Exploratory Development of Energetic and Pyrotechnic Combustion Characterization Apparatus

Dr. David Decker | Desert Research Institute

WP-2612

Objective

Project WP-2612 sought to develop and test measurement technology that would support the characterization of airborne particulate and gas-phase emissions generated from the use of metal-based energetics and pyrotechnic formulations.

The two technical objectives were to:

  1. Assemble a previously designed, but untested, measurement technology and test its operability at the Hypervelocity Laboratory facility at Naval Air Warfare Center, Weapons Division, China Lake.
  2. Characterize the practical limits of the measurement system and its potential for use in future efforts to characterize emission factors and human exposure concerns from open-burning/open-detonation (OB/OD), as well as the use of energetic materials, propellants, rocket motors, and pyrotechnics during live-fire training.

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Technical Approach

A sampling platform was designed and constructed with enough flexibility that it could be adapted to contain detonation or combustion test facility, as well as to take ground-level ambient samples of plumes from OB/OD activities or live-fire tests.

The main distinguishing features of the platform were: 1) the use of multiple mass flow controllers to precisely distribute particle laden air that has been routed from one sample port through a single PM10 (particulate matter with aerodynamic diameter smaller than 10 micrometers) selective inlet through to multiple filter collection and other measurement devices; 2) a relatively high flow rate of 113.5 liter per minute that enables collection of PM10 material on filter media in a relatively short period of time; 3) flexibility in adding or removing instrumentation with minimal impact; and 4) a rapid sample-air to clean-air port switching mechanism which provided the ability to instantly collect plume samples without any startup time or delays in achieving designed flowrates during sample platform operation.

The sampler filter ports were equipped with three types of filter media: Teflon™, quartz fiber, and Teflon-impregnated glass fiber followed by a selective resin column. A multi-stage impactor device was connected to a sampling port and used to collect size-segregated particle samples. Product gases including carbon dioxide, carbon monoxide, and oxides of nitrogen were sampled in real-time through a sample port using a gas analyzer. Particle concentration by light scattering was also measured in real-time. All real-time data were logged by a remote computer that also controlled the functions of the sampling apparatus. Filter media were analyzed with multiple techniques including gravimetric weighing to determine loading levels, X-ray fluorescence spectroscopy, ion chromatography, liquid chromatography, gas chromatography, and thermal optical reflectance/ transmittance.

Testing consisted of the detonation of thirteen test articles and one blank in a confined detonation test chamber (DTC), followed by sampling of the plume with the measurement platform. Twelve of the test articles consisted of combinations of two energetics (PBXN-113 and PBXN-114) and two casing materials for the article (brass and stainless steel with tin insert). The thirteenth test article was a booster-only device.

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Results

The main result of the test effort was the successful implementation of the sampling platform and the proof of its use in conjunction with the DTC. Overall, the platform performed as expected and the magnitude of the sample flowrate and ability to distribute the sample air to multiple filters and instruments made for very efficient and flexible sample collection. This experience also provided areas where improvement can be sought and enabled assessment of the testing platform for use in other configurations and for other energetic formulations.

  • The explosives tested in this study (~ 200 grams) resulted in extremely high initial concentrations of PM10, which caused overloading of filters for the first four tests. The issue was addressed by diluting the plume using the DTC active exhaust fan for five minutes before starting sample collection. In future efforts, it is recommended that a second sample line is used to monitor PM10 and gas constituent concentrations, before and after filter sample collection begins, in order to ensure optimal filter loading.
  • Each gas phase compound of interest should be analyzed using an independent analyzer; the multi-gas analyzer device used in this SEED project was designed for vehicle exhaust testing and the ranges of concentrations it can accommodate were limited.
  • Teflon™ filters are not ideally suited for sampling high concentrations of energetics products due to the tendency for the deposit to spall. Exploration of a different filter medium is suggested for future efforts.
  • The sampling platform can accommodate either a PM10 or a PM2.5 size selective inlet, but not both at the same time. Given the labor resources required from host facilities to perform these tests, it would be more efficient to simply build another platform so that both PM10 and PM2.5 samples could be collected simultaneously.

Additional results specific to the testing conducted included:

  • Compared to PBXN-113, the combustion of the carbon in PBXN-114 was less complete as indicated by a higher carbon monoxide to carbon dioxide ratio and more soot.
  • The major metal constituent from PBXN-113 was aluminum and the major constituent from PBXN-114 was tin, but this was likely a result of inserting a tin disc into the PBXN-114 test articles rather than a characteristic of the detonation of PBXN-114.
  • Several metals of interest were measured well above detection limits including lead, chromium, manganese, titanium, nickel, and strontium.
  • The main energetic HMX was detected in the first four test samples, when the plume was highly concentrated, but not in subsequent tests. Another commonly used compound in energetics, Ethanox, was not detected in any of the samples.
  • Particle size distributions indicated slightly different modes for PBXN-113 (0.3 – 0.5 micrometers) than PBXN-114 (broader mode between 0.3 and 1.8 micrometers).

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Benefits

In all, the platform used for measurements in this SEED project was flexible enough to be used in future DTC-based testing of explosives (< 200 grams) and propellants (small arms). In-situ testing in an outdoor setting expands the list of test article to include large energetic devices, pyrotechnics, large device propellants, and rocket motors. One caveat is that the plume must have a portion that is at or near ground level (i.e., can be reached by elevated ground-based sample line) for at least a short period of time, so that sample materials could be channeled through an inlet line. Other than pyrotechnics all of these test articles are available at NAWCWD China Lake along with personnel and facilities to conduct both indoor and outdoor testing.

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Points of Contact

Principal Investigator

Dr. David Decker

Desert Research Institute

Phone: 775-673-7353

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