Document Type : Original Article

Highlights

High-speed air-breathing propulsion systems, particularly ramjet and scramjet engines, represent key technologies for supersonic and hypersonic flight. Unlike conventional turbojet engines, these systems operate without rotating compressors or turbines and rely entirely on the dynamic compression of incoming air. In ramjet engines, combustion occurs at subsonic speeds inside the combustor, whereas in scramjet engines (Supersonic Combustion Ramjet), combustion takes place under supersonic flow conditions. Although this structural simplicity allows operation at extremely high Mach numbers, it introduces significant challenges in fuel–air mixing, flame stabilization, and combustion efficiency.

At high flow velocities, especially in scramjet configurations, the residence time of fuel inside the combustion chamber becomes extremely short. As a result, achieving stable combustion and complete fuel consumption is difficult. Among the various flame stabilization techniques, cavity-based flameholders have been widely adopted due to their ability to create recirculation zones that trap hot combustion products, increase fuel residence time, and enhance turbulent mixing. Previous investigations, including the experimental studies of Gruber and the comprehensive review by Ben-Yakar, demonstrated the effectiveness of cavity flameholders in stabilizing supersonic flames. More recent hydrogen-fueled investigations published in the International Journal of Hydrogen Energy have emphasized the sensitivity of combustion performance to cavity geometry.

Despite extensive research, most studies have focused on either ramjet or scramjet engines individually, and systematic comparisons under identical modeling frameworks remain limited. Furthermore, the combined effects of cavity height, length, and aft-wall angle under hot hydrogen combustion conditions have not been comprehensively evaluated. Therefore, the present study aims to numerically investigate the influence of these geometric parameters on combustion efficiency in both ramjet and scramjet engines using a unified computational approach.

Methodology

The computational domain consists of an inlet section, a combustor equipped with a wall-mounted cavity flameholder, and a downstream outlet region. The baseline cavity geometry includes a height of 9.8 mm, a length of 19 mm, and an aft-wall angle of 30°. Parametric variations are introduced by considering cavity heights of 6 mm and 12 mm, cavity lengths of 13 mm and 25 mm, and aft-wall angles of 15° and 45°. Hydrogen is selected as the fuel due to its high reactivity and suitability for high-speed propulsion systems.

Two operating conditions are considered: a ramjet configuration with subsonic combustor flow and a scramjet configuration with a Mach 2.5 supersonic inlet condition. All simulations are performed using ANSYS Fluent under compressible reacting flow assumptions. The governing equations include continuity, compressible Navier–Stokes, energy, and species transport equations.

Turbulence is modeled using the SST k–ω model, which is well suited for predicting boundary layer separation, shear layer development, and shock–boundary layer interactions in high-speed flows. Combustion is simulated using the Finite-Rate/Eddy-Dissipation model to account for both chemical kinetics and turbulence–chemistry interaction effects. Appropriate pressure-based boundary conditions are applied at the inlets and outlet depending on the engine configuration, while all walls are treated as no-slip and adiabatic.

Grid independence is verified using three mesh densities, and dimensionless static pressure distributions are compared to ensure numerical accuracy. Second-order discretization schemes and a coupled pressure–velocity solver are employed to enhance stability and accuracy. Combustion efficiency is defined based on hydrogen mass consumption between inlet and outlet, providing a direct quantitative measure of fuel utilization.

Discussion and Results

The numerical results demonstrate that cavity geometry significantly affects flow structure, recirculation strength, temperature distribution, and combustion efficiency in both engines. Increasing cavity height enhances combustion efficiency in both ramjet and scramjet configurations. A larger cavity height produces a stronger recirculation zone, increases residence time, and promotes improved fuel–air mixing. The effect is more pronounced in the ramjet engine due to longer subsonic residence time, which allows more complete heat release and reaction progression. In the scramjet configuration, although combustion efficiency increases with cavity height, the improvement is less significant because supersonic flow rapidly convects thermal energy toward the outlet.

Reducing cavity length generally leads to a moderate improvement in combustion efficiency. Longer cavities may cause partial fuel accumulation and spillage, weakening effective mixing within the shear layer above the cavity. Shorter cavities concentrate the recirculation region and strengthen mixing interactions. However, sensitivity to cavity length is lower compared to cavity height.

The aft-wall angle influences vortex structure, shear layer behavior, and flame anchoring location. Smaller angles tend to shift the recirculation zone downstream and may reduce mixing effectiveness, while larger angles intensify shear layer entrainment and improve flame stabilization. Nevertheless, excessively steep angles may introduce aerodynamic penalties, indicating that optimal geometric design requires balanced parameter selection.

A comparative analysis shows that the ramjet configuration is more sensitive to geometric modifications than the scramjet configuration. Because combustion occurs under subsonic conditions in the ramjet, changes in residence time and recirculation strength directly influence reaction completeness. In contrast, the scramjet operates under supersonic flow where limited residence time reduces the overall impact of geometric variations, although similar trends are observed.

Conclusion

A comprehensive numerical investigation was conducted to evaluate the effects of cavity flameholder geometric parameters on hydrogen combustion performance in both ramjet and scramjet engines. The results indicate that increasing cavity height significantly improves combustion efficiency by strengthening recirculation and increasing fuel residence time. Shorter cavity lengths provide moderate improvements by reducing fuel spillage and concentrating mixing regions. The aft-wall angle strongly affects vortex formation and flame stabilization, and appropriate optimization enhances combustion performance.

Overall, the ramjet engine exhibits greater sensitivity to geometric variations due to longer residence time and subsonic combustion conditions, whereas the scramjet shows similar but less pronounced trends under supersonic flow. The findings provide practical design guidelines for optimizing cavity flameholder geometry in

• Combustion