Numerical Investigation of Various Schemes of Cavity-Based Scramjet Flame Holders

Achieving efficient combustion in scramjet engines is challenging due to limited residence time, incomplete fuel–air mixing, and high total pressure losses. Traditional cavity-based flame holders improve stability but often suffer from excessive pressure drop and inefficient fuel utilization. Despite studies on cavity geometries, their impact across varying Mach numbers remains underexplored. This study investigates seven novel cavity configurations, namely, Peripheral Annular Channel with Internal Ramp, Inner Annular Channel with Peripheral Ramp, Inner Annular with Peripheral Annular Channel, Peripheral and Inner Dual Elliptical Cavity, Trapezoidal-Base Inward Triangular Cavity, Singular Inward Triangular Stepped and Outward Trapezoidal Cavity, and Concave Arc-Inset Trapezoidal Cavity, all with an L/D ratio of 3 for optimal mixing and flame-holding, chosen through bibliographical analysis. Through 56 computational fluid dynamics simulations across Mach 1.5–5.0, the Monolithic Inward Triangular Stepped and Outward Trapezoidal Cavity (Design 6) exhibited the highest combustion efficiency (99.9%) with a peak static temperature of 3700 K, while Design 5 reached 99.7% efficiency at 3300 K. Higher Exit Mach number recovery (2.7–5.02) was observed in Designs 1, 5, and 6, though shock train formation hindered recovery at higher Mach numbers. Design 6 also achieved the highest turbulent kinetic energy (70.49 kJ/kg), aiding sustained combustion, while Design 3 had the highest H₂O mass fraction (0.99594). These findings highlight the crucial role of cavity geometry in scramjet performance. Additionally, this study reveals how linear cavity geometries, like, Designs 5 and 6, enhance recirculation and fuel–air mixing, while nonlinear designs exhibit distinct flow physics and shock structures. By clarifying these effects, the research advances cavity-based scramjet flame holder development for supersonic transport, missile technology, and space exploration.