Experimental study of flame-spreading processes over thin aluminum sheets

An experimental study of flame-spreading behavior over thin aluminum (99% Al and 1% Mn) sheets was conducted in oxygen-enriched environments. This subject is of great interest since the occurrence of violent energy release (VER) phenomena in promoted ignition and combustion tests of brazed aluminum heat exchangers (BAHX) seems to be strongly coupled with flame-spreading processes. The objective of this study is to determine the dependency of flame-spreading rate of aluminum sheets as a function of oxygen pressure, oxygen purity, oxygen flow condition, sample thickness, and multiple layer configuration. The flame-spreading rate over aluminum sheets was measured by an array of fast-response lead-selenide (Pb-Se) IR photodetectors. The overall transient event of the ignition and flame spreading of aluminum sheets was recorded by both a high-speed movie camera and a CCD video camera. The initial chamber pressure was varied from 0.1 to 10 MPa. Two grades of oxygen gas were used with purities of 99.996% and 99.75%. Tests were conducted either with stagnant oxygen or under a continuous oxygen flow with oxygen velocity ranging from 5 to 15 m/s. The dimension of aluminum samples with three different thicknesses (0.15, 0.20, and 0.40 mm) was 2.5 cm by 43 cm. Besides tests with single aluminum sheets, tests were conducted using three sheets mounted with a controlled gap width of 1.6 mm. 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. The flame-spreading rates under high-purity oxygen (99.996%) environments are greater than those in commercial grade oxygen (99.75%). The threshold pressure for self-sustained flame propagation is lower under high-purity oxygen environments and also for samples with a smaller thickness. The oxygen flow can enhance the flame-spreading rate to some extent. There exists an optimum oxygen velocity beyond which the flame-spreading rate decreases with the increase of oxygen velocity. The mechanisms associated with aluminum combustion and VER phenomenon are also discussed.