Solid Propellant Flames Research Articles

Experimental investigation on the heat fluxes generated by AP/HTPB solid propellant flames

This article describes an experimental investigation into the heat fluxes generated by the combustion of AP/HTPB solid propellant samples inside 1‑MPa pressurized combustion chambers. Fluxmeters developed at ONERA and recently improved were used to measure the local heat flux densities. Two kind of fluxmeters were used: total fluxmeters for which the convective and the radiative components are measured and radiative fluxmeters for which the convective component is suppressed by the use of a sapphire window mounted on top of the fluxmeter. Additionally, the flame was visualized by a narrow-band filtered IR camera which provided quantitative and spatially resolved radiance field. From the IR camera recordings, a radiative heat flux equivalent to that measured by the radiative fluxmeters was constructed by means of the integration over the fluxmeters’ collection volumes of the spectrally equivalent radiance. A satisfactory agreement is thus found between the two radiative heat flux measurement methods.

Imaging Aluminum Particles in Solid-Propellant Flames Using 5 kHz LIF of Al Atoms.

Laser-induced fluorescence imaging of aluminum atoms (Al-PLIF) is used to analyze the spatio-temporal behavior of aluminized solid propellant combustion. Using alternating LIF and chemiluminescence emission images of the particles in the gaseous and liquid phase evolving close to and far above the dynamically varying propellant surface, sequences of images were recorded and analyzed. The good sensitivity achieved enabled us to track the dynamics of the flame in the vicinity of particles detected all along the flame extension and up to 1.5 MPa. Analysis of wide-field images enabled droplet velocity measurements due to the high LIF sampling rate (5 kHz). The observed typical plume structures were in good agreement with alumina-formation prediction and previous shadowgraphy visualization. High-resolution sequences of images showed gaseous distribution behavior around the molten particles. The Al vapor phase was thus found to extend between 3 and 6.5 radii around the particles. Particle detachment dynamics were captured just above the propellant surface.

Microwave plasma enhancement of multiphase flames: On-demand control of solid propellant burning rate

This effort explores the microwave-supported plasma enhancement of an aluminized, ammonium perchlorate composite solid propellant flame. The technique is enabled through novel alkali metal doping, which provides increased levels of ionization and allows efficient microwave energy deposition to the flame structure and subsequent perturbation of the steady-state propellant burning rate. Three potential modes of energy deposition are identified in composite propellants: (1) plasma-enhancement promoted by the presence of sodium, (2) strong absorption by high temperature metallic oxide products, and (3) direct absorption of energy by the propellant burning surface condensed phase. Equilibrium calculations are used to identify propellant compositions with high free-electron populations, and solution of the Boltzmann equation using the BOLSIG + code is used to show the significant effects of sodium doping on microwave absorption of the flame. Experimental emission spectroscopy and imaging of the microwave-enhanced propellant flame structure show evidence of these mechanisms. Propellant burning rates can be increased by up to 60% through the application of 1 kW, continuous 2.46 GHz microwave radiation. Propellant doping with 3.5% by weight of sodium nitrate shows significant improvement in the effectiveness of microwave application in modifying the propellant burning rate. This technique provides high levels of dynamic propellant combustion control from a solid-state system. The on-demand control of energetic material burning rates through low level doping and microwave irradiation is a promising technique which could lead to the development of a new class of ‘smart’ dynamically controllable energetic materials.

Thermokinetic Analysis and Performance Evaluation of Guanidinium Azotetrazolate Based Gas Generating Composition for Testing of Solid Rocket Motor Nozzle Closures

AbstractA gas generating composition (GGC) was designed with Guanidinium azotetrazolate (GZT) as fuel and Ammonium perchlorate (AP) as an oxidizer for burst testing of nozzle closure at rig level, simulating solid rocket motor ignition transient and propellant flame temperature. EXPLO5 software code was used for formulation studies. The selected GGC formulation was subjected to thermokinetic analysis for understanding the thermal decomposition behaviour as well as evaluating kinetic and thermodynamic parameters of decomposition.The non‐isothermal kinetic parameters of the composition over the extent of conversion were determined using two isocoversional methods viz; Flynn‐Wall‐Ozawa (FWO) and Kissinger–Akahira–Sunose (KAS). GGC in pellet form was fired using a pyrotechnic pressure generating device (PPG) in test rig level with nozzle closure. The nozzle closure was burst opened at a pressure of 0.985 MPa with a pressurization rate of 10.3 MPa s−1.

