Numerical study of carbonaceous nanoparticle formation behind shock waves

In this study the laser-induced incandescence (LII) diagnostics has been applied for sizing of carbon-encapsulated iron nanoparticles (CEINs). The carbon covered iron nanoparticles were synthesized by shock waves pyrolysis of the mixtures of Fe(CO)5 with C2H2 or C6H6 diluted with argon. Iron nanoparticles were formed in the shock tube behind incident shock waves at the temperatures of 700–1000 K. The pyrolysis of hydrocarbons behind reflected shock waves at the temperatures of 1400–2000 K resulted in formation of carbon shell over iron nanoparticles. At the end of the CEINs formation process, particles were heated by one pulse of Nd:YAG laser operated at wavelength 1064 nm with fluences varied in the range 70– 800 mJ/cm2 to collect LII signals. The LII model, which had been used previously for iron and carbon nanoparticles separately, was updated for carbon-encapsulated iron nanoparticles. Additionally, the nanoparticle samples were investigated by a transmission electron microscope (TEM). The iron core size and carbon shell thickness were measured by statistical treatment of microphotographs. The comparison of TEM and LII particle sizing results is discussed.

We present a new prototype acoustic chemometric approach for prediction of powder particle size distributions, intended for in-line implementation. The standard basic solutions demand that calibration be carried out on representative, ‘non-segregated’ reference powder samples. However, as practical powder flow with no segregation is extremely difficult to achieve with the precision needed for calibration, there will always be a significant uncertainty in the reference values relative to what is actually measured. The problem is flow segregation. In order to solve this problem, we have designed a completely new acoustic chemometric approach, which by way of contrast forces the flowing powder mass to segregate as much as possible by various mechanical means. The new approach measures the acoustic signals from an integrated series of segregated, part-sample characteristics. The calibration X-data matrix now becomes a three-way matrix, which demands a three-way calibration solution to ‘unscramble’ the latent information in the maximally segregated powder sample. Thus the problem is now the solution. Our earlier forays into this matter, which were based on two-way calibrations, have all been limited by a severe ‘particle size ratio’ bracket outside which destructive self-damping has effectively negated practical, useful accuracy and precision. The new approach allows a much greater range of contrasting particle sizes. Our first-generation results achieved by using three-way PLS-R as well as the standard two-way calibrations show that it is more precise than all earlier attempts and can be used for many-component mixtures without extensive further modifications. We also look at the feasibility of quantifying for prediction of in-line particle size distributions in an industrial environment. Copyright © 2000 John Wiley & Sons, Ltd.

Measurements of ignition delay times in cornstarch dust-air mixture

This report describes an optical sensing system for fine particle detection using a combination of two techniques: laser-induced breakdown spectroscopy (LIBS) and laserinduced incandescence (LII). We have improved laser measurement systems applied for small-particle analysis. We have determined elemental composition and density of particles using LIBS and have been able to measure particle size using LII. Controlling the power density of the light source of the present system allowed switching from LIBS to LII. LII temporal analysis was conducted using a streak camera, which was also used for LIBS analysis. The LII technique allows in situ measurement of the average primary particle size of nanoscale soot particles.

Three simple methods are presented for predicting particle size distribution (PSD) percentiles when subjecting a pharmaceutical crystallization wet-milled slurry to a single temperature cycle for two of Biogen's drug substances. The methods are (I) a mathematical manipulation of the wet-milled PSD based on particle death during dissolution; (II) a linear empirical model fit of product PSD percentiles as a function of the wet-milled PSD percentiles and the amount of material dissolved; and (III) a nonlinear model fit to the change in PSD as a function of the amount of material dissolved and the particle surface area/volume ratio. The three methods were verified on production scale batches. Method II's approach was also used for particle size control during manufacture by implementing a temperature solved from the model using the batch's wet-milled particle size percentiles and a final target size. Method III was found to be the most accurate at prediction because it models particle surface area to volume ratio influencing crystal dissolution and growth rates. Method II was found to be not as accurate as method III; however, it was easily implementable during process control at the production scale. Method I provided the quickest estimate of final particle size using only the initial PSD and a solubility expression. Method III was recommended for best particle size prediction.

