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This work reports the experimental investigations on direct initiation of detonation using multiple turbulent flame jets, with a special focus on the jet arrangement schemes and fundamental physics in the initiation processes. Results show that the direct initiation of detonation can be achieved by using turbulent jets even under the conditions that the tube diameter is much smaller than the empirical critical tube diameter with a mechanism of flame-shock-wall interaction. Conspicuous evidence has been shown that the probability of direct detonation initiation improves significantly near the detonatability limit using multi-jets compared to single jets. These results are found to be linked to several new phenomena observed when using the multiple jets to initiate the chamber. They are: 1) the unexpected rapid promotion of final-stage flame acceleration in ignition tubes by multiple jets, which is attributed to the fact that the expanding precursor shock waves propagate back into the adjacent tube and encounter the flame; 2) the enhancement of hot spot generation by multiple jets due to the precursor shock intersecting and the formation of an induction zone; 3) the obvious velocity loss in impinging jets initiation as a result of induced hot spots propagating in the burned gases.

The combustion characteristics of a hydrogen-fueled supersonic combustor featuring a large cavity length-to-depth ratio (i.e., 11) were examined by performing experimental trials while varying the fuel injector positions and equivalence ratios. During these trials, flame chemiluminescence images were acquired simultaneously from the side and bottom of the combustor under Mach 2.0 inflow conditions. The flame was observed to stabilize inside the cavity under all conditions. Proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) analyses of sequential flame chemiluminescence images demonstrated the important effects of oblique shocks induced by fuel injection and heat release on flame stabilization. Because fluctuations in the locations of the flame and of the intense heat release zone were not observed and no dominant frequency was identified in POD and DMD analyses, the present configuration was evidently able to suppress combustion instability. The present research provides preliminary guidance for exploring the feasibility of using cavity combustors with large length-to-depth ratios in scramjet engines.