## 4 Results and Discussion
### Flame Stabilization  
In Figure 6, three velocity conditions (5, 10, 20 m/s) for four H2 blending ratios (00H2, 30H2, 60H2, and 90H2) were considered. To obtain the lean blow-off limit equivalent ratio in every condition, firstly, a near blow-off condition (the equivalent ratio at 0.75) was employed to preheat the burner to the thermal equilibrium condition. Secondly, the fuel and air flow rates decreased synchronously until the flame was extinguished. The existing final flame equivalent ratio is defined as the lean blow-off limit equivalent ratio, both of which are marked in Figure 6 for all cases. It was observed that the blow-off limit could be extended to leaner conditions with increasing hydrogen ratios, indicating that the more H2 added, the better the stability of the flame. This can be firstly explained by the enhancement effects that increased H2 addition brings including increasing the flame laminar velocity to avoid the flame from petering out. Chemical effects that induce more active radicals may further increase flame stability. Moreover, the blow-off equivalence ratio slightly increases with an increase in the bulk velocity. This suggests that an increase in the bulk velocity can lead to disadvantages in terms of flame stability. More detailed analyses on this should be conducted. Therefore, to ensure comparability, the same equivalence ratio of 0.75 was selected for all cases. This criterion represents a stable condition for the four hydrogen ratios considered, according to the experimental results shown in Figure 6.  
The simultaneous flame location and flow structure provide a useful framework for studying the effect of the hydrogen ratio on flame stabilization and the flame-turbulence interaction. Vortices play an important role in the self-sustained turbulence process; thus, they are widely used to study flow structure [42]. The simultaneous flame location and vorticity field were analyzed, as shown in Figure 7, to clarify the stabilization mechanism for each hydrogen ratio. The flame location is defined by the contour of 10% of the maximum heat release rate (red lines). The shear layer is identifiable by the high vorticity values (\(\,\mathrm{l}\,\omega\,\mathrm{l}>3000\)), colored in blue or red. The border of the recirculation region is marked by stagnation streamlines (black lines). The leading edge is defined by the most upstream flame location, as designated in Figure 7.  
Figure 6: Blow-off limits \(\phi_{\mathrm{LBO}}\) for flames subjected to different hydrogen ratios. The hydrogen ratio affects the flame location and flow structure significantly. Figure 7 demonstrates that the leading edge is located inside the recirculation zone for all conditions. The location of the leading edge relative to the bluff body changes drastically when the hydrogen ratio changes. It can be seen that the recirculation zone is shorter at higher hydrogen ratios. As a result, the flame leading edge is closer to the edge of the bluff body. This results in a larger angle between the flame and the reactants outside the recirculation zone. This phenomenon was also described in a previous study [25]. The lift-off of the flame from the bluff body can be considered as the precursor of the final blow-off, which can be avoided by increasing the hydrogen ratio [43]. Moreover, the ISL moves closer to the burner axis with an increase in the hydrogen ratio. The vorticity at the flame root becomes larger, and the flame moves to the burner axis along the top surface of the bluff body. Figure 7 clearly illustrates that a flame can be stabilized within a higher-strain-rate region at a larger hydrogen ratio. In addition, the flame location overlaps more with the outer shear layer, and the region of high vorticity also gradually increases. To further assess the flame stabilization in the outer shear layer, the \(Q\) criterion was used to investigate the relation between the flame location and flow structure.