Propagation characteristics of unstable detonation waves in a round tube with annular perturbation

Abstract

The detonation propagation characteristics of the mixtures, 2H2 + O2+3Ar and CH4 + 2O2, were investigated. Accordingly, the mixtures were tested in round tube with inner diameter of D = 80 mm and annular tubes with widths of w = 25 mm, 15 mm, and 5 mm. The two mixtures represent stable mixture with regular cell pattern and unstable mixture with irregular cell pattern, respectively. Smoked foils were utilized to record cellular structure under various initial conditions. Subsequently, the length scale Ldsc was measured, which represents the length from the start of the test section to the position where the cellular structure changes drastically (the cell size obviously increases or the structure disappears). The results reveal that both mixtures can successfully propagate in round tube and annular tubes of 25 mm and 15 mm, but fail in 5 mm annular tube. The Ldsc value of 2H2 + O2 + 3Ar is higher than that of CH4 + 2O2 in 80 mm and 15 mm tubes, but it is opposite in 25 mm tube. Moreover, the relationship between Ldsc and hydraulic diameter DH was analysed. For a given tube, the values of Ldsc and Ldsc/DH increased when the initial pressure increased. And the variation trend of Ldsc and Ldsc/DH of CH4 + 2O2 is steeper. Furthermore, the mixtures 2H2 + O2 + 3Ar and CH4 + 2O2 resulted in over-driven detonation in 15 mm and 25 mm annular tubes, respectively. The ratio between the total reaction length (sum of the induction length and exothermic length) and the hydraulic diameter (DH/(Δi + Δe)) correspond to critical values of 18 for hydrogen-oxygen-argon and 6 for methane-oxygen, below which the detonation will fail.