Effect of Design Parameters on Flame Flashback Phenomenon Utilizing a Strut-Cavity Flame Holder :: A Numerical Study

Hypersonic air-breathing propulsion is now a viable and cost-effective method for launching payloads into orbit. This technology has significant advantages for both military operations and high-speed civilian transportation. However, challenges such as inefficient mixing, poor combustion, and flame stabilization issues arise due to short flow residence times. Addressing flame stability and combustion enhancement is crucial for effective scramjet designs. The underlying physical mechanism of flame flashback remains unclear, necessitating intensive research to understand its origins. In this study, we investigated the impact of design parameters on flame flashback using a newly developed configuration of a strut-cavity flame holder. By examining the influence of upper wall divergence, strut half angle, and cavity offset ratio, we gained insights into the mechanism of flame flashback.  The two-equation k-ω SST turbulence model was utilized for these simulations. The shear layer, originating from the cavity corner’s leading edge, impinges on the transverse fuel injection from the rearward step of the offset cavity. This impingement results in a complex flow field around the strut-cavity flame holder, characterized by interactions between shear-layer-shock phenomenon, cavity recirculation zones, and the trajectories of both the shear layer and fuel. During transient flow, a pre-combustion shock train reduces the flow speed, allowing heat release to influence the flow field and upstream flame propagation. Wall divergence contains the heat release and counteracts flashback speed caused by flow acceleration. Additionally, factors like thermal choking, low-speed zones, and boundary layer separation contribute to the interplay between the flow and flame speeds. Reducing the height of the rear wall of the cavity in relation to the cavity offset ratio created low-speed zone. This adjustment improved the interaction between the fuel jet and the surrounding fluid. The duration the fuel spent within the cavity, influenced by both the cavity size and the way the fuel jet interacted, was a key factor. It was noted that the cavity with an offset ratio was more likely to cause early unstart due to an increased flame flashback speed, disrupting the core supersonic flow.