Using optical techniques to measure aluminum burning in post-detonation explosive fireballs

Aluminum;Combustion;Explosive Fireball;Optical Measurements

Peuker, Jennifer

关键词: Aluminum;    Combustion;    Explosive Fireball;    Optical Measurements;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/30968/Peuker_Jennifer.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Metals are added to high explosives and propellants to increase the heat of explosion. Aluminum is commonly used because it has a high energy density, is relatively inexpensive, is easy to produce, and has a low toxicity. To optimize the performance and safety of aluminized explosives, it is necessary to understand where, when, and with what the aluminum is reacting in the explosive fireball. Efforts in aluminum combustion have focused on aluminum monoxide (AlO) emission because it is easy to measure, and it is a combustion marker in some cases; however, the first part of the current study has indicated that explosive fireballs are optically thick. Therefore, external measurements are biased toward conditions near the fireball surface. The objectives of the current study are twofold: (1) to further the understanding of aluminum combustion in an explosive fireball, specifically where, when, and with what the aluminum is reacting; and (2) to characterize AlO emission measurements from aluminized explosive fireballs in order to determine when and how AlO emission can be used as an indicator of aluminum combustion.Experiments were completed in six different environments–air, pure O2, pure CO2, pure N2, 40%/60% O2/N2 and 20%/80% CO2/N2–using four distinct aluminized charges of varying aluminum particle size–3, 10, and 40 micron–and loading amount–20 and 50 percent–by mass, to determine with what the aluminum is reacting. In addition, a charge containing 20 percent aluminum oxide was used as an inert comparison.Contrasting results from optical–emission spectroscopy, pyrometry and high speed imaging–and non-optical techniques–recovered residue analysis and overpressure measurements–is used to challenge typical interpretations of optical measurements of aluminized explosive fireballs. The effect of the aluminum particle location with respect to the explosive material was tested by using end-loaded charges, and by placing a layer of grease on the aluminized charge tip. Time-resolved overpressure measurements are used to determine when the aluminum is burning. Experiments employing an air-gap between the explosive charge and aluminum powder aid in determining how and when aluminum is activated and combusted in the initial blast wave and the subsequent fireball containing high pressure and high temperature detonation products.Tests in four environments–air, pure O2, pure CO2, and pure N2–show that even when AlO emission intensity is lower by 90 percent in N2 or CO2 than it is in air for a charge, it is possible to have significant–60 to 70 percent–aluminum particle oxidation. In addition, substantial AlO emission was measured in the absence of unburned aluminum–almost half of the peak AlO emission measured when unburned aluminum was present. Results show that AlO emission intensity measurements are skewed to higher AlO intensities by high transient temperatures within the first 30 microseconds when the peak AlO emission is usually measured. The aluminum particle location also affects the amount of AlO emission measured such that when more particles are on the fireball surface, then more AlO emission is measured. However, the end-loaded aluminum does not add to the energy output enhancement as much as the pre-loaded aluminum charges since the peak pressures and initial impulse are similar for different amounts of aluminum. A grease layer on the tip of the charge reduces the amount of AlO emission measured by 90 percent, but has the same energy output in the initial blast wave as the same charge not having a grease layer, indicating that the material at the tip of a charge changes the breakout and subsequent AlO emission production. In addition, the overpressure measurements indicate that four distinct stages of aluminum combustion exist.The first stage is the detonation and the activation of the aluminum. In the second stage the aluminum burns to enhance the blast wave which is indicated by higher peak pressures and initial impulses than a charge not containing aluminum. During the third stage, the aluminum continues to burn to increase the overpressure of the chamber. The fireball cools during the fourth stage and any aluminum oxidation does not add to the energy release. The variations in how much AlO emission is measured indicate that interpreting AlO emission measurements from explosive fireballs is not straightforward with respect to correctly determining the amount of aluminum combusted, how long the aluminum reacted, or the energy released.If aluminum is available to burn and AlO emission is measured, then the aluminum is burning–even taking into account AlO emission from the oxide layer.However, when no AlO emission is measured, it does not necessarily mean that the aluminum is not burning. When AlO emission is measured it indicates that the temperatures are high enough to sustain aluminum combustion which produces AlO, and that oxidizers are present which react to produce the AlO emission. The relative intensities for the same time frame of AlO emission measured could be indicators about the temperature or number of reactions occurring. AlO emission measured from explosive fireballs is a result of both anaerobic and aerobic reactions, and both types of reactions contribute to the aluminum combustion enhancement of the explosive charge. The aluminum combustion adds to the blast wave enhancement and the aluminum also burns to increase the overpressure of the chamber.