This paper examines scanning electron micrographs for the size of cells that appear on the burning surfaces of (1) single crystals and pressed pellets of ammonium perchlorate (AP), and (2) mixtures of fine AP particles with polymeric binder, quenched by rapid depressurization at different pressures. While the cellular structures in self-deflagration of AP are well known yet unexamined quantitatively, those on fine AP/binder mixtures are examined in the context of their mid-pressure extinction leading to plateau burning rate behavior of propellants containing them. The results are an experimental validation of the theoretical predictions of Margolis and Williams (1988) [1] based on diffusional-thermal coupling in the intrinsic stability of combustion of premixed solids. © 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Satyanarayanan R. Chakravarthy

Department of Aerospace Engineering

combustion

Explosives

Inorganic compounds

Mixtures

Mobile security

Propellants

Scanning electron microscopy

Single crystals

Solid propellants

CELLULAR INSTABILITIES

Experimental investigations

Experimental validations

Premixed flame

Rapid depressurization

Scanning electron micrographs

SOLID PROPELLANT COMBUSTION

Thermal coupling

Binders

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IIT Madras

Proceedings of the Combustion Institute

Nano-aluminium particles of ∼50 nm size, produced at this laboratory, are added to composite solid propellants based on ammonium perchlorate and hydroxyl-terminated poly-butadiene binder that exhibit plateau burning rate trends and those including burning rate catalysts. The nano-aluminized propellant burning rates are compared with corresponding micro-aluminized and non-aluminized ones in the 1-12 MPa pressure range. The mid-pressure extinction of the matrixes containing the fine-sized ammonium perchlorate particles in the propellant along with the binder is investigated in all the cases to understand the mechanism of plateau-burning. Further, the variations in aluminium content, the aluminium size (within nano- and micro-ranges), bimodal combination of nano- and micro-aluminium are considered. Ferric oxide and titanium dioxide are the burning rate catalysts considered. Large scale accumulation of aluminium is observed not only in micro-aluminized matrixes, but also in nano-aluminized ones as clusters of 1-5 μm size. Since aluminium is added at the expense of the coarse ammonium perchlorate particles to preserve the total-solids loading in the present formulations, addition of micro-aluminium decreases the burning rate; whereas, nano-aluminized propellants exhibit ∼80-100% increase in the burning rate under most conditions. The near-complete combustion of nano-aluminium close to the burning surface of the propellant provides heat feedback that controls the burning rate. Mid-pressure extinctions of matrixes and plateau burning rates of propellants are washed out when nano-aluminium is progressively added beyond 50% in bimodal aluminium blends, but low pressure-exponents are observed in the nano-aluminized propellant burning rates at elevated pressures. Adjusting the plasticizer content in the binder alters the pressure range of plateau burning rates in non-aluminized propellants. Catalysts increase the burning rate by ∼50-100% in non-aluminized and micro-aluminized propellants, but in nano-aluminized propellants, the μm-sized catalyst does not affect the burning rate significantly; whereas, the nanometre size catalysts increases the burning rate merely by ∼5-15%. © 2009 The Combustion Institute.

Combustion and Flame

Effect of nano-aluminium in plateau-burning and catalyzed composite solid propellant combustion

The burning rate trends of both pure binder sandwiches and those containing a mixture of binder with fine ammonium perchlorate particles are investigated over appreciable ranges of pressure and thickness of the middle lamina in the sandwich. The crucial aspect of this study is the wide variation of the fine ammonium perchlorate/binder ratio in the middle lamina of the sandwiches. The sandwiches with fine ammonium perchlorate particles have been quenched by rapid depressurization and their surface profiles have been carefully mapped for different conditions. The gas phase flame structure is estimated through numerical simulation, with the observed surface profiles and burning rates taken as inputs. It is found that, as the fine ammonium perchlorate particles are added to the binder in small quantities, they tend to dilute the fuel and increase the lamina thickness for maximum interaction between the leading edge flames, but the trend in the variation of the optimum thickness for maximum burning rate with increase in pressure is reversed when compared to the case of pure binder sandwiches reported earlier. The burning rates of these sandwiches decrease slightly relative to those of the pure binder sandwiches. With higher fine AP content, there is greater interaction between the leading edge flames due to the inward shift of the stoichiometric surface and the extension of the fuel-rich sides of the leading edge flames over the middle lamina, ultimately resulting in the establishment of the canopy premixed flame between them. This restores the optimum thickness trends with pressure as in the case of pure binder sandwiches. © 2008 The Combustion Institute.

