Optimization of global kinetics parameters for heterogeneous propellant combustion using a genetic algorithm

We examine the combustion of heterogeneous propellants for which, necessarily, the chemical kinetics is modelled using simple global schemes. Choosing the parameters for such schemes is a significant challenge, one that, in the past, has usually been met using hand-fitting of experimental data (target data) for global burning properties such as steady burning rates, burn-rate temperature sensitivity, and the like. This is an unsatisfactory strategy in many ways. It is not optimal; and if the target set is large and includes such things as stability criteria, or bounds, difficult to implement. Here we discuss the use of a general optimization strategy which can handle large data sets of a general nature. The key numerical tool is a genetic algorithm that uses MPI on a parallel platform. We use this strategy to determine parameters for HMX/HTPB propellants and AP/HTPB propellants. Only one-dimensional target data are used, corresponding to the burning of pure HMX (AP) or a homogenized blend of fine HMX (AP) and HTPB. The goal is to generate kinetics models that can be used in the numerical simulation of three-dimensional heterogeneous propellant combustion. The results of such simulations will be reported in a sequel.     

Binder melt: Quantification using SEM/EDS and its effects on composite solid propellant combustion

The present study reports the development of a novel technique to quantify binder melt on the surface of the propellant. Non-aluminized AP-HTPB propellants of 86% particulate loading are used to illustrate the technique. Elemental maps of unburnt and extinguished propellant surface are obtained using EDS (Energy Dispersive Spectroscopy). Overlap between the elements is identified and the elemental maps are processed to calculate AP and binder area exposed in unburnt and extinguished samples. The AP area exposed is found to be around 72.3% and 63.3% for unburnt and extinguished samples, respectively, indicating a reduction in AP exposed area with extinguished samples. This has been attributed to the binder melt discussed in literature but never quantified. Simulations have been carried out to analyze and understand the effects of this binder melt. A random packing algorithm is used to simulate propellant packs. Also, a methodology to account for binder melt layer is introduced and is used to capture AP exposed areas. Effect of binder melt in propellants with different solid loading and varying particle size is discussed. It is shown that fine AP particles are more prone to being covered by binder melt than the coarse AP particles. A possible explanation to the behavior of plateau burning propellants observed in literature has been provided through this analysis.     

Development and characterization of high performance solid propellants containing nano-sized energetic ingredients

This paper addresses the development of a pair of layered solid propellants suitable for use in a fast-core gun-propellant charge application. A baseline propellant combination was formulated using RDX particles and thermoplastic-elastomer binder as the major ingredients and CL-20 and nitroguanadine as separate additives for high- and low-energy propellants. The propellant’s burning rate was characterized and insufficient burning-rate ratio between the fast and slow baseline propellants was found. Impetus obtained from the combustion of the combined baseline propellants was also found to be far from the demanded value of 1300 J/g. Several modifications were made by introducing nano-sized aluminum particles and ultra-fine boron particles as well as high-energy oxidizer HNF into the propellant formulation. It was found that the addition of nano-sized aluminum particles can enhance the propellant burning rate only when the propellant contains oxidizers with a positive oxygen balance. Without the presence of positive oxygen balance oxidizer, the exothermic reaction of aluminum and boron particles occurs at a large distance from the burning surface introducing an energy-sink effect. The results obtained from the combustion of the advanced propellants show that an average impetus of 1299 J/g, a flame temperature of 3380 K with a burn rate ratio around 3 between the fast- and the slow-burning layers can be achieved. These conditions are desired for fast-core layered propellant applications. The impact sensitivities of the baseline, intermediate and advanced propellants were measured. The results show that addition of HNF and nano-sized aluminum exhibited improved impact sensitivity at levels that can be considered acceptable for deployment.     

