Theses and Dissertations from UMD

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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM

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2007 - 20102

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Subject: search.filters.subject.Asymptotic Analysis

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    Direct Numerical Simulation of Non-Premixed Flame Extinction Phenomena
    (2010) Narayanan, Praveen; Trouve, Arnaud C; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Non-premixed flame extinction phenomena are relevant in a variety of com- busting environments, including but hardly limited to diesel engines, pool fires, and fire suppression scenarios. These disparate phenomena are controlled by various parameters that contain information on flame stretch, heat losses, composition of the fuel and oxidizer supply streams, etc. Direct Numerical Simulation (DNS) is used in the present study to provide fundamental insight on diffusion flame extinction under non-adiabatic combustion conditions. The list of DNS configurations include: (C1) counterflow laminar flames with soot formation and thermal radiation transport; (C2) coflow turbulent flames with soot formation and thermal radiation transport; (C3) counterflow laminar and turbulent flames interacting with a mist-like water spray. Configurations C1 and C2 use single-step chemistry while configuration C3 uses detailed chemistry (all cases correspond to ethylene-air combustion). Configuration C1 is also treated using large Activation Energy Asymptotics (AEA). The AEA analysis is based on a classical formulation that is extended to include thermal radiation transport with both emission and absorption effects; the analysis also includes soot dynamics. The AEA analysis provides a flame extinction criterion in the form of a critical Damköhler number criterion. The DNS results are used to test the validity of this flame extinction criterion. In configuration C1, the flame extinction occurs as a result of flame stretch or radiative cooling; a variation of configuration C1 is considered in which the oxidizer stream contains a variable amount of soot mass. In configuration C1, flame weakening occurs as a result of radiative cooling; this effect is magnified by artificially increasing the mean Planck soot absorption coefficient. In configuration C3, flame extinction occurs as a result of flame stretch and evaporative cooling. In all studied cases, the critical Damkohler number criterion successfully predicts transition to extinction; this result supports the unifying concept of a flame Damköhler number Da and the idea that different extinction phenomena may be described by a single critical value of Da.
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    AN ASYMPTOTIC ANALYSIS OF SPONTANEOUS IGNITION OF HYDROGEN JETS
    (2007-04-26) lim, kianboon; Sunderland, Peter B; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Analytical work is presented for the spontaneous ignition of a hydrogen jet emanating from a slot into air. A similarity solution of the flowfield was obtained. This was combined with the species and energy conservation equations, which were solved using activation energy asymptotics. Limits of spontaneous ignition were identified as functions of slot width, flow rate, and temperatures of the hydrogen jet and ambient air. Two scenarios are examined: a cool jet flowing into a hot ambient and a hot jet flowing into a cool ambient. For both scenarios, ignition is favored with an increase of either the ambient temperature or the hydrogen supply temperature. Moreover, for the hot ambient scenario, a decrease in local fuel Lewis number also promotes ignition. The Lewis number of the oxidizer only has a weak effect on ignition. Because spontaneous ignition is very sensitive to temperature, ignition is predicted to occur near the edge of the jet if the hydrogen is cooler than the air and on the centerline if the hydrogen is hotter than the air.