Stefan T. Thynell

Thermal decomposition studies of energetic materials using confined rapid thermolysis/FTIR spectroscopy

An experimental setup for performing rapid thermolysis studies of small samples of energetic materials is described. In this setup, about 8 μL of a liquid sample or about 2 mg of a solid sample is heated at rates exceeding 1500 K/s to a set temperature where decomposition occurs. The rapid heating is achieved as a result of confining the sample between two closely spaced isothermal surfaces. The gaseous decomposition products depart from the confined space through a rectangular slit into the region of detection. The evolved gases are quantified using FTIR absorption spectroscopy by accounting for the instrument line shape. To illustrate the use of this setup, the thermolysis behaviors of three different energetic materials are examined. These materials include HMX, RDX, and HAN, all of which are considered as highly energetic propellant ingredients. The results obtained in this study of the temporal evolution of species concentrations from these ingredients are in reasonably close agreement with results available in the literature.

Condensed-phase kinetics of cyclotrimethylenetrinitramine by modeling the T-jump/infrared spectroscopy experiment

During the combustion of solid propellants, explosives, or pyrotechnics, the condensed phase experiences heating rates that may exceed 20,000 K/s. At such high heating rates, the thermal decomposition behavior of the energetic material could be affected by its rate of decomposition. To simulate the high heating rate environment, the T-jump experiment was developed for use with Fourier-transform infrared spectroscopy. The T-jump experiment utilizes electrical resistance heating of a thin Pt filament on which a small amount of the energetic test sample is placed. This work describes a heat transfer model of the filament and sample, a model of the currents control circuit, and global decomposition and heat release mechanisms of Cyclotrimethylenetrinitramine (RDX), which is an energetic ingredient .used in propellants and explosives. Comparisons of model calculations with experimental data reveal an excellent agreement. Similarly, the predicted time to rapid heat release for the highly energetic RDX sample also shows a good agreement with experimental results. Thus the use of the developed model in conjunction with experiments should be a useful tool in studying the thermal decomposition behavior of energetic materials under combustion-like conditions.

Discrete-ordinates method in radiative heat transfer

Abstract About 50 years ago, Chandrasekhar developed the discrete-ordinates method for analyzing radiation heat transfer within a plane-parallel medium. The effects of absorption, multiple scattering, and collimated incidence were considered. Since then, the numerical method has been utilized to study radiative heat transfer in one- and multi-dimensional rectangular, spherical and cylindrical geometries. In this article, the formulation of the discrete-ordinates method is described for computing radiative transfer in a one-dimensional (1D) slab and two-dimensional (2D) rectangular geometries, including the effects of absorbing, emitting and scattering constituents. Both the strengths and the weaknesses of this numerical approach are addressed. The accuracy of available results obtained by utilizing the discrete-ordinates method is compared with other methods of solution for several representative cases. It is evident that major advances have been made related to the development and application of the discrete-ordinates method in multi-dimensional systems. However, much more work remains to be done with the development of this classical technique, including, among others, a more effective treatment of nongray radiative properties of molecular gases and turbulence, as well as a removal of false scattering.

Transient Radiative Heat Transfer from a Plasma Produced by a Capillary Discharge

The objective of this work is to develop a better understanding of the transient behavior of radiative heat transfer from a plasma. The plasma generation occurred within a 3.2-mm-diam and 26-mm-long polyethylene capillary. Because of its high temperature and high pressure, the plasma evolved from the capillary into an ambient air environment as an underexpanded supersonic jet that interacted with a stagnation plate. Various diagnostic techniques were used. They include heat flux and pressure gauges mounted on the stagnation plate, heat flux gauges and silicon photodiodes mounted below the plasma jet, as well as current transducers interfaced with the electrical circuit. The heat flux gauges were manufactured via sputtering and calibrated using a standard convection oven. A fused-silica window, placed about 1 mm above the gauges, ensured that only the radiative heat flux transmitted by the window was deduced. The row of heat flux gauges mounted below the plasma provided an assessment of the fraction of the radiative heat flux transmitted by the fused-silica window. The results show that the peak of emitted radiant flux occurs immediately after the peak of the discharge of electrical energy, which usually occurred a relatively long time prior to arrival of the precursor shock on the stagnation plate.

Anisotropic Heat Conduction Effects in Proton-Exchange Membrane Fuel Cells

The focus of this work is to study the effects of anisotropic thermal conductivity and thermal contact conductance on the overall temperature distribution inside a fuel cell. The gas-diffusion layers and membrane are expected to possess an anisotropic thermal conductivity, whereas a contact resistance is present between the current collectors and gas-diffusion layers. A two-dimensional single phase model is used to capture transport phenomena inside the cell. From the use of this model, it is predicted that the maximum temperatures inside the cell can be appreciably higher than the operating temperature of the cell. A high value of the in-plane thermal conductivity for the gas-diffusion layers was seen to be essential for achieving smaller temperature gradients. However, the maximum improvement in the heat transfer characteristics of the fuel cell brought about by increasing the in-plane thermal conductivity is limited by the presence of a finite thermal contact conductance at the diffusion layer/current collector interface. This was determined to be even more important for thin gas-diffusion layers. Anisotropic thermal conductivity of the membrane, however, did not have a significant impact on the temperature distribution. The thermal contact conductance at the diffusion layer/current collector interface strongly affected the temperature distribution inside the cell.