Improved method of measuring pressure coupled response for composite solid propellants

Pressure coupled response is one of the main causes of combustion instability in the solid rocket motor. It is also a characteristic parameter for predicting the stability. The pressure coupled response function is usually measured by different methods to evaluate the performance of new propellant. Based on T-burner and “burning surface doubled and secondary attenuation”, an improved method for measuring the pressure coupled response of composite propellant is introduced in this article. A computational fluid dynamics (CFD) study has also been conducted to validate the method and to understand the pressure oscillation phenomenon in T-burner. Three rounds of tests were carried out on the same batch of aluminized AP/HTPB composite solid propellant. The experimental results show that the sample propellant had a high response function under the conditions of high pressure (~11.5 MPa) and low frequency (~140 Hz). The numerically predicted oscillation frequency and amplitude are consistent with the experimental results. One practical solid rocket motor using this sample propellant was found to experience pressure oscillation at the end of burning. This confirms that the sample propellant is prone to combustion instability. Finally, acoustic pressure distribution and phase difference in T-burner were analyzed. Both the experimental and numerical results are found to be associated with similar acoustic pressure distribution. And the phase difference analysis showed that the pressure oscillations at the head end of the T-burner are 180° out of phase from those in the aft end of the T-burner.     

Quench collection of nano-aluminium agglomerates from combustion of sandwiches and propellants

An experimental investigation has been carried out to measure the size of nano-aluminium agglomerates emerging from the combustion of nano-aluminized sandwiches and composite solid propellants. Nano-aluminium of median size of 50 nm produced in-house by the electrical wire explosion method is used in these samples. Propellants with different sizes of coarse and fine ammonium perchlorate are considered. Surface features of sandwiches and a propellant whose burning was interrupted by rapid depressurization are examined in a scanning electron microscope. The combustion products of the sandwiches and propellants are quenched close to the burning surface and collected in a quench collection set-up. The surface features of rapid-depressurization quenched sandwiches exhibit relatively large nano-aluminium clusters—of the order a few micrometres—particularly in the binder lamina. Quench-collected nano-aluminium exhibits significant agglomeration, but only a small fraction of the agglomerates are in the 1–3 μm range, except for both the coarse and fine AP particles used in the formulation being large, but even there they do not exceed ∼5 μm in size. This is expected to be benign for reduced smoke propellant applications from exhaust signature point of view, and to decrease the specific impulse losses without sacrificing the energetics of the propellant.     

A numerical study of periodic sandwich propellants with oxygenated binders

We examine sandwich propellants constructed from sheets of pure ammonium perchlorate (AP) interleaved with an AP/binder blend, and construct solutions numerically using a code that fully couples gas-phase and solid-phase processes via an unsteady moving interface. This code has been used elsewhere to simulate the burning of random packs of spherical AP particles embedded in binder. We show that for a stoichiometric configuration, variations of the burning rate with α (a measure of the oxygenation of the AP/binder blend) are not monotonic, but display a weak maximum; and variations of the burning rate with sandwich thickness are monotonic for small α, but display a minimum for large α (e.g. α?=?0.5). When the equivalence ratio is varied, the burning rate displays a maximum on the fuel-lean side when α is small, on the fuel-rich side when α is large. These results, and the manner in which the sandwich topography varies with the different parameters, suggest that the configuration could be invaluable for validating the model ingredients and parameter values of heterogeneous propellant combustion codes.     

Experimental data and model predictions of aluminium agglomeration in ammonium perchlorate-based composite propellants including plateau-burning formulations

Sixteen propellant formulations based on ammonium perchlorate (AP), hydroxyl-terminated polybutadiene, and aluminium particles have been tested for size distribution of aluminium agglomerates emerging from their burning surface. The formulations are based on a bimodal size distribution of AP particles. Ten of the formulations exhibit one or two plateaus/mesa in their burning rate variation with pressure (zero/negative pressure exponent of burning rate). The relevant formulation variables, namely, coarse and fine AP sizes and coarse-to-fine ratio, aluminium size and content, and two different curing agents, have been varied. Tests are performed in the 1–10 MPa pressure range. A direct correlation between burning rate and agglomerate size exists for propellants with normal burning rate trends but a neutral or inverse correlation is observed for those exhibiting plateau burning behaviour. Larger the parent aluminium size, lesser the agglomeration, as expected; but the effect of aluminium content is non-monotonic. The coarse AP size influences the aluminium agglomerate size as expected from the pocket model regardless of plateau burning effects. The agglomerate size decreases with increase in fine AP size, however. A computer model developed earlier at this laboratory for prediction of aluminium agglomerates based on three-dimensional packing of particles and deduction of AP particles with attached leading edge diffusion flames is applied to the present formulations. The model under-predicts the agglomerate size, only marginally for propellants that do not exhibit plateau burning rate trends, but substantially, otherwise. This is because it does not take into account effects of binder melt flow and is independent of the curing agent of the binder.     

