The transition from particle flamelet burning to premixed flame burning of ammonium perchlorate (AP)/hydrocarbon (HC) binder propellants is identified with detachment of the oxidizer/fuel (O/F) flamelet from the stoichiometric tip of the diffusion field over the AP particle. The conditions (pressure, particle area, local stoichiometry) for detachment are revealed by singular regions of the burning rate versus pressure curves for selected bimodal propellants.

Satyanarayanan R. Chakravarthy

Department of Aerospace Engineering

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

The Details of Combustion of Ammonium Perchlorate Propellants: Leading Edge Flame Detachment

Combustion Science and Technology

Combustion of sandwiches made of alternating laminae of ammonium perchlorate (AP) and a matrix of hydrocarbon binder mixed with fine AP and nano-aluminium particles is considered in the 1–12 MPa pressure range. Companion non-aluminized sandwiches are studied for comparison. The nano-Al particles are produced from the electrical wire explosion process in the size range ∼45 nm, and added to the matrix containing either 5 or 45 µm AP particles. Three categories of middle lamina thicknesses—thin (100–170 µm), intermediate (200–280 µm), and thick (370–480 µm)—are examined in all the sandwiches. High-speed high-magnification imaging of the combustion is performed, and surface profiles and features of quenched sandwiches are investigated, besides burning rate measurements. The sandwich burning rates are compared with those of the corresponding matrices and AP laminae burning alone as well. The results imply the close location of lamina leading edge flames (L-LEFs) over the lamina interface edges, leaving the outer AP laminae inclined steeply relative to the matrix middle lamina, in the nano-aluminized cases relative to the non-aluminized versions. This observation suggests that the L-LEFs control the burning rates of the sandwiches even in the case of the fast burning matrix that contains nano-Al. The non-aluminized sandwiches containing the 5 µm AP show higher burning rates than their 45 µm AP counterparts because of the enhanced heat feedback available to the matrix burning surface from the extended fuel-rich branches of adjacent L-LEFs in the former case. Therefore, the enhancement in burning rate is minimal with the inclusion of nano-Al in sandwiches with smaller fine AP size, whereas the nano-aluminized sandwiches containing larger fine AP particles exhibit such enhancement. These effects affirm previous results on propellants with smaller fine and coarse AP sizes, where the burning rate is not enhanced by nano-Al. © 2017 The Combustion Institute

Combustion and Flame

Combustion mechanism of composite solid propellant sandwiches containing nano-aluminium

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.

Proceedings of the Combustion Institute

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

This paper reports the production of nanoaluminum particles using the wire explosion process at our laboratory. Electrical energy is applied to the aluminum wire in an argon atmosphere to yield spherical particles around 50 nm in size. The material is also characterized by a number of methods. The nano-Al is included in bimodal ammonium Perchlorate composite solid propellants, and their burning rates are compared with those containing normal micron- sized aluminum and nonaluminized propellants in a total of 21 formulations in the 1-12 MPa pressure range. Nanoaluminized propellants show ∼100% increase in burning rate relative to nonaluminized or microaluminized ones, but only when large-sized coarse ammonium Perchlorate particles are used. Burning-rate plateaus observed in non- and microaluminized propellants are washed out with the addition of nano-Al, but the burning rates of nanoaluminized propellants register low pressure exponents in the elevated pressure range. The results suggest that the heat feedback from the diffusion-limited combustion of nano-Al particles near the propellant burning surface controls the propellant burning rate when sufficiently large parts of the burning surface are made up of the nanoaluminized fine ammonium-perchlorate/binder matrix.

Journal of Propulsion and Power

Production, characterization, and combustion of nanoaluminum in composite solid propellants

The process of agglomeration of aluminum particles on the burning surface of an ammonium perchlorate-based composite propellant is modeled using a computer algorithm. A random pack of particulate ingredients of given size and mass specifications is cast on the computer to simulate the propellant microstructure as reported previously. The aluminum particles are tracked as they emerge at a regressing burning surface, accumulate into filigrees, and get ignited by the near-surface leading-edge oxidizer-binder diffusion flamelets attached to the exposed areas of certain ammonium Perchlorate particles. An approximate heat transfer model is incorporated to estimate the ignition delays radially inward and outward from the leading-edge flamelets into the filigrees accumulated over ammonium Perchlorate particles and surrounding binder-fine ammonium Perchlorate matrix layers. The delay influences the number of parent aluminum particles constituting a filigree, and consequently the size of the agglomerate that the filigree rolls up into. The implementation of the algorithm is validated against experimental results available in the literature, which were specifically obtained to investigate the relationship between the decrease in the agglomerate size and attachment of leading-edge flamelets over the fine ammonium Perchlorate particles in the propellant with increase in pressure.

Computer model of aluminum agglomeration on burning surface of composite solid propellant

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.

Journal	Data powered by TypesetCombustion Science and Technology
Publisher	Data powered by TypesetTaylor and Francis Inc.
ISSN	00102202
Open Access	No