The effect of instability of detonation on the propagation modes near the limits in typical combustible mixtures

At the near-limit conditions, the behavior of detonation propagation manifests unstable modes; those modes are classified into three types, i.e., steady mode, fluctuation mode and fast failure mode. This work aims to elucidate essential physical relation between those unstable propagation modes and the instability of detonation. The velocity of the combustion wave in five mixtures (i.e., C2H4-6N(2)O, CH4-2O(2), 2H(2)-O-2, 70% and 85% Ar diluted C2H2-2.5O(2)) are experimentally measured to analyze velocity behavior near the limits. The stability parameter chi is computed from CHEMKIN package and chemical kinetics. According the detonation cellular pattern and the calculation of chi, C2H4-6N(2)O and CH4-2O(2) are considered as unstable mixtures (with highly irregular cells), 2H(2)O(2) is characterized as stable mixture (with regular structure), 70% and 85% Ar diluted C2H2-2.50(2) are determined to be highly stable mixtures (with highly regular pattern). In the above unstable/stable/highly stable mixtures, the detonation propagation behavior at the near-limit condition presents Slow Decay/ Fluctuation/ Fast Failure modes. Evidences confirm that instability (or cell regularity) dominates the propagation modes as detonation approaches the limits. The competition mechanism between detonation instability and boundary effect resulting in the slow decay of propagation velocity as the condition is outside the limits. However, for stable mixtures, due to the instability has very limited effect on the propagation of detonation, this competition mechanism is lacking, and resulting in detonation rapidly attenuates to a fast flame. In addition, the neutral stability curve (Ng et al., Combust Theor Model 2005;9:385-401) is an appropriate separatrix between Fast Failure/Fluctuation modes, a new second stability curve is proposed in this work, which is the boundary of Fluctuation/Slow Decay modes.