Using level sets for creating virtual random packs of non-spherical convex shapes

Random packs of spheres have been used to model heterogeneous and porous material morphologies during simulations of physical processes such as burning of coal char, convective burning in porous explosives, and regression of solid rocket propellant. Sphere packs have also been used to predict thermo-mechanical properties, permeability, packing density, and dissolution characteristics of various materials. In this work, we have extended the Lubachevsky–Stillinger (LS) sphere packing algorithm to create polydisperse packs of non-spherical shapes for modeling heterogeneity in complex energetic materials such as HMX and pressed gun propellants. In the method, we represent the various particle shapes using level sets. The LS framework requires estimates of inter-particle collision times, and we predict these times by numerically solving a minimization problem. We have obtained results for dense random packs of various convex shapes such as cylinders, spherocylinders, and polyhedra, and we show results with these various particles packed together in a single pack to high packing fraction.     

Holographic Observation of AluminumAgglomerates near Surface Area of SolidRocket Propellants during Burning Process

l.IntroductionMetalaluminumisusual1yaddedtosolidrocketpropellantstoincreasethrustandspecificimpu1se.Inaddition,a1uminumoxideparticlesproducedinhightemPeratureburningareacaninturnsuppresshighfrequencycombustioninstabilities.Fromthetheo-reticalcombustionmode1ofsolidproPellants['2j,oxiderandbinder,maincompositionofsolidproPellants,arefirstdecomPOsedandvaPOrizcdintheburningprocere,sothatthealuminumparticlesareexposedonthesurfaceofsolidproPCllants.SomeoftheexPOsedaluminumparticlesescaPerapedl…     

On the structure of the combustion wave of nitroglycerine-based propellants

The parameters of the temperature distribution in the combustion wave of nitroglycerin-based propellants N and NB are analyzed and compared. The aim of the study is to explain the known experimental fact that the size of hotspots and the critical quenching diameter for propellant NB (more rapidly burning) are larger than those for propellant N. It is demonstrated that, at a given burning rate, the burning surface temperature, heat conduction zone thickness, temperature gradient near the burning surface, and the dark zone temperature for propellants N and NB are the same, but the fizz zone thickness for NB is approximately twice as wide as that for N. The dependence of the ratio of the hotspot size to the fizz zone thickness is described by a single power law for both propellants. It is also shown that the hotspot size can be defined as the distance between two consecutive transverse waves, which, in turn, is determined by the delay in the initiation of each following wave.     

Ammonium Nitrate as an Eco–Friendly Oxidizer for Composite Solid Propellants: Promises and Challenges

Ammonium nitrate (AN) has received attraction globally not only as a nitrogenous fertilizer but also as an oxidizer in gas generators and propellants. Nowadays, great attention is being focused on the development of composite solid propellants with green oxidizers in realizing eco–friendly combustion products. The ammonium perchlorate (AP), which is the work horse oxidizer in composite propellant, needs replacement due to its environmental and human health issues. In this context, AN is regarded as an alternative to AP because of its easy availability and environmentally friendly chlorine free combustion products. However, AN has its own inherent drawbacks such as hygroscopicity, room temperature phase transition, and low burning rate. Recently, several studies have been focused on its phase stabilization and burning rate modification so as to develop solid propellants with improved properties. The knowledge of thermal characteristics of AN is a crucial factor for its applications in propellants and gas generators. This article details the different aspects of polymorphism, phase stabilization, thermal decomposition, hygroscopicity, specific impulse, and burn rate modification of AN and also addresses ways to overcome the inherent weakness of AN as a propellant oxidizer in formulating an effective propellant composition.     