Detection of iron atoms by emission spectroscopy and laser-induced fluorescence in solid propellant flames.

Planar laser-induced fluorescence on atomic iron is investigated in this paper, and a measurement strategy is proposed to monitor the fluorescence of iron atoms with good sensitivity. A model is proposed to fit the experimental fluorescence spectra, and good agreement is found between simulated and experimental spectra. Emission and laser-induced fluorescence measurements are performed in the flames of ammonium perchlorate composite propellants containing iron-based catalysts. A fluorescence signal from iron atoms after excitation at 248nm is observed for the first time in propellant flames. Images of the spatial distribution of iron atoms are recorded in the flame in which turbulent structures are generated. Iron fluorescence is detected up to 1.0MPa, which opens the way to application in propellant combustion.

The influence of oxidizer content in the solid propellant on the combustion condition in the flame

We have reported on the detection of the axial distribution at the height of the flame of OH radials in an alcohol burner, in premixed methane/oxygen, and in solid propellant flames using laser-induced fluorescence (LIF) spectroscopy. In this experiment, the influence of substances with different oxidizer contents on the OH fluorescence intensity was investigated. The results indicated that the oxidizer content exerted a strong influence on the combustion condition in the flame.

Dark zones of solid propellant flames: Critically assessed datasets, quantitative model comparison, and detailed chemical analysis

The formation of propellant dark zone (DZ) structures in the gaseous flames above many solid propellants has been a subject of recurrent interest to us for about 20 years. The DZ structure is controlled by small molecule chemistry. The DZ chemistry is very important in controlling both the flame structure at low pressure (10–100 atm) and burning rates at high pressure (above ∼500 atm), even though DZs collapse at higher pressures. We developed a detailed, frequently updated mechanism to model it. This report reviews prior work, introduces our most recent modeling results, and documents the first quantitative tests of several key assumptions. All relevant experimental literature is critically assessed to identify datasets for testing our model. Comparison of predictions and experimental results shows reasonable agreement, and thus we advocate use of our mechanism. But the precision of both experiments and predictions is not tight, nor are there many test datasets. Further experimentation is needed, and types of study that would be of greatest benefit are suggested. A detailed discussion of the chemistry that controls DZ structure is presented.

Modeling of Propellants Containing Ultrafine Aluminum

In the study of heterogeneous propellants that contain large amounts of fine aluminum and ammonium perchlorate, it is appropriate to distinguish between the matrix (a homogenized blend of fine particles and binder) and larger particles that are embedded in this matrix. Then the burning properties of a pure matrix are required. We construct a one-dimensional model for this purpose. Key ingredients include the determination of the thermal conductivity and pyrolyis law of the matrix, and an accounting of the radiation field generated by the fine aluminum in the gas phase. Experimental results of Dokhan et al. (The Ignition of Ultra-Fine Aluminum in Ammonium Perchlorate Solid Propellant Flames, AIAA Paper 2003-4810, July 2003) are used for calibration and in comparisons.

Formation of dark zone and its temperature plateau in solid-propellant flames: A review

A comprehensive review on the formation of dark-zone temperature plateaus in solid-propellant flames has been conducted. Various key physiochemical processes involved in the formation of a dark zone were investigated for a broad range of propellants, including nitramine, ammonium dinitramide, and double-base propellants. The effects of propellant formulation, surface condition, burning rate, and preconditioned temperature, as well as ambient pressure and external stimuli, were investigated systematically. Dominant chemical pathways responsible for the formation of a dark zone were identified. Major species in the dark zone were tabulated for all the solid propellants of concern. Apart from NO, important species in a dark zone are CO for double-base, HCN for nitramine, and N 2O for ADN propellants. These chemical species typically undergo reactions with high activation energies and their chemical-induction and flow-residence times are instrumental in explaining the existence of a dark-zone temperature plateau. The transport velocity must be sufficiently high to allow for chemical species to travel farther from the propellant surface before they overcome the activation energies to react, thereby giving rise to a temperature plateau. Conversely, if the transport velocity is low, then reactions take place much closer to the propellant surface, producing a very short or even no dark zone. The flame stand-off distance decreases as the pressure increases because of the increase in the reactant concentrations and gas densities, which in turn increase the rate of reactions and reduce the transport velocity.

