Flame-spreading Process over Thin Aluminum Sheets in Oxygen-enriched Environments

An experimental study of flame-spreading process over thin aluminum (99% Al and 1% Mn) sheets was investigated in oxygen-enriched environments. The objective of this study was to determine the dependency of flame-spreading rate over aluminum sheets as a function of initial chamber pressure, sample thickness, oxygen purity, oxygen flow condition, and sample orientation. The reaction mechanism of aluminum in oxygen was also studied by examining the recovered partially-burned sample using a scanning electron microscope (SEM) coupled with an energy dispersive spectrometer (EDS). The flame-spreading rate over aluminum sheets was measured by an array of fast-response lead-selenide (Pb-Se) IR photodetectors. The initial chamber pressure was varied from 0.1 to 6.3 MPa. Two grades of oxygen gas were used with purities of 99.996% and 99.75%. In terms of the effect of pressure on the flame-spreading rate, as the initial chamber pressure was increased, the flame-spreading rate was found to increase to a maximum, decrease to a minimum, and then increase again. Based upon the comparison of flame-spreading rates in horizontal, upward, and downward orientation, the flame-spreading process over aluminum sheets was found to be dominated by the solid-phase heat conduction mechanism. The continuous oxygen flow showed a strong influence on the flame-spreading behavior, and it was demonstrated that the flame can be blown off when the counter-current flow velocity exceeds a critical value. The flame-spreading rates under high-purity (∼99.996%) oxygen environments were found to be significantly greater than those in commercial grade (∼99.75%) oxygen. In addition, the oxygen content in the white ceramic-type nodules formed on the burned edge of the recovered partially-burned sample is much higher than that on the unburned surface. These imply that there exist heterogeneous reactions between aluminum and either oxygen or gaseous aluminum sub-oxides on the burning surface.