Jad Sadek

Date of Graduation

2015

Document Type

Thesis

Degree Type

MS

College

Statler College of Engineering and Mineral Resources

Department

Mechanical and Aerospace Engineering

Committee Chair

V'yacheslav Akkerman

Committee Co-Chair

Patrick Browning

Committee Member

Cosmin Dumitrescu

Abstract

Obstructed pipes constitute one of the most relevant configurations for extremely fast premixed flame acceleration and deflagration-to-detonation transition. While the flame propagation through obstacles is often associated with turbulence and/or shocks, a conceptually-laminar and shockless mechanism of extremely fast flame acceleration in semi-open "tooth-brush"-like obstructed pipes has been developed by Bychkov et al [ Phys. Rev. Lett. 101 (2008) 164501]. Namely, a flame front is ignited at the closed end of a pipe, with the flame propagating towards the open pipe end. This acceleration scenario is devoted to a powerful jet-flow, which is produced by delayed combustion in the spaces between the obstacles. This mechanism is scale-invariant (Reynolds-independent), with turbulence playing only a supplementary role in the flame evolution. In the present work, the Bychkov formulation is extended from semi-open channels and tubes to open or vented ones, for the sake of the industrial needs fulfillment, and in order to describe the recent experiments at Karlsruhe Institute of Technology (KIT), Germany [http://arxiv.org/abs/1208.6453]. Both two-dimensional channels and cylindrical tubes are studied. It is demonstrated that flames accelerate extremely fast in open/vented obstructed pipes, with tubes providing stronger acceleration as compared to channels of the same width. The acceleration mechanism is qualitatively the same as that for the semi-open pipes with the ignition at the closed end: namely, it is conceptually-laminar, shockless, and Reynolds-independent, being associated with the delayed burning in pockets between the obstacles. Although the acceleration rate is large enough in open obstructed pipes, it is nevertheless lower than that in the semi-open ones, because the flame-generated flow spreads both upwards and downwards of the flamefront when both pipe ends are open. Starting with obstructed pipes within the inviscid approximation, the analysis subsequently incorporates the viscous forces (hydraulic resistance), comparing their roles with that of the jet-flow driving the acceleration. It is shown that, on the contrary to the common belief, hydraulic resistance is not required to drive the flame acceleration. In contrast, this is a supplementary effect, which actually moderates the acceleration. Besides, hydraulic resistance can be responsible for the initial delay, before the flame acceleration onset, observed in the experiment.

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