Evaluation of a distributed fuelling scheme for a Mach 8 axisymmetric scramjet using large eddy simulation

• Wall-modelled LES is used to simulate supersonic turbulent combustion in a Mach 8 ethylene-fuelled scramjet. • Distributed tandem-cavity fuelling is compared against a single-cavity baseline, validated against experimental data. • The influence of turbulent Schmidt number on RANS initialisation and LES combustion behaviour is quantified. • Distributed tandem-cavity fuelling enhances a scramjet’s operational stability by reducing susceptibility to thermal choking and subsequent scramjet unstart. This paper examines supersonic turbulent combustion in an ethylene-fuelled Mach 8 axisymmetric scramjet using US3D with wall-modelled large eddy simulation (LES). A tandem cavity scramjet combustor with distributed fuelling was compared against a single cavity design with one fuelling location, which also served as a validation study. LES successfully replicated experimentally observed combustion and extended beyond the experimental test window to reveal the potential of domain unstart. Each LES was initialised from Reynolds-Averaged Navier-Stokes (RANS) solutions using turbulent Schmidt numbers ( Sc t ) of either 0.3 or 0.75. While S c t = 0.30 produced a steady-state RANS solution matching experimental ‘steady’ pressure profiles, only the S c t = 0.75 LES captured the experimentally observed transient combustion behaviour resulting in domain unstart. These Sc t -varied initialisations were applied to the tandem cavity domains which found the S c t = 0.3 domain produced robust scram-mode combustion, while S c t = 0.75 yielded marginal ignition behaviour. Combustion regime and Damköhler number analyses indicated that an ignition aid could enable the validated S c t = 0.75 condition to evolve and replicate the S c t = 0.3 case. LES demonstrated that distributed fuelling in tandem cavities can achieve stable, scram-mode combustion with reduced susceptibility to thermal choking and scramjet unstart. Although the ignition-supported LES predicted lower combustion efficiency (40%) than RANS (65%), the design’s distributed heat release supports improved flow stability and operational resilience.