Optimization of the deflagration to detonation transition: reduction of length and time of transition

摘要

The aim of this experimental investigation is the study of Deflagration to Detonation Transition (DDT) in tubes in order to (i) reduce both run-up distance and time of transition (L (DDT) and t (DDT)) in connection with Pulsed Detonation Engine applications and to (ii) attempt to scale L (DDT) with lambda(CJ) (the detonation cellular structure width). In DDT, the production of turbulence during the long flame run-up can lead to L (DDT) values of several meters. To shorten L (DDT), an experimental set-up is designed to quickly induce highly turbulent initial flow. It consists of a double chamber terminated with a perforated plate of high Blockage Ratio (BR) positioned at the beginning of a 26 mm inner diameter tube containing a "Shchelkin spiral" of BR approximate to 0.5. The study involves stoichiometric reactive mixtures of H-2, CH4, C3H8, and C2H4 with oxygen and diluted with N-2 in order to obtain the same cell width lambda(CJ) approximate to 10 mm at standard conditions. The results show that a shock-flame system propagating with nearly the isobaric speed of sound of combustion products, called the choking regime, is rapidly obtained. This experimental set-up allows a L (DDT) below 40 cm for the mixtures used and a ratio L (DDT)/lambda(CJ) ranging from 23 to 37. The transition distance seems to depend on the reduced activation energy (E (a)/RT (c)) and on the normalized heat of reaction (Q/a(0)2). The higher these quantities are, the shorter the ratio L (DDT)/lambda(CJ) is.