Journal
COMBUSTION AND FLAME
Volume 225, Issue -, Pages 406-416Publisher
ELSEVIER SCIENCE INC
DOI: 10.1016/j.combustflame.2020.11.005
Keywords
Pyrotechnics; Energetic materials; Metal combustion; Alkali metal-doping; Plasma-assisted combustion
Categories
Funding
- SERDP [WP19-1163]
- NSWC-Crane Naval Innovative Science and Engineering Program
- Air Force Office of Scientific Research [FA9550-15-1-0195, FA9550-17-1-0167]
- Iowa State University Alliance for Graduate Education and the Professoriate program
- NASA Iowa Space Grant Consortium [NNX16AL88H]
- NASA [NNX16AL88H, 900227] Funding Source: Federal RePORTER
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The study demonstrates the modulation of light emission intensity of pyrotechnic flames using 2.46 GHz microwave energy, achieving increased atomic photo emission from alkali species. The findings suggest that microwave illumination of alkali-containing pyrotechnic flames may be a useful strategy to achieve dynamic control of light emission intensity, especially in low-temperature pyrotechnic plumes.
Creating next-generation pyrotechnic emitters capable of dynamically controllable light output requires a paradigm shift away from emission control via formulation. This work demonstrates the ability to modulate light emission intensity of a pyrotechnic flame with 2.46 GHz microwave energy within a multimodal cavity. Stoichiometric mixtures of three pyrotechnic systems are investigated: magnesium fuel with oxidizers of NaNO3, KNO3, or CsNO3. Using time-resolved visible and infrared emission spectroscopy and high-speed color videography, microwave illumination of flames is found to produce enhanced atomic photo emission from the alkali species. Emission increases of up to similar to 120% are demonstrated, which are predominantly in the visible and near-infrared wavelengths and have little effect on flame emission chromaticity. At near- and mid-IR wavelengths, gray body continuum enhancement is observed with moderate enhanced emission from CO2 and H2O bands. Sustained light emission from microwave illumination of combustion products long after pyrotechnic extinguishment was also demonstrated. A simplified model of microwave-enhanced visible and near-IR emission is presented and shown to be consistent with the observed trend of elevated emission enhancement for lower wavelength alkali transitions. Sensitivity analysis is performed which suggests a lower equilibrium population of electronically excited alkali atoms is primarily responsible for maximizing the degree of light emission enhancement of applied microwave fields, especially in extinguished low-temperature pyrotechnic plumes. These findings suggest microwave illumination of alkali-containing pyrotechnic flames may be a useful strategy to achieve dynamic control of light emission intensity. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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