Melvyn C. Branch

Combustion of Magnesium with Carbon Dioxide and Carbon Monoxide at Low Gravity

The burning behavior and x8f ame structure of magnesium in pure carbon dioxide and pure carbon monoxide atmospheres in low-gravity conditions are investigated. Cylindrical specimens are suspended by a thermocouple wire and are radiatively ignited. Spherical x8f ames are obtained during steady-state burning of the metal sample with increasing metal-oxide accumulation in an outer shell. Burning times twice as long as in normal gravity are observed, revealing a diffusion-controlled reaction. The burning time is proportional to the square of the metal sample diameter. Combustion of magnesium with carbon monoxide is not possible without constant heating of the sample. A one-dimensional, quasi-steady numerical model of the spherically symmetric diffusion x8f ame using elementary gas-phase reactions and detailed transport property calculations shows qualitative agreement with the observed structure of the x8f ames. It predicts a maximumtemperature close to the vaporization–decomposition point of themetal oxide, as well as the coexistence of the gaseous and condensed phases of the oxideproduct. It also predicts a diffusion-controlledreaction formagnesiumburning in oxygen, air, and carbon dioxide and provides an accurate comparison of the burning rates of these systems. The discrepancies between the numerical simulation and the experimental observations may be attributed to the absence of accurate condensation, radiation, and surface-reaction models.

Structure and chemical kinetics of flames supported by solid propellant combustion

The structure and chemical kinetics of gaseous flames are important aspects of the combustion of solid propellants. Advanced diagnostics have led to specific knowledge of solid propellant decomposition products, and advanced computer programs have allowed detailed chemical kinetic modeling of these products reacting with each other. There has therefore been considerable effort aimed at measuring and modeling the structure of premixed gas flames of fuels and oxidizers representative of the chemistry occurring in actual solid propellants. Most of the recent modeling work for representative gaseous flames can be traced to a mechanism first developed to model air pollution. Comprehensive mechanisms have also been developed to model the entire gas-phase chemistry process above burning propellant surfaces. These comprehensive mechanisms also are largely traceable to the air pollution modeling mechanism. Russian researchers have also made significant advances in the comprehensive modeling of solid propellant flame structures. Overall, the current state of knowledge of the gas-phase structure of solid propellants is incomplete, but encouraging agreement exists between modeling and experimental data. Recommendations are made for additional flame and reaction studies and for a standard mechanism to use in all gaseous flame modeling.

Ignition and combustion of bulk titanium and magnesium at normal and reduced gravity

An investigation of the effect of gravity on ignition and combustion of metals has been conducted with bulk titanium and magnesium samples. A 1000-W xenon 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 critical and ignition temperatures are obtained from thermocouple records. Qualitative observations and propagation rates are extracted from high-speed cinematography. Emission spectra of gas-phase reactions are obtained with an imaging spectrograph/diode array system. High applied heating rates and large internal conduction losses generate critical and ignition temperatures that are several hundred degrees above the values obtained from isothermal experiments. Because of high conduction and radiation heat losses, no appreciable effect on ignition temperatures with reduced convection in low gravity is detected. Lower propagation rates of the molten interface on titanium and of ignition waves on magnesium are obtained at reduced gravity. These rates are compared to theoretical results from heat conduction analyses with a diffusion/convection controlled reaction. The close agreement found between experimental and theoretical values indicates the importance of the influence of natural convection-enhanced oxygen transport on combustion rates. Lower oxygen flux and lack of oxide product removal in the absence of convective currents appear to be responsible for longer burning times of magnesium diffusion flames at reduced gravity. The accumulation of condensed oxide particles in the flame front at low gravity produces a previously unreported unsteady explosion phenomenon in bulk magnesium flames. This spherically symmetric explosion phenomenon seems to be driven by increased radiation heat transfer from the flame front to an evaporating metal core covered by a porous, flexible oxide coating.