Date of Graduation

2017

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

V'yacheslav Akkerman

Committee Member

Hailin Li

Committee Member

Arvind Thiruvengadam

Abstract

Continuous fire safety hazards stimulate in-depth learning and understanding of what causes an initially slow premixed flame (deflagration) front to accelerate and eventually detonate. Such flame acceleration (FA) and deflagration-to-detonation transition (DDT) are intriguing phenomena that have both fundamental interests and practical relevance. On one hand, it is the desire to prevent FA and DDT to avoid or, at least, mitigate unwanted explosions or fires. On the other hand, FA and DDT can be utilized, constructively, in the novel energy-efficient combustion technologies such as micro-combustors, pulse-detonation or rotation-detonation engines. A flame accelerates in tubes or channels, with acceleration being most intensive in obstructed pipes. The latter fact has been known for a while, but this acceleration was typically devoted to turbulence or shocks. In contrast, the Bychkov scenario of FA in channels, equipped with a tooth-brush-like array of tightly-packed obstacles, is shockless and conceptually laminar, with turbulence playing only a supplementary role. In spite of the laminar nature, this FA is extremely strong and leads to DDT.;The geometry of the Bychkov mechanism is the following: one end of an obstructed channel is closed, while the other end is open or vented; a flame embryo is ignited at the closed end and then it accelerates towards the open one. This mechanism has been identified and quantified analytically and substantiated by the comprehensive computational simulations. However, the Bychkov theory and modeling employed various simplifications, including that of equidiffusive combustion, i.e. the Lewis number, Le (the thermal-to-mass diffusivities ratio) is unity. While the latter is a conventional approach in combustion science, flames are usually non-equidiffusive in the practical reality, with Le belonging to the key parameters controlling the flame dynamics and morphology. Consequently, there has been a critical need to scrutinize the impact of the Lewis number in obstructed channels, which is addressed in the present thesis.;Specifically, acceleration of non-equidiffusive flames in obstructed channels is investigated by means of computational simulations of the reacting flow equations with fully-compressible hydrodynamics and Arrhenius chemical kinetics. A detailed parametric study is performed for the Lewis number in the range 0.2 ≤ Le ≤ 2.0, blockage ratio (BR) being 1/3~2/3, the spacing between the obstacles Deltaz/R = 1/4~1/2, and the channel width 48 ≤ D/Lf ≤ 96, where Lf is the thermal flame thickness. It is shown that Le > 1 flames accelerate slower than equidiffusive ones, due to a flame thickening. In contrast, Le < 1 flames acquire stronger distortion, associated with the diffusional-thermal combustion instability, and thereby accelerate much faster than at Le = 1.

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