Physical and chemical effect mechanism of steam dilution on H2-O2 coaxial diffusion flames

Steam dilution in H 2 –O 2 coaxial diffusion flames induces a non-monotonic flame response that differs fundamentally from the monotonic inert-dilution baseline of N 2 . This study integrates high-speed OH* chemiluminescence imaging with 3D multi-physics decoupling simulations based on the artificial-species method to clarify the physicochemical coupling mechanism of steam dilution. Experimental results show that steam dilution drives a rise-then-fall evolution in flame morphology, with flame height and projected area reaching maxima at about 20% dilution, accompanied by a more elongated and spatially coherent reaction zone. In the low-to-moderate dilution regime, steam reorganizes flame instability into a more distributed and ordered form by suppressing high-frequency unsteadiness and preserving large-scale structural coherence; at stronger dilution, the flame transitions toward near-limit unsteadiness and modal fragmentation. Under the representative high-dilution condition (50%), 3D decoupling reveals that the near-limit flame response is governed by an asymmetric competition between global thermophysical suppression and localized kinetic compensation, particularly the third-body effect, which reinforces the near-nozzle reaction layer. Stretch analysis further shows that this fluid–chemical coupling asymmetrically reconstructs the kinematic balance of the flame surface: the net-stretching response is mainly governed by the normal branch, whereas the net-compressive response is dominated by shear and is highly sensitive to direct chemical effects. These findings provide a mechanistic basis for the stability-oriented design and optimization of wet H 2 –O 2 combustion systems. Novelty and significance statement Steam-diluted hydrogen-oxygen combustion enables precise temperature control for clean energy systems, yet the causal link between steam’s physicochemical effects and flame instability remains underexplored. For the first time, this study quantitatively isolates the physical and chemical roles of steam in H 2 –O 2 coaxial diffusion flames. High-speed experimental diagnostics reveal a non-monotonic transition in macroscopic flame topology and unsteady dynamics. Combined with artificial-species turbulent flame simulations, this study further clarifies the dominant physicochemical competition underlying the representative near-limit high-dilution response, particularly the balance between global thermophysical suppression and localized chemistry-induced compensation associated with the third-body effect. These findings provide a solid theoretical basis for optimizing wet hydrogen combustion strategies and burner design.