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Wada, Naoyuki,Kim, Chang-Hwan,Yoh, Jack J.,Hamashima, Hideki,Hokamoto, Kazuyuki The Japan Institute of Metals 2012 Materials Transactions Vol.53 No.1
<P>An experimental method to synthesize titanium dioxide (TiO<SUB>2</SUB>) using high-power laser in water was performed. A high-power Nd:YAG pulsed laser was used for the synthesis, with the laser energy fixed at 1 J/pulse. This laser was focused on a titanium wire set in water. This investigation recovered nano-sized anatase phase titanium dioxide with well crystallized structure. Pulsed bubbles generated in the water were confirmed by optical measurement, and their collapse may have induced high pressures and temperatures. The bubbles generated was approximately spherical in shape, with an estimated maximum size of 3.7 mm generated about 200 µs later after focusing the laser. Recovered powders were confirmed as single anatase phase titanium dioxide by XRD analysis. Effects of bubbles on synthesis and crystallization are also suggested.</P>
Ambekar, Anirudha,Yoh, Jack J. Elsevier 2019 Proceedings of the Combustion Institute Vol.37 No.3
<P>This study reports an experimental investigation into the chemical kinetics of several commercial pyrotechnic compositions. Differential Scanning Calorimetry (DSC) was utilized to elucidate the thermo-kinetic characteristics of four multicomponent pyrotechnic compositions. The combustion process of typical pyrotechnics is primarily driven by condensed phase reactions including processes such as phase change, decomposition, and oxidation. The multicomponent nature of practical pyrotechnics results in a particularly complex interaction between the components when heated. A thermo-kinetic study was performed to simulate the heating experienced by the pyrotechnics before the combustion zone. The physical processes occurring within these temperature limits provide important insight into the overall combustion rate. The non-isothermal DSC experimental technique combined with isoconversional methods, such as Friedman and Starink methods were utilized to evaluate the apparent chemical kinetics parameters for these propellants. The observations from the DSC study and isoconversional kinetic analysis provided an insight into the phenomenology of the combustion process of pyrotechnics. The problem of highly variable activation energy due to the presence of multiple reactions was addressed through a mechanistic deconvolution using nonlinear regression technique. The study confirmed the prominence of oxidizer decomposition on overall combustion reaction kinetics.</P>
A reduced order model for prediction of the burning rates of multicomponent pyrotechnic propellants
Ambekar, Anirudha,Yoh, Jack J. Pergamon 2018 Applied thermal engineering Vol. No.
<P><B>Abstract</B></P> <P>This study reports a reduced order model for the prediction of the burning rate of pyrotechnic compositions. The combustion process of most pyrotechnics is primarily driven by condensed phase reactions. A priory estimation of the burning rate of pyrotechnics with multiple components may not be possible using the established methods. The study provides a simplified approach based on integral analysis of a proposed combustion wave structure for estimating the burning rate when the pyrotechnic composition, pure component thermo-physical properties, and thermo-kinetics parameter are known. The proposed combustion wave assumes a staged combustion process where the oxidizer undergoes decomposition in a broad reactive zone while fuel combustion occurs in a thin surface region. This approach takes account of the effective thermal conductivity as well as porosity of the pyrotechnic matrix. The pyrotechnic compositions studied here are expected to burn conductively at atmospheric pressure with little or no overpressure. The phenomenology of the combustion process of energetic materials is elucidated, and the reduced order model is validated through a case study.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Reduced order analytical model for burning rate of pyrotechnic compositions. </LI> <LI> Linear burning rate of multi-component granular porous pyrotechnics predicted. </LI> <LI> Conductive combustion regime with primary reactions occurring on the surface. </LI> <LI> Technique accounts for the propellant conductivity, heat of reaction, and porosity. </LI> <LI> Case study predictions for KClO<SUB>4</SUB> and KNO<SUB>3</SUB> based pyrotechnics reasonably accurate. </LI> </UL> </P>
Characterization of display pyrotechnic propellants: Colored light
Ambekar, Anirudha,Kim, Minsung,Yoh, Jack J. Pergamon 2017 Applied thermal engineering Vol. No.
<P><B>Abstract</B></P> <P>Pyrotechnic flames utilized as a source of illumination are typically characterized by the color and the luminous intensity of the flame. In the current study, five display pyrotechnic formulations utilized to produce colored flames have been investigated experimentally and theoretically. The experimental study was focused on photographically recording the colored flame of fireworks propellants burning at ambient conditions in order to quantify the flame color and the luminous intensity. This study reports the application of a digital single reflex camera as a colorimeter as well as a luminance meter. Theoretical estimation of the flame color was carried out by equilibrium thermochemical analysis followed by additive color mixing of the spectral emitters deduced to be present in the combustion products. The experimentally observed chromaticity and theoretical predictions were found to be reasonably close validating the methodology for the color prediction.</P>
Simulating sympathetic detonation using the hydrodynamic models and constitutive equations
김보훈,김민성,Taeboo Sun,Jack J. Yoh 대한기계학회 2016 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.30 No.12
A Sympathetic detonation (SD) is a detonation of an explosive charge by a nearby explosion. Most of times it is unintended while the impact of blast fragments or strong shock waves from the initiating donor explosive is the cause of SD. We investigate the SD of a cylindrical explosive charge (64 % RDX, 20 % Al, 16 % HTPB) contained in a steel casing. The constitutive relations for high explosive are obtained from a thermo-chemical code that provides the size effect data without the rate stick data typically used for building the rate law and equation of state. A full size SD test of eight pallet-packaged artillery shells is performed that provides the pressure data while the hydrodynamic model with proper constitutive relations for reactive materials and the fragmentation model for steel casing is conducted to replicate the experimental findings. The work presents a novel effort to accurately model and reproduce the sympathetic detonation event with a reduced experimental effort.
Shock to detonation transition analysis using experiments and models
Kim, Bohoon,Kim, Minsung,Yoh, Jack J. Elsevier 2017 Proceedings of the Combustion Institute Vol.36 No.2
<P><B>Abstract</B></P> <P>The performance of a pyrotechnic device consisting of donor/acceptor pair separated by a thin inert material or a gap relies on the shock sensitivity of the energetic materials and detonation shock attenuation in the gap. Despite its common use, full-scale numerical simulation of the device configured in an explosive train is seldom reported because the proper modeling of the entire process requires precise capturing of extreme pressure waves from a donor charge during its attenuation in the inert gap before triggering an acceptor charge and accurate description of high strain rate dynamics of both reactive and inert solids. We developed a hybrid particle level-set based multi-material hydrocode with reactive flow models for Donor (Pentolite) – Gap (PMMA) – Acceptor (aluminized RDX). The complex shock interaction, critical gap thickness, and Go/No-go characteristics of the explosive train were quantitatively investigated. An additional detailed simulation of a miniaturized pyrotechnic initiator in a train of Donor (HMX) – Gap (steel) – Acceptor (aluminized RDX) revealed the existence of a critical gap thickness for successful operation of a small device.</P>