Effects of steam dilution on flame characteristics and vortex field mechanisms in normal and inverse diffusion Hydrogen-Oxygen flames

Steam-diluted hydrogen-oxygen combustion is a key technology for future hydrogen utilization. Steam dilution significantly affects heat release and mixing, altering flame characteristics. Normal diffusion flames (NDFs) and inverse diffusion flames (IDFs) in coaxial jets exhibit distinct flame evolution patterns under steam influence. To elucidate the coupling between combustion mode and steam dilution, this study combines experiments and numerical simulations to clarify the distinct mechanisms of steam dilution on NDFs and IDFs. Static characterization shows that steam dilution significantly alters the unfiltered broadband flame emission structure and OH* distribution near the flame root in NDFs. When the steam dilution exceeds 40 %, the emission peak shifts downstream, while the spatial distribution in IDFs remains stable. Dynamic analysis demonstrates that steam dilution intensifies transient flame front wrinkling and shortens the OH* oscillation period in NDFs. In contrast, IDFs dynamics are less sensitive to dilution, with minimal disruption to the transient flame front structure. Multiphysics analysis of combustion reveals that near-field vortex evolution in NDFs is governed by viscous diffusion, vortex stretching and tilting, volume expansion, and baroclinic torque, with viscous diffusion dominating downstream. In IDFs, viscous diffusion controls near-field dynamics, while vortex stretching and tilting, volume expansion, and viscous diffusion jointly influence the downstream region. Steam dilution enhances the misalignment between density and pressure gradients within the flame, thereby amplifying the baroclinic torque effect in NDFs. Conversely, the baroclinic torque mechanism is negligible in IDFs.