Studies On The Cylindrical Converging Shock Induced Ignition

Shock-Induced-Ignition (SII) has obvious advantages to study the ignition and reaction wave propagation process of combustible gas, in which many previous works have been carried out for the investigation of planar-shock-induced-ignition (PSII) due to its efficient performance in a shock tube. Thanks to the studies on PSII, various valuable understandings of the chemical reaction kinetics of premixed combustible gases, especially the ignition delay characteristics, have been achieved and relatively well developed. Therefore, this paper will investigate another form of SII, namely converging shock induced ignition (CSII). In contrary to the case of planar shock (PS),a converging shock (CS) intensifies gradually as it propagates forward and the ignition environment behind the converging shock changes continuously along the space. Such kind of ignition environment may provide some fascinating behaviours of ignition and reaction wave propagation characteristics. Based on the above consideration, we have carried out the following works:First, a new method of generating cylindrical shock to perform CSII is proposed.The shock dynamics theory is used to design the configuration of the converging channel so that the disturbances generated in the process of shock convergence is reduced remarkably. Thus a nearly ideal ignition environment can be constructed.Furthermore, it is convenient to change the relevant parameters which determine the ignition environment. For this approach, the experimental results have demonstrated its feasibility and superiority. In addition, nearly perfect match of the experimental results with that of quasi-one-dimensional numerical simulation indicates that the ignition and reaction wave propagation process has a satisfactory one-dimensional feature.Therefore this approach is confirmed to investigate the chemical reaction kinetics and the reaction wave propagation characteristics in the ignition environment constructed by the converging shock.After the successful development of the method,a basic phenomenon of converging-cylindrical-shock-induced-ignition (CCSII) has been investigated in detail.In terms of the ignition process, the experimental results show that there is a distance between the initial ignition position and the converging shock. It is found by theoretical analysis and numerical simulation that the total ignition time contains two parts. One is determined by the movement process of the converging shock and the other is determined by the chemical reaction kinetics. The comprehensive effect of the two parts of ignition times is that there is a minimum point of ignition time along the shock movement direction. On the other hand, in terms of the reaction wave propagation process, the experimental results show that for a period of time after ignition a shock-flame complex forms at some distance upstream of the initial ignition position. The shock-flame complex finally develops into the self-propagating CJ detonation. The reaction wave propagation process can be depicted in detail by numerical simulation. It is found that a weak detonation with a super-CJ speed forms from the initial ignition position and propagates along the ignition time spatial distribution curve. Its speed decreases gradually to the local sound speed in the propagation process. Then a strong coupling of the chemical reaction kinetics and the gas dynamics process renders the shock-flame complex forms and develops into the self-propagating CJ detonation. It is noted that there is no obvious overdriven phenomenon in the whole reaction wave propagation process which is different from the direct initiation of detonation,deflagration-to-detonation-transition (DDT) or the detonation initiation induced by the planar shock.Further, the effects of the variation of the intensity of incident planar shock, the variation of the component of gas, the variation of the initial pressure of gas etc. on the ignition and reaction wave propagation process have been investigated respectively.First, in terms of the effect of the intensity of incident planar shock, with an increase of the intensity of incident planar shock, the initial ignition position moves upstream and the converging shock intensity and the post-shock temperature of fluid particle at the initial ignition position decrease. When the intensity of incident planar shock increases to a certain degree, a super-CJ acceleration phenomenon of the reaction wave arises.With further increase of the intensity of incident planar shock, in addition to the reaction wave propagating upstream forms at the minimum point of ignition time, one more reaction wave propagating downstream forms at the contact surface. The two reaction wave interacts with each other after encountering. On the other hand, the outlet of the converging channel restricts the enhancement of the converging shock. With the decrease of the intensity of incident planar shock, there exists a critical point of detonation initiation. When the intensity of incident planar shock is higher than the critical point, after forming at the downstream of outlet, the shock-flame complex can propagate into the converging channel and finally develop into the detonation. Because of the disturbances generated at the outlet, there exists overdriven phenomenon in the reaction wave propagation process. When the intensity of incident planar shock is lower than the critical point, after forming at the downstream of outlet, the shock-flame complex finally elapses because of the effect of the expansion waves at the outlet.Second, in terms of the variation of the component of gas, when the dilution degree of the inert gas to the fuel increases, the initial ignition position moves downstream and the converging shock intensity and the post-shock temperature of fluid particle at the initial ignition position increase, meanwhile the formation of the shock-flame complex takes longer time and the detonation initiation process becomes milder. Besides, it is also found that when the initial pressure of gas increases to a certain degree, the movement direction of the initial ignition position reverses.