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Friday, September 26

Formation of Preheated Zone Ahead of a Flame and
Origins of the Deflagration-to-Detonation Transition

Michael A. Liberman, Uppsala University, Sweden
Computer Science and Mathematics Division Seminar
11:00 AM, Computational Sciences Building (5600), Room L-202
Contact: Yehuda Braiman (braimany@ornl.gov), 865.241.2065

Abstract

The quantitative prediction of deflagration-to-detonation transition is considered as one of the most basic unsolved problems in combustion and detonation. Predicting the occurrence of the deflagration-to-detonation transition is of great practical importance because of its destructive potential, addresses safety issues, and due to possible practical applications, for example, for design and optimization of pulse detonation engines. Results of the recent experimental studies and numerical simulations of deflagration-to-detonation transition (DDT) in channels are outlined. For the first time a self-consistent theory is obtained describing flame accelerating in a channel and physical mechanisms underlying the deflagration-to-detonation transition and explaining the major experimentally observed features of DDT. It is shown that the transition to detonation occurs due to formation of a preheated zone ahead of the flame. Different mechanisms of the preheated zone formation are discussed and analyzed. Experimental studies and numerical simulations indicate that the deflagration-to-detonation transition has three separate stages. During the first stage flame accelerates creating conditions for the formation of preheated zone ahead of the flame front. The second stage is formation of the reactivity gradient. The third stage is the actual onset of the detonation wave itself. The mechanism for the detonation wave formation, given the appropriate formation of a preheated zone, is universal and involves the reactivity gradient formed from the flame in the presence of preheated zone. If the domain of the preheated zone is wide and temperature is high, then the flame structure rearranges the induction-time distribution to one that bears little resemblance of the initial temperature distribution in the flame. In the presence of the preheated zone the flame transforms itself into the temperature gradient needed for triggering detonation onset through the Zel'dovich gradient mechanism. The developed theory answers the questions about how DDT depends on the flame velocity, order of the reaction, etc. One of the major unresolved questions about the possibility of unconfined DDT can be understood in light of the preheated zone formation by the flame folding due to the hydrodynamic flame instability.