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Physical Mechanism of Ultrafast Flame Acceleration
Institute of Physics, Umeå University.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
Department of Applied Mechanics, Chalmers University of Technology.
2008 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 101, no 16, 164501-1-164501-4 p.Article in journal (Refereed) Published
Abstract [en]

We explain the physical mechanism of ultrafast flame acceleration in obstructed channels used in modern experiments on detonation triggering. It is demonstrated that delayed burning between the obstacles creates a powerful jetflow, driving the acceleration. This mechanism is much stronger than the classical Shelkin scenario of flame acceleration due to nonslip at the channel walls. The mechanism under study is independent of the Reynolds number, with turbulence playing only a supplementary role. The flame front accelerates exponentially; the analytical formula for the growth rate is obtained. The theory is validated by extensive direct numerical simulations and comparison to previous experiments.

Place, publisher, year, edition, pages
2008. Vol. 101, no 16, 164501-1-164501-4 p.
Keyword [en]
Computer simulation, Direct numerical simulation, Reynolds number, Analytical formulas, Channel walls, Flame accelerations, Flame fronts, Physical mechanisms, Ultrafast
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-9148DOI: 10.1103/PhysRevLett.101.164501ISI: 000260141300023Scopus ID: 2-s2.0-54849405187OAI: oai:DiVA.org:kth-9148DiVA: diva2:25287
Note
QC 20100915. Uppdaterad från submitted till published (20100915).Available from: 2008-09-24 Created: 2008-09-24 Last updated: 2017-12-13Bibliographically approved
In thesis
1. Flame Dynamics and Deflagration-to-Detonation Transition
Open this publication in new window or tab >>Flame Dynamics and Deflagration-to-Detonation Transition
2008 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Various premixed flame phenomena are studied by means of direct numerical simulations of the complete system of hydrodynamic equations. Rigorous study of flame dynamics is essential for all premixed combustion problems where multidimensional effects cannot be disregarded.The present thesis consists of six parts. The first part deals with the fundamental problem of curved stationary flames propagation in free-slip tubes of different widths. It is shown that only simple "single-hump" slanted stationary flames are possible in tubes wider than some stability limit. The flame dynamics is shown to be governed by a large-scale stability mechanism resulting in a highly slanted flame front.The second part of the thesis is dedicated to studies of acceleration and fractal structure of outward freely propagating flames. It is shown that the development of Landau-Darrieus instability results in the formation of fractal-like flame front structure. Two-dimensional simulation of radially expanding flames displays a radial growth with 1.25 power law temporal behavior. It is shown that the fractal excess for 2D geometry obtained in thenumerical simulation is in good agreement with theoretical predictions.In third part the flame acceleration in tubes with non-slip at the walls is studied in the extremely wide range of flame front velocity. Flame accelerates from small initial velocity to supersonic speed in the laboratory reference frame. Flame acceleration undergoes three stages: 1) initial exponential acceleration in the quasi-isobaric regime, 2) almost linear increase of the flame speed to supersonic values, 3) saturation to a stationary high-speed deflagration velocity, which is correlated with the Chapman-Jouguet deflagration speed. The saturation velocity is in line with previous experimental results.In fourth part the role of viscous stress in heating of the fuel mixture in deflagration-to-detonation transition in tubes is studied both analytically and numerically. The developed analytical theory determines temperature distribution ahead of an accelerating flame. The heating effects of viscous stress and the compression wave become comparable at sufficiently high values of the Mach number. Viscous stress makes heating and explosion of the fuel mixture preferential at the walls.In fifth part we reveal the physical mechanism of ultra-fast flame acceleration in obstructed channels used in modern experiments on detonation triggering. It is demonstrated that delayed burning between the obstacles creates a powerful jet-flow, driving the acceleration. The flame front accelerates exponentially; theanalytical formula for the growth rate is obtained. The theory is validated by extensive direct numerical simulations and comparison to previous experiments.The last part of the thesis concerns the transition from deflagration to detonation. It is found that in sufficiently wide free-slip channels and for sufficiently fast flames Landau-Darrieus instability may invoke nucleation of hot spots within the wrinkled flame folds, triggering an abrupt transition from deflagrative to detonative combustion. Results on DDT in channels with non-slip at the walls are also presented.

Place, publisher, year, edition, pages
Stockholm: KTH, 2008. viii, 75 p.
Keyword
flame, premixed, instability, fractal, deflagration, detonation, DDT, simulation
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-4875 (URN)978-91-7415-115-2 (ISBN)
Public defence
2008-10-03, Sal B2, Materialvetenskap, Brinellvägen 23, KTH, Stockholm, 13:00 (English)
Opponent
Supervisors
Note
QC 20100915Available from: 2008-09-30 Created: 2008-09-09 Last updated: 2010-09-16Bibliographically approved

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