AP/HTPB laminate propellant flame structure: Fuel-lean intrinsic instability

Two-dimensional laminate propellant flames of ammonium perchlorate (AP) and hydroxyl-terminated polybutadiene (HTPB) have been observed using infrared (IR) and ultraviolet (UV) emission and transmission imaging. Under fuel-lean conditions and at slightly elevated pressures (4 atm), intrinsic instability has been observed in the form of a leading-edge flame kernel whose location oscillates laterally about the central fuel binder layer. A mechanistic explanation for this behavior is described in terms of local gas-phase equivalence ratio, surface geometry, and gas–solid thermal coupling. The flame structure under these conditions is unique in having a leading-edge flame kernel that appears to be more spatially distinct from the trailing diffusion flame than under nonoscillatory conditions. Other results are reported, including gas-phase rotational and vibrational temperature estimates based on HCl emission imaging spectroscopy. These results add to a growing set of flame and burning surface observations being assembled for the purpose of comprehensive validation of multi-dimensional AP composite propellant computational combustion models.     

Effect of binder melt flow on the leading edge flames of solid propellant sandwiches

This study reports the effect of binder melt flow on the burning behaviour, specifically the burning rate controlling sites referred to as the leading edge flames (LEFs), of different types of sandwich propellants, namely, pure, micro-aluminized and nano-aluminized binder. The distance between the LEFs anchored over the lamina interface edges of sandwiches is measured from the combustion images captured under high spatio-temporal resolution. Similarly, the extent of binder melt flow is also measured from the quenched surfaces of sandwiches. The burning rate experiments are performed as well on sandwiches with different middle lamina contents and thicknesses at pressures of 2, 4 and 7 MPa. Two different curing agents are considered to examine the melt flow behaviour of the binder. The curing agent significantly influences the inter-LEF distance mainly in the case of pure binder sandwiches, however, its effect is negligible in aluminized binder sandwiches because of the presence of Al particles that impedes the flow to appreciable extent. Substantial protrusion of the middle lamina relative to the lamina interfaces is observed in micro-aluminized binder sandwiches due to significant accumulation of Al particles on the burning surface. In the case of nano-aluminized binder sandwiches, such protrusion is relatively marginal since nano-Al particles burn quickly, which enables the gas phase flame to locate close to the burning surface, although the extent of Al accumulation is considerably more than in the former case. This causes the nano-aluminized binder sandwiches as a whole to burn significantly faster than the other two cases in the pressure range (<7 MPa) where the LEFs predominantly control the sandwich burning rates.     

The effects of SiO2 and TiO2 on the two-phase burning behavior of aqueous HAN propellant

Silica and titania nanoparticles were included at mass loadings of 1% and 3% in aqueous HAN propellants to evaluate their effects on liquid- and gas-phase decomposition and combustion. Both the liquid-phase and overall burning rates of propellant formulations were indirectly measured in a constant-volume strand burner filled with Argon from pressures of 3–22?MPa using a novel, pressure-based method developed by the authors in recent work. This approach provides overall burn times for propellants such as aqueous HAN which continue to burn beyond the disappearance of the liquid, making it superior to methods based solely on visual observation which only monitor the liquid surface regression. The presence of silica nanoparticles increased the liquid-phase burning rate in the low- and medium-pressure regimes (<10?MPa) and increased the overall burning rate at all pressures evaluated. The maximum amount of burning rate enhancement was realized at the lowest evaluated pressure (3?MPa) which corresponded to 80% and 670% increases in the liquid-phase and overall burning rates, respectively, for a silica loading of 1%, and 160% and 830% increases in the liquid-phase and overall burning rates, respectively, for a silica loading of 3%. The presence of titania did not measurably affect the liquid-phase burning rate, but it did increase the overall burning rate in the low-pressure regime (<5.7?MPa). This low-pressure overall burning rate enhancement was not amplified by further titania loading from 1% to 3% and was maximized at the lowest evaluated pressure (3?MPa) which corresponded to a 500% increase in the overall burning rate. The observed enhancements of the propellant's liquid-phase and overall burning rates were attributed to the presence of catalytic processes which diminish at higher pressures. This work represents the first time nanoparticle additives have been utilized to tailor the combustion of liquid HAN-based monopropellants.     

