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Title:High temperature spectroscopic measurements of aluminum combustion in a heterogeneous shock tube
Author(s):Lynch, Patrick T.
Director of Research:Glumac, Nick G.
Doctoral Committee Chair(s):Glumac, Nick G.
Doctoral Committee Member(s):Krier, Herman; Kyritsis, Dimitrios C.; Scheeline, Alexander
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Abstract:Aluminum is an important energetic material that burns with a variety of oxidizers with a high reaction enthalpy. It is a top performer for energy density if oxidizer mass is considered as well as the fuel. Because of this, it is often used as an energetic additive in anaerobic conditions because it reacts in what typically are products of primary fuels and oxidizers and further increases chamber temperatures in solid rocket motors or blast overpressure in enhanced blast weapons. Despite decades of research, gaps still exist in the knowledge of how aluminum burns, especially with the recent trend of using smaller particles (10 micron and below). While aluminum is a very energetic material, its rate of oxidation is relatively low, and efforts are made to not only increase the heat release rate of particles reacting with oxidizer, but also to increase peak combustion temperature. Experimental studies were performed measuring the combustion characteristics of aluminum in the heterogeneous shock tube. The heterogeneous shock tube provides unmatched control of temperature, pressure, oxidizer concentration, etc., in which to test energetic materials of different particle sizes. The burning time diameter exponent, n as in t_b ~ d^n, in aluminum particle combustion was measured to be as low as, or lower than 0.3, in conditions in the transition regime between kinetic and diffusion limited particles. This anomalous result, as well as observed increases of burn time with pressure when using water vapor as an oxidizer had been attributed to a pressure dependence on ignition of particles in these conditions or broad overlapping size distributions. Both theories were tested and rejected. Furthermore, an acrylic end section was implemented on the shock tube which provides complete optical access to the final 61 cm of the shock tube. This optical access allowed high speed images (50k fps) of the particle motion, ignition, and combustion. Results are presented which give a more complete understanding of burntime variability in the heterogeneous shock tube, owing to the contribution of initially wall-bound particles vs. those that are in the free stream upon the passage of the incident shock, bright clusters of rapidly moving burning particles, and non-uniform cloud distributions in the tube, all previously undifferentiated by shock tube burntime methods. Absorption spectroscopy was used to probe the ground state of Aluminum monoxide (AlO), a gas phase combustion intermediate, and Al vapor in order to quantify the amount of Al and AlO present under conditions where these species were not observed in emission previously, notably in most conditions with nano-aluminum particles. At least three regimes of combustion were observed for nano-aluminum combustion. At temperatures above 2000 K, particles burn with AlO and Al vapor present. Between 1500 K and 2000 K, particles burn with Al vapor present, but without detectable AlO. Between 1200 K and 1500 K, particles burn without either vapor phase component present. These results had important implications for two proposed mechanisms for nano-aluminum ignition and combustion. Optical measurements of the peak combustion temperature from AlO consistently measure near 3200 K in micro-Al combustion, even though higher temperatures are seen near larger particles. One proposed limiting factor was the volatilization temperature of alumina. The reference literature is divided on this temperature, which previously was measured with large extrapolations. The volatilization temperature was measured by measuring the extinction cross-section of nano- and micro-alumina at non-resonant wavelengths at different ambient temperatures. The volatilization temperature at 1 atm appears to be at least as high as 4000 K and does not appear to be a limiting temperature in micro-Al combustion Finally, two other sets of measurements were made to support high temperature measurements of aluminum combustion, especially in the optically thick conditions commonly observed in propellant plumes or explosive fireballs. The first was a measurement of the alumina emissivity spectral dependence, which is absolutely necessary to make pyrometric measurements. A significant temperature dependence was observed in the emissivity spectral dependence. Additionally, spatially resolved fiber optic emission probing was used in the optically thick fireballs from aluminized explosives. Using the probes allowed observations inside the fireball and avoided biases from temperature inhomogeneity and ambient air interactions in these explosive fireballs.
Issue Date:2010-08-31
Rights Information:Copyright 2010 Patrick T. Lynch
Date Available in IDEALS:2010-08-31
Date Deposited:2010-08

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