The conditions that are required for DDT are studied in the present thesis by focusing on the final phase of the onset of detonation. A series of experiments were carried out where high speed turbulent deflagrations were generated downstream of a perforated plate by reflecting a CJ detonation upstream from the plate. Four different detonation tubes were used to investigate scaling effects. Four different fuels (acetylene, hydrogen, propane, and ethylene) were used with either pure oxygen or air as an oxidizer to investigate the effect of mixture sensitivity on the onset of detonation. In some cases, fuel-oxygen mixtures were diluted with either nitrogen or argon. Perforated plates with various hole diameters and hole spacings were used in order to control the turbulence parameters in the reacting downstream flow. A theoretical study was also carried out to determine the various flow fields as well as the ignition processes via turbulent mixing. / It is found that a high speed turbulent deflagration traveling at about the CJ deflagration speed of the mixture (around 800 to 1200 m/s, depending on the given mixture) is required for the onset of detonation in a smooth tube. This critical ve locity is found to be rather insensitive to boundary conditions, which supports the conclusion that critical deflagrations are driven by the expansion of the combustion products (similar to a CJ detonation) and are dependent on the energetics of the mixture rather than on transport rates. The ignition mechanism in a critical deflagration differs from that of a CJ detonation and is effected by the turbulent mixing of combustion products with the unburned mixture. Therefore, intense turbulence is required in the reaction zone. Since the initial plate generated turbulence decays, the sustained propagation of a critical deflagration wave in a smooth tube relies on "flame generated turbulence" where the high temperature sensitivity of the mixture can mantain the turbulence in the reaction zone. It is also found that the critical deflagration is comprised of a leading shock wave followed closely by an extended turbulent reaction zone rather than the conventional shock wave followed by a distinct thin flame. / Once a critical deflagration wave is established, the onset of detonation requires the generation of transverse waves that couple to the reactions and are sustained by the chemical energy release as in a cellular detonation. The onset of detonation can be effected by the amplification (via the Rayleigh criterion) of repeatedly reflected transverse waves resonating with the reaction zone in a confined tube. It can also occur via the formation of a local explosion centre where an overdriven detonation bubble and the accompanying retonation wave is formed in the turbulent reaction zone.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:QMM.102486 |
| Date | January 2006 |
| Creators | Chao, Jenny C., 1976- |
| Publisher | McGill University |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Format | application/pdf |
| Coverage | Doctor of Philosophy (Department of Mechanical Engineering.) |
| Rights | © Jenny C. Chao, 2006 |
| Relation | alephsysno: 002572153, proquestno: AAINR27762, Theses scanned by UMI/ProQuest. |
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