Macrokinetics of Combustion of Monodisperse Agglomerates in the Flame of a Model Solid Propellant

The paper describes a procedure for studying the macrokinetics of combustion of agglomerates in a solid propellant flame using special samples of a model propellant generating monodisperse agglomerates. Empirical dependences of the incompleteness of aluminum combustion in the combustion products of a propellant based on ammonium perchlorate and HMX on time and pressure were established. The mass fraction of oxide accumulated on a burning agglomerate versus the degree of aluminum conversion was determined. For fine agglomerates (310–350 μm), this fraction decreases with increase in the degree of conversion. For large agglomerates (400–540 μmum), it increases, and, hence, the mass of large agglomerates increases as aluminum burns out. Because of accumulation of oxide, the agglomerate size does not change markedly in the examined range of parameters.

Line-of-Sight Variations of Temperature and Species in Solid Propellant Flames

The purpose of this work is to deduce line-of-sight variations of temperature and species concentrations in high-pressure, solid-propellant e ames by using spectral transmittances acquired by Fourier transform infrared spectrometry. To deduce these variations, an inverse technique was developed. For its validation, temperature and mole-fraction proe les within nitramine-composite propellant e ames at low pressures were compared with similar measurements made by using e ne-wire thermocouples and a microprobe mass spectrometer. Subsequently, it was applied to spectral transmittance data acquired for a high-pressure, self-sustained solid-propellant e ame. Several conclusions were made. First, at about 3 ‐4 mm above the surface, one must account for line-of-sight variations. Second, the deduced centerline temperatures were within 50 K of those measured using e ne-wire thermocouples. Finally, the deduced centerline concentrations of CO and NO established a dark-zone behavior, which is expected of nitramine-composite propellant e ames. However, to deduce the line-of-sight variation of other infrared-active species, further improvements in the database of the spectral absorption coefe cients must be made.

Experimental study of solid-propellant ignition transient and flame spreading under convective flows

A project on unsteady three-dimensional numerical simulation of solid-propellant rocket motors is being developed by this research group. In this general approach, ignition and flame spreading are critical points. Experimental investigations were performed to support the modeling approach. The paper describes the first step of this experimental investigation. Attention is focused on the ignition delay and flame-propagation velocity for a flat sample of a solid propellant containing ammonium perchlorate and hydroxyl-terminated polybutadiene. Different measuring techniques are implemented, which can be extended to more complex geometries and to different operating conditions, according to the needs of the numerical-simulation approach.

Species and Temperature Profiles of Propellant Flames Obtained from FTIR Absorption Spectroscopy

The use of Fourier transform infrared absorption spectroscopy to obtain species and temperature profiles within solid-propellant flames is described. Measurements were performed at a spectral resolution of 2 cm-1 and a spatial resolution of 1 mm within the flame zone of an RDX-composite propellant. A nonlinear, least-squares routine was developed to deduce the absolute species concentrations of CO, NO, H2O, CO2 HCN, CH4 and N2O as well as the rotational temperature. The deduced temperature profile revealed the presence of darkzone/ luminous flame-zone structures. The mole fraction of CO increased while the mole fraction of HCN decreased from the beginning of the dark zone to the luminous flame zone. Data on the spatial variation of species concentration and temperature within these zones are important for the development and validation of chemical kinetic mechanisms describing the combustion of solid propellants.

