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https://hdl.handle.net/2142/87736
Description
Title
Edge-Flames and Combustion at the Microscale
Author(s)
Kessler, David A.
Issue Date
2006
Doctoral Committee Chair(s)
Short, Mark
Department of Study
Theoretical and Applied Mechanics
Discipline
Theoretical and Applied Mechanics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, Fluid and Plasma
Language
eng
Abstract
"Numerical studies are performed to examine several phenomena relevant to the dynamic behavior of laminar flames in devices whose critical dimensions are at or below classical quenching limits. The behavior of premixed and nonpremixed edge-flames in tune-periodic counterflows is modeled using a constant-density approximation and a single-step Arrhenius reaction rate. Results of numerical simulations indicate that the unsteady strain rate is generally a destabilizing influence that expands the range of Damkohler numbers for which instabilities occur for small Lewis numbers and affects the quenching limits of edge-flames with a Lewis number of unity. The thermal coupling between the combustion field and conductive burner walls in externally-heated, straight-tube, single-pass microburners is studied numerically for premixed and nonpremixed reactants. The transient dynamics of the premixed flame fronts are controlled by the properties of the wall material. The level of external heat loss dictates the ultimate steady-state behavior, allowing periodic extinction/reignition, stable oscillations, and stationary flames. A new type of ""tuning fork"" flame structure is formed in nonpremixed microburners clue to the interaction of the thermal layers formed along the heated channel walls with the mixing layer of the reactants. External heat losses cause this flame structure to oscillate. The speed and quenching limits of premixed flames in a thin, nonadiabatic channel calculated using a constant-density model are compared with those obtained using a model that allows density variations due to thermal expansion of the combustion gases. Two-dimensional numerical simulations show that at low levels of heat loss, stretching caused by the thermal-expansion-induced flow causes an increase in flame surface area and propagation speed. However, thermal expansion is shown to facilitate quenching at lower levels of heat loss. Finally, the development of a two-dimensional simulation of the flow in a long, thin channel with side-wall mass injection using the full compressible Navier-Stokes equations is described. Initial results suggest that finite Mach number effects contribute to edge-flame formation when the local Mach number of the flow approaches unity. A pulsating edge-flame is observed for a fuel mixture with Lewis number equal to two."
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