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https://hdl.handle.net/2142/21616
Description
Title
Modelling of microgravity ignition phenomena
Author(s)
Lee, Changjin
Issue Date
1992
Doctoral Committee Chair(s)
Buckmaster, John
Department of Study
Aerospace Engineering
Discipline
Aerospace Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Aerospace
Engineering, Mechanical
Language
eng
Abstract
The structure and stability of flame balls and the characteristics of the outwardly propagating spherical flames of sub-limit mixtures are studied both analytically and numerically in the context of an asymptotic analysis in which $\theta\to\infty$, and (1 $-$ Le) = O(1/$\theta$), where $\theta$ is the activation energy and Le is Lewis number. For flame balls, the heat loss mechanism necessary for stable solutions is the radiation from burnt gas. If heat loss is less than quenching value, there are two stationary solutions, but the small branch solution is always unstable to the one dimensional disturbances regardless of Lewis number when Le $\leq$ 1. And the large branch solution is stable within a certain range of heat loss, if Lewis number is less than the critical Lewis number Le$\sb{\rm c}$ (Le $<$ Le$\sb{\rm c}\/<$ 1). The flame balls with sufficiently large radius are always unstable to three dimensional disturbances, independent of Le. These results, therefore, reveal that flame balls can only be possible if Le $<$ Le$\sb{\rm c}$ where Le$\sb{\rm c}$ is identified very close to unity. The three dimensional stability is not Lewis number but near field heat losses driven.
For spherically propagating flames, the evolution of flame after the flame initiation subject to radiative heat loss and confinement effect(pressure rise of unburned mixture due to finite chamber volume) was studied using numerical calculation of asymptotically derived model. The Lewis number effect (Le $<$ 1) enhances flame intensity but radiative cooling attenuates the intensity through the inflow velocity behind flame front. The pressure rise enhances the burning as flame radius is comparable to the chamber radius. The exchange of these two competitive mechanism leads to Self-Extinguishing Flames (SEFs), and the confinement effect causes flames to be extremely sensitive to the initial data.
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