Study of Solid Propellant Flame Structure by Mass-Spectrometric Sampling

Abstract A method of mass-spectrometric sampling (MSS) of solid propellant (SP) flames for SP flame structure investigation is described. Two types of instruments have been developed: 1) microprobe sampling and 2) molecular-beam mass-spectrometry (MBMS)using time-of-flight mass spectrometry (TOFMS). Addressing an important question concerning estimated errors of probe measurements, detailed investigations on substantiating the sampling technique for flames with narrow combustion zones have been carried out. Examples of the mass-spectrometric probing technique applications to study the flame structure of the following solid propellants and their ingredients are presented: RDX, HMX, sandwich type systems based on HMX and double-base propellants. Applications of MSS to study of unstable combustion of SP are described.

Propellant dark zone concentrations via multichannel ir absorption

A multichannel infrared absorption technique has been developed and applied to the study of steadystate, self-sustained combustion of solid propellant flames at elevated pressure. An infrared light source. windowed pressure chamber, and spectrometer-diode array detector formed the basic experimental setup with which in situ absorption measurements were made. Spectra were obtained over the wavenumber range of 2300 to 6000 cm −1 . Within this range, absorptions for HCN, H 2 O, N 2 O, CO, CO 2 , and CH 4 have been detected through the use of a 1024-element platinum silicide array detector with the ability to read complete spectra in 10 ms. Experimental dark zone results for a nitramine, propellant (XM39) and a double-base-like propellant (JA2) have been obtained, and these absorption spectra analyzed with a full spectrum, multivariate, nonlinear, least-squares-fitting pe program that makes use of the HITRAN data base for the required molecular information. In some cases, the absorption spectra of the molecule had sufficient detail and fidelity to allow determination of both temperature and absolute concentration. This occurred for HCN and H 2 O in XM 39 propellant and for H 2 O and CO in JA2 propellant. Dark zone temperatures (1185 K for XM39 and 1433 K for JA2) obtained in this fashion are in good agreement with reported values. All six of the species were found in XM39, and absolute concentration or upper limits are given. No evidence of HCN or N 2 O absorption was observed in the dark zone of JA2, thus, four dark zone species concentrations are reported for this propellant. Comparisons of species concentration and dark zone temperature measurements are made with other published measurements under conditions of steady-state. self-sustained propellant combustion.

Structure and chemical kinetics of flames supported by solid propellant combustion

The structure and chemical kinetics of gaseous flames are important aspects of the combustion of solid propellants. Advanced diagnostics have led to specific knowledge of solid propellant decomposition products, and advanced computer programs have allowed detailed chemical kinetic modeling of these products reacting with each other. There has therefore been considerable effort aimed at measuring and modeling the structure of premixed gas flames of fuels and oxidizers representative of the chemistry occurring in actual solid propellants. Most of the recent modeling work for representative gaseous flames can be traced to a mechanism first developed to model air pollution. Comprehensive mechanisms have also been developed to model the entire gas-phase chemistry process above burning propellant surfaces. These comprehensive mechanisms also are largely traceable to the air pollution modeling mechanism. Russian researchers have also made significant advances in the comprehensive modeling of solid propellant flame structures. Overall, the current state of knowledge of the gas-phase structure of solid propellants is incomplete, but encouraging agreement exists between modeling and experimental data. Recommendations are made for additional flame and reaction studies and for a standard mechanism to use in all gaseous flame modeling.

SOLID PROPELLANT FLAME CHEMISTRY AND STRUCTURE

The application of optical diagnostic techniques, predominantly Planar Laser Induced Fluorescence (PLIF) and UV-Visible absorption, to the study of solid propellant flame chemistry, structure, and kinetics is described. Measurements outlined include time resolved 2D images of species concentrations and flame structure during transient ignition of solid propellants, calibrated ID and 2D species profiles during deflagration of solid propellant energetic ingredients, ID and 2D temperature profiles in solid propellant flames, multi-pass absorption measurements through solid propellant flames, and 2D images of species concentrations and diffusion flame structure during ignition and deflagration of sandwiches of solid propellant oxidizers and binders. Species measured under some or all of these conditions include CN, CH, C2, OH, NH, NO, NO2, NCO, and H2CO. Temperature has been obtained from fitting rotational and vibrational population distributions for OH, CN, and NO. These studies are a useful adjunct to the overall task of understanding the mechanism of solid propellant combustion and developing a computer code for the a-priori prediction of propellant ballistic properties.

