Criterion modeling of local ignition and global flame establishment in a wide-range scramjet with torch-assisted ignition

Future hypersonic aerospace vehicles equipped with wide-range scramjets require both high performance and compact configurations. While the design of combustor flow channel with large expansion ratios reduces engine axial dimension, it simultaneously introduces ignition failure challenges due to increased mainstream flow velocity, particularly for the large-scale scramjets where flame propagation from the walls to supersonic mainstream faces critical difficulties. To accurately predict ignition performance, a novel dual-stage criterion model for local flame generation and global flame establishment is established. The model integrates three core technical components. 1) The pre-ignition flow and mixing state is predicted by a one-dimensional Lagrangian-Eulerian multiphase solver. 2) Local flame generation is represented using a simplified quasi-steady reactor model in the cavity recirculation zone with the consideration of entrainment, reaction consumption and evaporative cooling. 3) Global flame establishment is assessed by quantifying post-ignition thermal choking at the cavity trailing edge. Both failed and successful global flame establishment cases from ground direct-connect tests under Mach 6.0 condition are successfully reproduced using the model. Parametric analyses provide quantitative ignition design guidance, revealing strongly Mach-dependent critical torch mass flow rates for local ignition (0.470, 0.275, and 0.040 kg/s for Mach 4.0, 5.0, and 6.0, respectively), nearly invariant critical ignition temperatures (1251-1265 K), and increasing critical cavity heat release rates required for global flame establishment via thermal choking (1.27, 1.49, and 3.77 MW). Cavity depth primarily enhances local fuel consumption and flame kernel formation with limited influence on global choking, whereas increased strut blockage promotes air entrainment, elevates net heat release, and facilitates thermal throat formation.