Infrared Emitted Intensity Measurements from Burning Aluminum Droplets in Solid Propellants

Abstract

Infrared emitted intensities (absolute) were measured from burning aluminum droplets in aluminized composite solid propellants near the burning surface of the propellant from 1 to 50 atm. A high-speed, focal-plane array camera was used for mid-infrared imaging at a wavelength of 3.42 μm. At 1–5 atm high-resolution images were able to resolve three potentially important regions of infrared emission: molten metal, molten oxide cap on the droplet surface, and oxide smoke in the detached flame envelope. At this wavelength, which is of particular importance due to the strong influence of gas composition on oxide stoichiometry and emissivity, spectral intensities ranged from 0.2–3.6 W/m² μm sr at 1 atm increasing to 2.0–5.5 W/cm² μm sr at 50 atm. The large variation in emitted intensity at each pressure was attributed to variations in oxide cap emission and oxide smoke emission. Molten metal emission, which was relatively uniform over the droplet surface, corresponded well with fundamental predictions for an opaque, optically smooth surface if the droplet temperature was assumed to be several hundred degrees Celsius below the metal boiling point. Oxide cap emission, on the other hand, was quite non-uniform over the droplet surface, exhibiting significant volumetric (path length-dependent) effects. Oxide caps were optically thick enough over most slant paths to augment metal emission significantly. Smoke envelope emission was less than 10% below 5 atm.

Keywords:

• Aluminized solid propellants
• Aluminum radiation
• Burning aluminum
• Spectral intensity measurements
• Thermal emission

Support from ATK Launch Sytems, Inc. (formerly ATK-Thiokol Propulsion) and the Department of Energy, Center for Advanced Simulation of Rockets at the University of Illinois (B341494) is appreciated. Kim Chesterfield helped with the nichrome ribbon calibration and Jessica Mullen with the IR camera. Thanks to the National Research Laboratory of Metrology in Tsukuba, Japan for providing the database of metal emissivities and to Prof. Nick Glumac for the use of the two-color pyrometer.