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.

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Measurements of high-temperature spectral emissivities of aluminum oxide were made within a heterogeneous shock tube over the spectral range of 650-900 nm. The spectral emissivity of optically thin micrometer-scale alumina particles scaled approximately as λ −1.4 from 2800 to 3500 K. Results from optically thin clouds of nanoscale alumina showed that a λ −1.2 dependence, closer to the λ −1 predicted by the Rayleigh limit, is appropriate over the same spectral and temperature ranges. For temperatures below the melting point of alumina, the emissivity of nanoalumina shows a significant temperature dependence. The effect multiple scattering has on the apparent emissivity is studied, and it is determined to contribute to the discrepancy between the current and previous works. A Monte Carlo simulation showed qualitative agreement with the experimental work. It was found that, at small to moderate optical depths, scattering is responsible for a small change in the spectral distribution of particle emissivity. At large optical depths, absorption has a much stronger effect on the apparent spectral emissivity. It was determined that choice of optical depth can strongly affect the results of pyrometry measurements. Nomenclature C = fit parameter F = calibrated spectral signal I = radiant intensity, W∕sr · m 2 L = length scale, m or cm l = length scale, m or cm k = absorptive index n = fit parameter R = randomly generated number R 2 = coefficient of determination T = temperature, K z = model coordinate, cm or m ϵ = emissivity κ = absorption coefficient λ = wavelength, nm or μm σ = scattering coefficient τ = optical depth Subscripts abs = absorption value bb = blackbody value cal = calibration value exp = experimental value new = new value prev = previous value sca = scattering value κ = absorption value λ = spectral dependence σ = scattering value