Determination of the Temperature in a Solid Propellant Flame by analysis of emission spectra

AbstractTwo methods are described to determine the temperature in a solid propellant flame by analysis of emission spectra. First, experimental spectra of NH and CN radicals were compared with calculated spectra using a least squares fit routine with the parameters temperature T and halfwidth of the line profile Δλ. Second, in flames where no or only small band intensities were observed, the occurring continuous spectra were fitted to a Planck radiation. The emission coefficient ϵ was there by suggested as a function of wavelength λ and temperature T. The first technique was applied to the nitrmine propellant HMX, the second to a composite propellant consisting of 15% AI, 15% GAP and 70% ammonium perchlorate. TO detect the emission, propellant strands were burnt in an optical bomb under nitrogen atmosphere at various pressures (octogen at 0.1 MPa, composite at 2 MPa). To calibrate the intensities of the continous spectra tungsten strip lamp was used. The maximum time resolution of the used spectrometer system is about 10 ms. The determined temperature range was from 2300 K to 3000 K, which is about 20% below the corresponding adiabatic temperature (nitramine propellant: 2900 K, composite propellant: 3500 K).

Determination of temperature and no concentrations through the dark zone of solid-propellant flames

Absorption spectroscopy has been used to characterize the steady-state flame structure of solid propellants over a pressure range of 0.3 to 2.0 MPa. An arc-lamp light source and two spectrometer-intensified photodiode-array detectors formed the basic experimental setup from which NO and OH absorption spectra were acquired. Vibrationally resolved transitions in the A 2 Σ-X 2 II electronic system of NO from 230 to 250 nm comprise the absorption spectra from which temperatures and absolute NO concentrations are determined in the dark-zone region of the solid-propellant flames and rotationally resolved transitions in the A 2 Σ-X 2 II (0,0) vibrational band system of OH from 306 to 311 nm comprise the absorption spectra from which limunous-flame temperatures are determined. Both NO and OH are sufficiently well characterized that the spectra can be least-sequeres fitted with respect to a variety of parameters which include an instrument response function, an absorption baseline, a wave-length nonlinearity as well as the temperature and concentration. Three different propellants (JA-2, M9, and HMX2) that exhibit a dark zone have been studied as a function of pressure. The dark-zone temperatures ranged from about 1300 K to 1500 K and NO concentrations varied from 13 to 30 mole per cent depending on the propellant type and pressure. Temperatures in the luminous flame region reached values in agreement with the computed equilibrium, adiabatic flame temperatures.

Optical constants of Al2O3 smoke in propellant flames

An in situ light scattering and extinction technique was developed to determine the optical constants (n — ik) and mean optical size (1/32) of molten AhOa smoke particles in propellant flames. Direct transmittance and bidirectional transmittance and reflectance measurements were made using scattered laser light on aluminimized solid propellant flames at visible and near infrared wavelengths (Xi = 0.6328 /*m and \2 = 1.064 /mi). The optical properties of the molten AliOs smoke combustion product were obtained from the light scattering and extinction measurements by inverse solution of the radiative transfer equation. A mean optical size of d^i = 0.97 j*m was obtained, which agrees well with other reported values. The values of n obtained for molten AhO3 at 2680 K (nm,\i = 1.65 and /iOT,X2 = 1-64) were significantly less than the values which have been reported for solid AhOs at temperatures just below the melting point of 2320 K (ns,\i = 1.82 and ns,\i = 1.81) indicating that a substantial decrease in n occurs upon melting. This decrease in n can be attributed to the expansion that takes place upon melting and is in good qualitative agreement with the predictions of the Lorentz-Lorenz equation. The value of k obtained for molten AhOa at 2680 K was 0.006 ± 0.004 (at both wavelengths), which is in reasonable agreement with other reported values. A dispersion analysis was also performed to fit this and other data over the spectral region from 0.5 to 5.0 jim and for temperatures from 2320 to 3000 K.