Single Particle Combustion of Pre-Stressed Aluminum.

Kevin J Hill,Joseph Kalman,Ephraim Washburn,Michelle L Pantoya

doi:10.3390/ma12111737

Abstract

An approach for optimizing fuel particle reactivity involves the metallurgical process of pre-stressing. This study examined the effects of pre-stressing on aluminum (Al) particle ignition delay and burn times upon thermal ignition by laser heating. Pre-stressing was by annealing Al powder at 573 K and quenching ranged from slow (i.e., 200 K/min) identified as pre-stressed (PS) Al to fast (i.e., 900 K/min) identified as super quenched (SQ) Al. Synchrotron X-ray Diffraction (XRD) analysis quantified an order of magnitude which increased dilatational strain that resulted from PS Al and SQ Al compared to untreated (UN) Al powder. The results show PS Al particles exhibit reduced ignition delay times resulting from elevated strain that relaxes upon laser heating. SQ Al particles exhibit faster burn times resulting from delamination at the particle core-shell interface that reduces dilatational strain and promotes accelerated diffusion reactions. These results link the mechanical property of strain to reaction mechanisms associated with shell mechanics that explain ignition and burning behavior, and show pre-stressing has the potential to improve particle reactivity.

Highlights

Aluminum (Al) is an important solid fuel for propulsion and pyrotechnic applications because of its high (85 GJ/m3 ) stored chemical energy, and its burning behavior at a range of particle sizes and environments are of great interest
The largest percent decrease in burn time compared to untreated Al (UN Al) for both Pre-stressing aluminum (PS Al) and super quenched (SQ) Al occur when only two standard deviations of data are considered, indicating outliers in the data that may be a function of agglomeration
The PS Al has the largest decrease in ignition delay, with little difference depending on how many standard deviations were considered

Summary

Introduction

Aluminum (Al) is an important solid fuel for propulsion and pyrotechnic applications because of its high (85 GJ/m3 ) stored chemical energy, and its burning behavior at a range of particle sizes and environments are of great interest. Ignition delay and burn times of Al particles are important to understand because reduced Al particle burn time and ignition delay can provide added energy to facilitate propellant surface burning regression or reduce particle agglomeration on the burning propellant surface [1]. A promising approach to improve Al particle ignition delay and burn time is pre-stressing [2,3]. The induced stresses place the shell in compression, and change the dynamics of shell failure during ignition and subsequent combustion. Considerable studies have been undertaken on pre-stressed Al particles under impact [2,3], but no studies have been conducted on the combustion of pre-stressed Al particles under thermal loads for single particle conditions

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Conclusion