Plasma ignition and combustion of JA2 propellant

Experiments were performed to investigate the effects of various parameters on plasma-driven ignition and combustion of a double-base propellant under closed-chamber conditions. The parameters varied include input electrical energy, nozzle length and inner diameter, nozzle exit to propellant distance, as well as propellant sample thickness. Chamber pressure was measured to determine the ignition delay and to deduce the regression rate. High-speed images of the plasma jets and combustion event were also recorded. At low plasma energies, rapid, plasma-driven burning occurred, but self-sustained burning was not achieved. With moderate plasma energies, combustion of the propellant exhibited a two-stage burning behavior: one stage of plasma-driven rapid burning that occurred during the plasma pulse and a second stage of slower self-sustained burning that occurred with a clear delay after the first stage. When plasma energy was increased further, the two-stage behavior became less distinct and eventually disappeared, leaving only one stage of burning. Nozzle length and diameter affected the ignition and combustion characteristics as a result of energy losses from the plasma as it flows through the nozzle. The propellant burning behavior is also affected by both nozzle-sample distance and sample thickness. High-speed images revealed vigorous motion of gases in the closed chamber, which was induced by the plasma jet. Also, the images showed what appeared to be JA2 fragments; this observation was confirmed by recovery of fragmented propellant after some of the tests.

Interaction of Capillary Plasma with Double-Base and Composite Propellants

Experiments were conducted to characterize the electrothermal capillary plasma and its interaction with a double-base propellant, JA2, and two nitramine composite propellants, M43 and XM39, in closed-chamber and open-air conditions. Pressure-time histories were recorded during ignition and burning of the propellants in the closed chamber. Experimental results indicate significant differences in ignition and combustion of the propellants. The composite propellants exhibited a two-stage burning behavior: one stage of rapid burning driven by the plasma and a second stage of slower self-sustained burning, which occurred with a clear delay after the first stage. During the burning that is driven by the plasma, the burn rate was largely independent of propellant type. Optical microscopic images of recovered propellant samples showed clear physical changes in surface and possibly in subsurface structure, an indication of in-depth melting, vaporization, and possibly chemical reactions. Plasma-induced mass losses for the three propellants were obtained from open-air testing and compared to the values calculated based on the pressure data from closed-chamber tests.

Study of Plasma-Propellant Interaction During Normal Impingement

This paper describes an effort to investigate the transient process of an electrothermal plasma normally impinging on a plate or a solid propellant. The objective of this effort was to develop a better understanding of the fundamental aspects of this process to facilitate the development of the plasma ignition system for use in electrothermal chemical propulsion applications. The plasma is produced through an electrical discharge occurring within a polyethylene capillary. The high-temperature and high-pressure plasma exited from the capillary into an ambient air environment and impinged normally onto a plate. Pressure transducers mounted on the plate were used to obtain the stagnation pressure of the plasma jet, and a multiple CCD imaging system was used to visualize this highly transient process. Tests were also performed with the stagnation plate replaced by a solid propellant sample. Plasma-induced surface changes were examined using a scanning electron microscope, mass loss of the propellant was measured, and decomposition species were analyzed using a triple quadrupole mass spectrometer. The results show that when the distance between the capillary exit port and the plate was varied, changes in plasma jet structure, stagnation pressures, and mass losses of the propellant were significant.

Hot fragment conductive ignition of nitramine-based propellants

Hot fragments generated during the impact of shaped-charge jets may penetrate the cartridge casing and induce propellant ignition. The major objective of this study is to acquire a better understanding of hot fragment conductive ignition behavior of nitramine-bas ed XM39 and M43 propellants. The confinement effect of the cartridge is simulated by an enclosure within which the pyrolysis products can accumulate and significantly increase pressure. Both theory and experiments showed that the ignition threshold was a strong function of confinement. At low pressures, the XM39 propellant was more susceptible to ignition, because its binder decomposition is more exothermic than that of the M43 propellant. At highly confined conditions, which allow for chamber pressurization, the M43 ignition threshold became lower than that of XM39 propellant. The reduction of the ignition threshold is caused mainly by exothermic reactions between CH2O and NO2 species, which are enhanced under gas accumulation conditions.

Analysis of RDX-TAGzT pseudo-propellant combustion with detailed chemical kinetics

A detailed model of steady-state combustion of a pseudo-propellant containing cyclotrimethylene trinitramine (RDX) and triaminoguanidinium azotetrazolate (TAGzT) is presented. The physicochemical processes occurring within the foam layer, comprised of a liquid and gas bubbles, and a gas-phase region above the burning surface are considered. The chemical kinetics is represented by a global thermal decomposition mechanism within the liquid by considering 18 species and eight chemical reactions. The reactions governing decomposition of TAGzT were deduced from separate confined rapid thermolysis experiments using Fourier transform infrared spectroscopy and time-of-flight mass spectrometry. Within the gas bubbles and gas-phase region, a detailed chemical kinetics mechanism was used by considering up to 93 species and 504 reactions. The pseudo-propellant burn rate was found to be highly sensitive to the global decomposition reactions of TAGzT. The predicted results of burn rate agree well with experimental burn-rate data. The increase in burn rate by inclusion of TAGzT is due in part from exothermic decomposition of the azotetrazolate within the foam layer, and from fast gas-phase reactions between triaminoguanidine decomposition products, such as hydrazine, and oxidiser products from the nitramine decomposition.