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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>
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>
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>
Kim, Bohoon,Choi, Sanghun,Yoh, Jack J. Elsevier 2019 Combustion and Flame Vol.210 No.-
<P><B>Abstract</B></P> <P>An investigation of shock–particle interactions in reactive flows is performed using an Eulerian hydrodynamic method with a hybrid particle level-set algorithm to handle the material interface dynamics. The analysis is focused on the meso- to macro-scale numerical modeling of a granular metalized explosive containing randomly distributed metal particles intended to enhance its blast effect. The reactive flow model is used for the cyclotrimethylene-trinitramine (RDX) component, while thermally induced deflagration kinetics describes the aerobic reaction of the metal particles. The complex interfacial algorithm, which uses aligned level sets to track deforming surface between multi materials and to generate the random shape of granule elements, is described for aluminized and copperized RDX. Then, the shock-induced collapse of metal particles embedded in the condensed phase domain of a high explosive is simulated. Both aluminized and copperized RDX are shown to detonate with a shock wave followed by the burning of the metal particles. The energy release and the afterburning behavior behind the detonating shock wave successfully identified the precursor that gave rise to the development of deflagration of the metal particles.</P>