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Fast emission spectroscopy for a better understanding of pyrotechnic combustion behavior

: Weiser, V.; Eisenreich, N.


Propellants, explosives, pyrotechnics 30 (2005), Nr.1, S.67-78
ISSN: 0340-7462
ISSN: 0721-3115
ISSN: 1521-4087
Fraunhofer ICT ()

Pyrotechnic mixtures react mostly fast and under a high release of thermal energy and heat radiation. A spectral analysis of this emission allows deep insight into the reaction process by the non-intrusive determination of the reaction temperatures and observation of the main reaction species and their concentrations. In the presentation various pyrometric and spectroscopic measurement techniques and data analysis and interpretation procedures are introduced using the spectral range form the ultraviolet, visible and near to medium infrared in combination with image processing of video and IR-camera signals. The available scan rate of several hundred spectra per second is sufficient to investigate: flame structure of high pressure strand burners from rocket propellants, gun powders and gas generators reaction processes in bulk pyrotechnic mixtures like igniter mixtures, flares, incendiary compositions, etc. moving and burning of single particles ignition processes of gun powders propagation of gas explosions in-situ observations of reactions in closed burning chambers via fiber optics The quantitative intensity calibration of the recorded spectra series is carried out by relation to reference spectra of a black body radiator in units of power per wavelength, area and steradian. The evaluation uses the BAM code of ICT to model bands of reaction products. The code calculates NIR/IR-spectra (1-10 mu m) of non homogenous gas mixtures of H2O (with bands around 1.3, 1.8, 2.7 and 6.2 mu m), CO2 (with bands near 2.7 and 4.3 mu m), CO (4.65 mu m), NO (5.3 mu m) and HCl (3.5 mu m) taking into consideration also emission of soot particles. It is based on the single line group model and makes also use of tabulated data of H2O and CO2. Rotationally resolved emission spectra of diatomic flame radicals in UV/Vis-range like OH, CH, C-2, NH correlate directly to the energies regarding electronic, vibrational and rotational energy transitions and therefore enable the estimation of flame temperatures. Taking into account that there are many unknown parameters influencing the emission spectrum of an inhomogenous gas mixture, only a simplified model can be applied. Therefore, it was assumed that the flame consists only of one hot emitting layer of undefined thickness, constant temperature, constant concentration of the various gases and soot particles in thermal equilibrium. These assumptions lead to a reasonable number of parameters, which can be determined by a least squares fit procedure fitting calculated spectra to experimental data. With this restriction, temperature and concentration length of major combustion products can be determined. Additionally, it is found that many pyrotechnic reactions emit a nearly grey continuum in the NIR spectral range that can be used for temperature determination. For spatial allocation of the spectroscopic emission signals the intensity distribution of simultaneously recorded video or IR-camera frames were used.