Study On Behaviors Of Deflagration And Detonation Propagation After Interaction With Head-on Disturbances

An investigation was carried out for the behaviors of detonation and deflagration propagation after the interaction of head-on disturbances. Two kinds of so called "head-on" disturbances were chosen in the work, one is the disturbance generated by a perforated plate located on the way of detonation propagation, suffering from which the detonation will be decayed into a deflagration. The other is the head-on collision of a detonation with an opposite propagating shock wave, after which the combustion field behind the leading shock of the detonation will be enhanced by the incoming shock wave. Both of the disturbances play a similar role, that is, to destroy the condition of self-sustained detonation propagation almost instantaneously, so that the flow field will be forced to undergo an unstable process, which is the main interest of present work.The experiments were conducted in a detonation tube, whose diameter is 65mm and 4.5m in length. During the tests of detonation-perforated plate interaction, a perforated plate of 5mm thick separated the tube into two parts 2.5 m away from the ignition end. On the other hand, the head-on collision of detonation and shock wave was generated with an additional driver of shock tube connected to the other end of the detonation tube, in which the ignition of the detonation was triggered by the incoming shock wave, in order to guarantee the detonation and the shock wave to meet at the windows. A stoichiometric acetylene-oxygen mixture (M1), as well as 80% Ar diluted acetylene-oxygen mixture (M2), were tested during the operations. Right downstream the perforated plate, there were a pair of 300 mm long 2 mm wide slice windows for streak schlieren photography. The averaged structure and its evolution of the initially transmitted deflagration waves thus could be recorded in the film as an x-t diagram. Meanwhile, pressure transducers and ion probes mounted on the downstream tube traced the further development of the flame and wave front. A global reaction model of 4 reactant species and 3 reaction steps was proposed for a fast numerical calculation, in which the characteristic scales of induction time, heat release, as well as products were reasonably included.For the interaction of detonation with perforated plate, the main interest was focused on deflagration-detonation transition. It was found that, with the increase of initial pressure in Ml gas mixture, the deflagration wave varies from a laminar structure to a turbulent one, after which the transition to detonation will occur within a dozen of diameter length. The deflagration can also be switched into detonation when the initial pressure is high enough in M2 gas mixture, where transition takes place within a couple of diameter length with no obvious turbulence occurrence, which implies that the initial disturbance plays an important role for the transition.Thanks to the advantage of nearly 1 -D disturbance generated by the perforated plate, it was possible to demonstrate clearly that there is a critical state of deflagration with up to 50-60% of CJ detonation speed before the transition. And the deflagration can hardly exist with a speed in between the above state and the CJ detonation speed. The analysis of present work showed that, the deflagrations with laminar structure usually can not stand the attenuation of background Taylor rarefaction and keeps on slowing down, whereas the turbulent one is capable of accelerating and running up to a detonation wave. Therefore, it might be explained that for Ml gas mixture, the deflagration who is capable of propagating for a relatively long distance with 50-60% CJ detonation speed could be attributed to the competition between the turbulence enhancement and the Taylor rarefaction attenuation.From the streak schlieren visualization results, it was clearly observed for the first time that, after head-on collision of planar shock with detonation, the transmitted detonation wave in shock-compressed flow is always followed by a fan of rarefaction wave, which implies the transmitted equilibrium detonation wave to be in CJ state. It was also revealed that, (1) The cell size of the detonation decreases obviously and distributes more regularly after the collision, and furthermore, the length scale along the flow direction is compressed even shorter by the incoming shock wave; and (2) There exists an unstable process during the collision, the detonation wave decouples at the initial stage of the collision, followed by a quick switch into overdriven detonation wave which then approach gradually to the steady CJ state, and this process develops non-uniformly for real cellular detonation. The combination of experiment, numerical simulation and theoretical analysis was well performed in this part of the work.