Emission thermometry of microwave-assisted alkali-doped propellant combustion

Abstract

Recently, the application of high-power microwave fields to sodium-doped aluminized solid propellants has been shown to increase the atmospheric burning rate by up to 60%. Two main mechanisms have been proposed to increase the burning rate: (1) plasma enhancement by the presence of low-ionization threshold sodium and (2) the dielectric thermal runaway heating of metallic-oxide products. To examine the mechanisms of microwave energy deposition in the solid propellant burning strands, optical emission measurements were made during the application of a 2.46 GHz, 870 W microwave field in a single-mode microwave resonator. Flame temperature measurements are made via gas-phase AlO B2Σ+→X2Σ+(Δv=−1) electronic emission spectroscopy, spatially-resolved two-color imaging pyrometry of the condensed phase, and spatially-resolved two-line sodium emission thermometry. These experiments indicate an increase in the gas phase AlO temperature of ∼130 K, and a ∼1200 K increase in the sodium gas-phase temperature during the application of the microwave field. Gray body condensed-phase temperatures of burning aluminum particles indicated little temperature change, but the product plume temperature increased by ∼700 K. These temperature measurements of the vapor and condensed phase products indicate several important pathways for the thermalization of the field energy throughout the propellant flame structure, offering opportunities for further optimization of electromagnetically tunable energetic material combustion.