Deflagration-to-detonation transition in gases in tubes with cavities

Abstract

The existence of a supersonic second combustion mode — detonation — discovered by Mallard and Le Chatelier and by Berthelot and Vieille in 1881 posed the question of mechanisms for transition from one mode to the other. In the period 1959–1969, experiments by Salamandra, Soloukhin, Oppenheim, and their coworkers provided insights into this complex phenomenon. Since then, among all the phenomena related to combustion processes, deflagration-to-detonation transition is, undoubtedly, the most intriguing one. Deflagration-to-detonation transition (DDT) in gases is connected with gas and vapor explosion safety issues. Knowing mechanisms of detonation onset control is of major importance for creating effective mitigation measures addressing two major goals: to prevent DDT in the case of mixture ignition, or to arrest the detonation wave in the case where it has been initiated. A new impetus to the increase in interest in deflagration-to-detonation transition processes was given by the recent development of pulse detonation devices. The probable application of these principles to creation of a new generation of engines put the problem of effectiveness of pulse detonating devices at the top of current research needs. The effectiveness of the pulse detonation cycle turned out to be the key factor characterizing the Pulse Detonation Engine (PDE), whose operation modes were shown to be closely related to periodical onset and degeneration of a detonation wave. Those unsteady-state regimes should be self-sustained to guarantee a reliable operation of devices using the detonation mode of burning fuels as a constitutive part of their working cycle. Thus deflagration-to-detonation transition processes are of major importance for the issue. Minimizing the predetonation length and ensuring stability of the onset of detonation enable one to increase the effectiveness of a PDE. The DDT turned out to be the key factor characterizing the PDE operating cycle. Thus, the problem of DDT control in gaseous fuel–air mixtures became very acute. This paper contains results of theoretical and experimental investigations of DDT processes in combustible gaseous mixtures. In particular, the paper investigates the effect of cavities incorporated in detonation tubes at the onset of detonation in gases. Extensive numerical modeling and simulations allowed studying the features of deflagration-to-detonation transition in gases in tubes incorporating cavities of a wider cross section. The presence of cavities substantially affects the combustion modes being established in the device and their dependence on the governing parameters of the problem. The influence of geometrical characteristics of the confinement and flow turbulization on the onset of detonation and the influence of temperature and fuel concentration in the unburned mixture are discussed. It was demonstrated both experimentally and theoretically that the presence of cavities of wider cross section in the ignition part of the tube promotes DDT and shortens the predetonation length. At the same time, cavities incorporated along the whole length or in the far-end section inhibit detonation and bring about the onset of low-velocity galloping detonation or galloping combustion modes. The presence of cavities in the ignition section turns an increase in the initial mixture temperature into a DDT-promoting factor instead of a DDT-inhibiting factor.