Coupling of leading edge flames in the combustion zone of composite solid propellants

This paper presents burning rates as a function of pressure of several propellant formulations based on ammonium perchlorate (AP) and hydroxyl-terminated polybutadiene cured by isophorone diisocyanate, many of which exhibit significantly low (nearly zero or negative) values of the pressure exponent of the burning rate in distinct pressure ranges, termed as plateau burning rate trends. The propellants contain a bimodal distribution of AP particles with the size of the coarse and fine particles within narrow ranges whose mean values are widely separated. Two mean sizes of fine particles were considered for the propellant formulations in the present work, namely, 5 and 20 μm. These choices are based on the mid-pressure extinction behavior exhibited by the matrix of fine AP and binder contained in the propellants but when tested alone over a wide range of fine AP size and pressure. The propellants that include the fine AP/binder matrixes exhibiting a mid-pressure extinction, in turn, exhibit the plateau burning rate trends within the corresponding pressure ranges. A plateau is also observed at elevated pressures in the burning rates of some formulations, which is related to the diminishing relative importance of the near-surface leading-edge region of the oxidizer/fuel diffusion flame in the gas-phase combustion zone. The choice of the coarse AP size influences the exact pressure range within the mid-pressure extinction domain of the matrix where the propellant exhibits the plateau burning rate trends. © 2007 Springer Science+Business Media, Inc.

Combustion, Explosion and Shock Waves

Ammonium perchlorate-based composite solid propellant formulations with plateau burning rate trends

This paper examines afresh features of ammonium-perchlorate (AP) combustion. Here an AP model, which predicts most experimental observations, is proposed. The one-dimensional aero-thermo-chemical field is captured through the solution of mass, energy, and species conservation equations. The unsteady one-dimensional phase heat transfer accounting for regression is solved for in the condensed phase. The uncertain parameters for AP pyrolysis, gas-phase reaction, and extent of surface exothermicity are chosen so that the most certain of the measured parameters of AP combustion, namely, pressure index of stable combustion (0.77 between 2 and 7 MPa), initial temperature sensitivity of burn rate, σ p (about 0.0021 to 0.0015 K -1), range of surface temperatures measured and suggested in several studies (∼850 to 875 K), and low-pressure deflagration limit (LPDL) at 300 K (∼2 MPa), are correctly predicted. The results obtained show the pressure index of AP combustion to be 0.77 and σ p to be 0.0024-0.0023 K -1. The LPDL is caused by a combination of loss of liquid layer and transient conduction into the condensed phase and not by heat loss. Copyright © 2006 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

Journal of Propulsion and Power

Revisiting the modeling of ammonium-perchlorate combustion: Development of an unsteady model

The onset of the midpressure extinction of certain formulations (termed matrixes) of ammonium perchlorate (AP) of monomodal particle size distribution and hydroxyl-terminated polybutadiene binder, in the pressure range around 2-5 MPa, is examined. These matrixes, besides being tested in isolation, have been included in between AP laminas to form sandwiches and mixed with coarse AP particles to form high solids-loading (87.5%) non-aluminized propellants. The burning rates of the sandwiches show abnormal trends with pressure such as low or negative exponents in ranges corresponding to the onset of the midpressure extinction of their respective matrixes. The propellants exhibit this behavior to a lesser degree. Quenched surfaces (self-extinguished or intentionally interrupted during burning) of all the three types of samples were analyzed using a scanning electron microscope, and the burning history of the samples was captured with a high-speed digital camera. The results indicate the prevalence of intermittent burning of the matrixes as the pressure is varied across the boundary between continuous burning and self-extinction (burn/ no-burn boundary) of the matrixes. The burning surfaces are marked by extreme three dimensionality coupled with a redistribution of the fine AP particles and the binder. The observations are explained based on the combined effects of the need for the AP particles and the binder to accumulate relative to each other on the burning surface depending on the difference in their pyrolysis rates and the existence of the binder in a molten state on the burning surface.

Intermittent Burning of Ammonium Perchlorate-Hydrocarbon Binder Monomodal Matrixes, Sandwiches, and Propellants

Experimental investigation of cellular instability in Ammonium Perchlorate (AP) and fine AP-binder mixtures

Journal	Data powered by TypesetProceedings of the Combustion Institute
Publisher	Data powered by TypesetElsevier Ltd
ISSN	15407489
Open Access	No