Ignition and combustion of bulk metals under elevated, normal and reduced gravity conditions

Content Provider	Semantic Scholar
Author	Abbud-Madrid, Angel Branch, Melvyn C.
Copyright Year	1995
Abstract	Angel Abbud-Madrid, Melvyn C. Branch, and John W. DaffyCenter for Combustion ResearchMechanical Engineering DepartmentUniversity of ColoradoBoulder, Colorado 80309-0427IntroductionAlthough experiments on combustion of metals can be traced back as early as 1782, it was not untilviolent reactions were observed between titanium (Ti) and oxygen (02) in the 1950's that a significantnumber of studies focused on the compatibility of metals with oxygen (ref. 1). Later, studies focusedprimarily on metal powders due in part for the use of metal additives in solid rocket propellants. Morerecently, attention has been given to the flammability properties of bulk, structural metals, due to thelarge number of accidents involving burning metal components in oxygen systems. In spite of all theinvestigations conducted on this field to date, there is still a lack of basic understanding of the processescontrolling ignition and burning of bulk metals compared to the literature available for gaseous reactions.The difficulty of providing accurate models stems from the variety of complex variables affecting metal-oxygen reactions such as: metal type, phase changes, oxygen pressure and solubility in metals,mechanical and thermal properties of oxide layers, gravitational force, experimental apparatus andconfiguration, surface tension effects, and sample size to name a few.This research effort is aimed at providing further insight into this multi-variable dependent phenomenaby looking at the effects of gravity on the ignition and combustion behavior of metals. Since spacecraftare subjected to higher-than-lg gravity loads during launch and reentry and to zero-gravityenvironments while in orbit, the study of ignition and combustion of bulk metals at different gravitationalpotentials is of great practical concern. From the scientific standpoint, studies conducted undermicrogravity conditions provide simplified boundary conditions since buoyancy is removed, and makepossible the identification of fundamental ignition mechanisms. The effect of microgravity on thecombustion of bulk metals has been investigated by Steinberg, et al. (ref. 2) on a drop tower simulator.However, no detailed quantitative work has been done on ignition phenomena of bulk metals at lower orhigher-than-normal gravitational fields or on the combustion characteristics of metals at elevated gravity.The primary objective of this investigation is the development of an experimental system capable ofproviding fundamental physical and chemical information on the ignition of bulk metals under differentgravity levels. The metals used in the study, iron (Fe), titanium (Ti), zirconium (Zr), magnesium (Mg),zinc (Zn), and copper (Cu) were selected because of their importance as elements of structural metals andtheir simple chemical composition (pure metals instead of multi-component alloys to avoid complicationin morphology and spectroscopic studies). These samples were also chosen to study the two differentcombustion modes experienced by metals: heterogeneous or surface oxidation, and homogeneous or gas-phase reaction. The experimental approach provides surface temperature profiles, spectroscopicmeasurements, surface morphology, x-ray spectrometry of metals specimens and their combustionproducts, and high-speed cinematography of the heating, ignition and combustion stages of the metalspecimen. This paper summarizes the results obtained to date from experiments conducted undernormal and high-gravity conditions.* This work is supported by the National Aeronautics and Space Administration, Lewis Research Center underGrants NAG3-I257 (May 1991 -November 1994) and NAG3-1685 (December 1994 -December 1997).
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Language	English
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