Effects of detonation instability and boundary layer on flame propagation behavior in millimeter-scale smooth tubes

Abstract

Small pre-detonators have potential applications in practical pulse detonation engines or rotating detonation engines. The effects of detonation stability parameter and area divergence on flame propagation in capillary tubes are experimentally analyzed using high-speed cinematography. The four mixtures (namely, stoichiometric methane/oxygen, ethylene/oxygen, propane/oxygen and stoichiometric acetylene/oxygen/50% Ar mixtures) used have different values of stability parameter. The inner diameters of the tubes (namely, 0.5, 1.0, 2.0, and 4.0 mm) investigated provide a variation in area divergence at initial pressures p0 ranging from 5 to 100 kPa. The results show that stability parameter and area divergence play positive and inhibitory roles, respectively, in the process of flame propagation. Five propagation modes are observed (namely, steady detonation, stuttering detonation, galloping detonation, deflagration, and no flame), among which the stuttering detonations have three different forms, depending on the product of stability parameter and area divergence. For the most-unstable mixture (stoichiometric methane/oxygen, with the largest values of stability parameter), the range of the product of stability parameter and area divergence in which steady detonation can occur is the narrowest. The results also indicate that the stoichiometric ethylene/oxygen and stoichiometric propane/oxygen, with similar values of the product of stability parameter and area divergence, have the same propagation modes. Further analysis suggests that the Chapman–Jouguet deflagration-to-detonation transition (DDT) distance L decreases with increasing of stability parameter and area divergence for all four mixtures. Compared with the other three mixtures, stoichiometric methane/oxygen, which has the smallest values of L because it is the most unstable mixture with the greatest propensity for hotspot formation, is most likely to undergo DDT under the same initial and boundary conditions. These conclusions agree well with the various characteristics of L obtained for larger tubes.