In the process of deflagration-to-detonation transition (DDT) in reactive gases, the flame typically accelerates first to the choked flame condition (known as a Chapman-Jouguet deflagration), where it propagates at the sound speed with respect to the product gases. Subsequently, the choked flame may transit to a detonation. In the present study, the transition length from choked flames to detonations was measured experimentally in laboratory-scale experiments in methane, ethane, ethylene, acetylene, and propane with oxygen as oxidizer. The choked flames were first generated following the quenching of an incident detonation after its interaction with cylindrical obstacles with two different blockage ratios, 75\% and 90\%. Comparison with a recently proposed model confirms that these are Chapman-Jouguet deflagrations. The subsequent acceleration was monitored via large-scale time-resolved shadowgraphy. The mechanism of transition was found to be through the amplification of transverse waves and hot spot ignition from local Mach reflections. The transition length was found to correlate very well with the mixture's sensitivity to temperature and pressure fluctuations. These fluctuations could be connected to a unique parameter (X), introduced by Radulescu. The parameter is the product of the non-dimensional activation energy (Ea/RT) and the ratio of chemical induction to reaction time (ti/tr). Mixtures with a higher X were found to be more prompt to hot spot ignition and amplification of the fast flame into detonations. The run-up distance for unstable mixtures was found to be much shorter than anticipated from a model neglecting the fluctuations in a 1-D framework. The run-up distance was also correlated to the detonation cell size, yielding LDDT ~ 7 - 50 cells, with the proportionality coefficient depending on X and the obstacle blockage ratio. Finally, a unique correlation for the run-up distance is proposed, yielding LDDT ~ 3000 c tr, where c is the sound speed in the shocked non-reacted gas, valid for large X.
| Identifer | oai:union.ndltd.org:uottawa.ca/oai:ruor.uottawa.ca:10393/34592 |
| Date | January 2016 |
| Creators | Ahmed, Mohamed Saifelislam Abdelgadir |
| Contributors | Radulescu, Matei |
| Publisher | Université d'Ottawa / University of Ottawa |
| Source Sets | Université d’Ottawa |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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