( C D ) bubble analogy is insufficient, as the C D varied with flame regime. These findings provide a quantitative basis for interpreting buoyancy-induced near-limit H 2 -O 2 flame propagation and demonstrate the need for a new evaluation framework for flame-speed determination under normal-gravity near-limit conditions.