On the burning behavior of radiatively-ignited bulk titanium and magnesium in low gravity

A preliminary investigation on the effect of reduced gravity on ignition and combustion of metals has been conducted with bulk titanium and magnesium samples. The present work describes the technical characteristics of the experimental system used, the analytical techniques employed, and the results obtained from the ignition of samples under normal and low gravity conditions. A 1000 W xenon short-arc lamp irradiates the top surface of cylindrical specimens, 4 mm in diameter and 4 mm high, in a quiescent, pure-oxygen environment at 1 atm. Reduced gravity is obtained from an aircraft flying parabolic trajectories. Values of transition, critical and ignition temperatures are obtained from thermocouple records. Qualitative observations and burning rates are extracted from high-speed cinematography. Emission spectra of gas-phase reactions are obtained with an imaging spectrograph/diode array system. Preliminary results show that magnesium samples have shorter ignition times and slower burning rates at low gravity, while titanium samples exhibit longer ignition times and slower burning rates under the same conditions. An explanation for these phenomena is introduced, which is based on the effect of buoyancy-induced convection on heat transfer and oxygen transport mechanisms. In addition, the effect of high applied heating rates on critical and ignition temperatures is discussed. Graduate Student, Member AIAA Professor, Member AIAA Professor, Associate Fellow AIAA Introduction In the past four years, metal combustion studies at the University of Colorado have focused on the effects of gravity (g) on the ignition and burning behavior of a variety of bulk, pure metal specimens. The impetus behind this effort is the interest to understand the flammability properties of structural metals found in a variety of oxygen systems in spacecraft. Since these systems are subjected to higher-than-one g loads during launch and reentry and to a zero-gravity environment while in orbit, the study of ignition and combustion of bulk metals at different gravitational accelerations is of great practical concern. From the scientific standpoint, studies conducted under low gravity conditions provide simplified boundary conditions since buoyancy is removed, and make possible the identification of fundamental ignition mechanisms. In the early stages of our research effort, a computational model of the proposed experimental system was developed to explore the effects of gravity and pressure on the heating phase of a metal sample under non-oxidizing conditions. The model includes the effects of conduction, convection and radiation heat transfer in the solid specimen and the surrounding gas. The results from the model predict a decreasing value on the steady-state temperature of the metal specimen (in an asymptotic fashion), slower heating rates and progressively longer delays for ignition as gravity is increased from zero to higher g's. After completing the normal gravity tests we proceeded to explore the effects of high g's on the ignition and combustion of titanium (Ti) samples. A geotechnical centrifuge was used to generate gravity levels above the terrestrial value. It was found that shorter ignition times and faster burning rates were possible in bulk metal samples subjected to gravity levels greater than 10 g's. Copyright © 19% by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. This result, contrary to the heat transfer arguments proposed by the model, was attributed to the faster rates of oxygen transport produced by natural convection. The influence of microgravity on the combustion of bulk metals has been investigated by Steinberg, et al. on a drop tower simulator. All the metals and alloys tested (including titanium, aluminum, magnesium, zinc and iron), using the promoted ignition test, supported combustion in the absence of gravity. The study found that in microgravity, the regression rate of the melting surface of a sample ignited in the bottom is significantly faster than in normal gravity. In this particular case, a faster propagation of a reaction front was possible because the combustion products do not detach as it occurs in upward propagation in 1 g. Glassman, et al. made a number of theoretical predictions on the possible role of gravity on the burning of magnesium (Mg) and aluminum (Al) droplets by developing a quasi-steady, vaporphase diffusion flame theory including flame radiation, transport of condensed oxide products, and evaporation of the metal. They argued that when the amount of condensed oxide in the flame front increases, its outward transport becomes governed by gravity, the interaction of the external flow field with the gas layer surrounding the flame, by inertial forces experienced by the droplet and by collisions between droplets. To date, no experimental work has confirmed the above theories by studying the ignition of bulk metals on low gravity fields. This preliminary study is a first step to fill this void by presenting the first experimental findings on the effect of reduced gravity on the ignition of metals in bulk form. Objectives and Approach For this phase of our investigation, the primary objective is to collect the first evidence on the effect of reduced gravity on the ignition and combustion characteristics of bulk metals. 11 was decided early on the design process to use one of NASA's experimental aircraft as a microgravity simulator to provide the required amount of reduced-gravity time to heat up and bum the metal samples (approximately 20 seconds). In consequence, an experimental apparatus had to be designed to withstand the variable g environment in the aircraft, provide a totally automated operation of the process to reduce human interaction and error, and collect as much information as possible during the brief periods of reduced gravity allowed in the aircraft. Details of the experimental system used are given in the next section. The analytic techniques to be used in the study are: a) metal surface thermometry, b) surface and flame visualization, c) emission spectroscopy, and d) surface morphology and chemical analysis. The metals selected for this study, titanium (Ti) and magnesium (Mg), were chosen because of their importance as elements of structural metals and their simple chemical composition (pure metals instead of multi-component alloys to avoid complication in morphology and spectroscopic studies). These samples were also chosen to study the two different combustion modes experienced by metals: heterogeneous or surface oxidation and homogeneous or gas-phase reaction. In addition, these specimens exhibit different melting and boiling temperatures, heats of combustion, oxide formation processes and flame temperatures. The experimental approach provides surface temperature profiles, transition, critical and ignition temperature values, pressure records, spectroscopic measurements, surface morphology, x-ray spectrometry of metals specimens and their combustion products, and high-speed cinematography of the heating, ignition and combustion stages of the metal specimen. These techniques are applied for both normal and low gravity conditions. Experimental System and Procedures Figure 1 shows a schematic of the various components used in the Ignition and Combustion of Metals (ICOM) experiment. This system is used in both the normal and the low gravity tests. The non-intrusive, ignition source consists of a 1000 W xenon lamp with 250 W of effective broadband output radiation power. The high-intensity light comes out of the lamp in a highly collimated beam and is then intercepted by an aspheric lens. The beam is then focused down to a 4 mm spot on the top surface of a metal sample providing 1.75 MW/m2 power density. An electric shutter was installed to block the light beam for precise sample heating. American Institute of Aeronautics and Astronautics