Extinction of a propellant upon contact with a wall or substrate

The burning of nitroglycerine-based propellants N and NB under unsteady conditions, when the sample burns pressed to a heat conducting lateral wall or placed onto a heat conducting substrate, was studied. The experimental results were analyzed within the framework of the concept of the existence of tangential waves that propagate over the burning surface, giving rise to chaotically migrating burning spots. The dependences of the unburnt propellant layer thickness on the thermal activity of the wall material and on the burning rate were obtained.     

The numerical simulation of two-dimensional aluminized composite solid propellent combustion

A numerical framework is presented which examines, for the first time, the burning of two-dimensional aluminized solid propellants. Aluminized composite propellants present a difficult mathematical and numerical challenge because of complex physics and topological changes that occur at the propellant surface. For example, both mathematical models and appropriate numerical solvers must describe the regressing burning surface, aluminium particle detachment and evolution throughout the gas-phase flow field, surface tension effects, ignition and combustion of aluminium particles, phase change effects, agglomeration of aluminium particles, radiation feedback to the propellant surface, to name a few. All of these effects must be modelled in a consistent manner. A numerical framework for which these effects can be included in a rational fashion is currently being developed. This framework includes the level set method to capture the solid–gas interface as well as particle motion in the gas phase. Some preliminary calculations of the two-dimensional combustion field supported by a disc pack with embedded aluminium particles are presented.     

Experimental observations of the dependence of impedance tube behavior upon gas phase losses and propellant self-noise

Some of the unexpected behavior observed during admittance measurement of burning solid propellants in a modified impedance tube set-up is discussed. Specifically, repeated tests conducted with the same solid propellant resulted, unexpectedly, in different standing wave structures in the impedance tube when the exhaust configuration was changed. This resulted in the calculation of different admittances at the propellant surface. It is shown in this paper that the observed experimental trends can be explained when the presence of gas phase damping and a propellant self-noise are taken into consideration in the development of a simplified analytical model describing the behavior of the impedance tube.     

Influence of the composition and ionizing radiation on the stability of combustion of composite propellants

The results of an experimental study of the acoustic admittance of the burning surface of composite propellants performed with the use of a two-end combustion chamber (T-chamber) are presented. The effects of the composition of the composite propellant (type of fuel-binder, content of aluminum powder, burning rate catalysts) and of ionizing γ-radiation on the acoustic admittance, which characterizes the tendency of the combustion chamber to high-frequency instability, are analyzed.     

高密实含能颗粒床对流燃烧过程的实验研究

高密实含能颗粒床对流燃烧过程的实验研究张小兵，金志明，袁亚雄（南京理工大学动力学院南京２１００９４）关键词对流燃烧；含能颗粒床；实验研究１引言在高能燃料火箭推进系统和粒状火药炮膛内，火药颗粒燃烧的热量传递机理是强迫对流。当装填密度高到一定程度时，火焰...     

Flame structure of HMX/GAP propellant at high pressure

The chemical and thermal structure of a HMX/GAP propellant flame was investigated at a pressure of 0.5 MPa using molecular beam mass spectrometry and a microthermocouple technique. The pressure dependence of the burning rate was measured in the pressure range of 0.5–2 MPa. The mass spectrometric probing technique developed for flames of energetic materials was updated to study the chemical structure of HMX/GAP flames at high pressures. Eleven species, including HMX vapor, were identified, and their concentrations were measured in a zone adjacent to the burning surface at pressures of 0.5 and 1 MPa. Temperature profiles in the propellant combustion wave were measured at pressures of 0.5 and 1 MPa. Species concentration profiles were measured at 0.5 MPa. Two main zones of chemical reactions in the flame were found. The data obtained can be used to develop and validate combustion models for HMX/GAP propellants.