Study of ignition-driven flame propagation and energy-release evolution in an evaporation-tube annular combustor

A micro-turbojet converts liquid-fuel chemical energy into propulsion and power and often employs an evaporation-tube annular combustor to enhance evaporation and establish an acceptable turbine-inlet thermal field. Yet the coupled effects of the airflow field, ignition start-up transients, and axial/circumferential flame propagation on energy release and the transition to steady combustion remain unclear. This work provides a stage-quantified, time-resolved map of the ignition-to-steady transition and establishes an energy-release diagnostic based on HRR-isothermal area-OH timing. In this study, large-eddy simulation is performed and complemented by a self-built combustor-rig experiment; the time-averaged outlet temperature is validated against measurements. The flow structure, ignition-to-steady transition, and the co-evolution of high-temperature zones and OH radicals are examined. Coupled evaporation-tube and swirl-hole jets generate a primary-zone recirculation vortex that provides favorable residence and mixing conditions for flame-kernel stabilization and subsequent axial/azimuthal propagation; ignition actuation has limited influence on the recirculation intensity, while flame-front merging is associated with a slight increase in recirculation intensity. The first kernel forms at 8.4 ms near tube #1 and propagates in both directions, completing circumferential merging at tube #7 by 13.2 ms; steady combustion is established after 16 ms. During merging, the counterclockwise branch, whose propagation direction is generally consistent with the swirl-hole-induced azimuthal flow, advances faster with a thinner front than the clockwise branch. The 1300 K preheating-layer isothermal area and the heat-release rate peak at 13.2 ms, followed by the 1781 K high-temperature isothermal area at 13.6 ms, and both stabilize after 16 ms. Mixture fraction-temperature phase-space analysis further shows that flame merging is associated with the rapid expansion of the high-temperature region within a relatively narrow, reaction-favorable mixture-fraction range of approximately 0.04-0.16, rather than with a continuous increase in local maximum temperature. OH peaks 1.0 ms after heat release (14.2 ms) and evolves from localized clusters to filament/sheet-like multi-band patterns under stable combustion.