Visualization study of ignition modes behind bifurcated-reflected shock waves

Experimental study on the non-equilibrium condensation of a liquid film after shock wave in a vertical diaphragmless shock tube

The process of reflection of shock waves (SW) from a solid wall in a two-component mixture of condensed materials is studied within the framework of mechanics of heterogeneous media. The velocity of a reflected SW and the values of the parameters behind its front are analytically determined as functions of the velocity of the incident wave and the initial parameters of the mixture. It is shown that the absolute value of the velocity of the reflected SW can be greater than the velocity of the incident SW in mixtures with a small content of the light component and at low velocities of the incident shock wave. The nonmonotonic character of the dependence of pressure in the final equilibrium state behind the incident SW on the initial volume concentration of particles is demonstrated. The velocity of the incident SW is estimated for the case where a similar effect is also observed behind a reflected SW. It is established that, for weak shock waves, the dependence of the amplification factor of the reflected SW on the initial volume concentration of the light component is nonmonotonic and has a local maximum. It is noted that, as the velocity of the incident SW increases, the effect of compacting of the mixture (increase in concentration of the heavy component) behind the reflected SW becomes much less pronounced than in a passing SW.

Numerous experimental investigations on the reflection of plane shock waves over straight wedges indicated that there is a domain, frequently referred to as the weak shock wave domain, inside which the resulted wave configurations resemble the wave configuration of a Mach reflection although the classical three-shock theory does not provide an analytical solution. This paradox is known in the literature as the von Neumann paradox. While numerically investigating this paradox Colella & Henderson [1] suggested that the observed reflections were not Mach reflections but another reflection, in which the reflected wave at the triple point was not a shock wave but a compression wave. They termed them it von Neumann reflection. Consequently, based on their study there was no paradox since the three-shock theory never aimed at predicting this wave configuration. Vasilev & Kraiko [2] who numerically investigated the same phenomenon a decade later concluded that the wave configuration, inside the questionable domain, includes in addition to the three shock waves a very tiny Prandtl-Meyer expansion fan centered at the triple point. This wave configuration, which was first predicted by Guderley [3], was recently observed experimentally by Skews & Ashworth [4] who named it Guderley reflection. The entire phenomenon was re-investigated by us analytically. It has been found that there are in fact three different reflection configurations inside the weak reflection domain: • A von Neumann reflection – vNR, • A yet not named reflection – R, • A Guderley reflection – GR. The transition boundaries between MR, vNR, R and GR and their domains have been determined analytically. The reported study presents for the first time a full solution of the weak shock wave domain, which has been puzzling the scientific community for a few decades. Although the present study has been conducted in a perfect gas, it is believed that the reported various wave configurations, namely, vNR, R and GR, exist also in the reflection of shock waves in condensed matter.

This paper demonstrates an application of artificial neural network (ANN) for determination of underflow and overflow densities of hydrocyclone separators. The discussions are extended and further results are presented for the prediction of particle size distributions in the underflow and overflow streams. The fit of the experimental results against the predicted results are illustrated and a statistical analysis is made. It is shown that, once the history of the operations are known, the ANN proves to he a useful tool for predicting future separation efficiencies. This approach has a potential to eliminate the need for installation of expensive on-line instruments for density measurements and particle size analyses. This approach can be applied in similar situations in the mineral processing industry.

Computational study of an incident shock wave into a Helmholtz resonator

We present computational results of two different cases on the evolution of shock-SF6 heavy bubble interaction. And the shock-focusing processes and jet formation mechanisms are analyzed by using the high resolution of computation schemes, and the influence of reflected shock wave is also investigated. It is concluded that there are two stages in the shock-focusing process behind the incident shock wave. However, different jet formation mechanisms are found behind the reflected shock wave in case 2, owing to the gas being with a rightward velocity before the reflected shock wave impinges. The multi-influences of different shock waves and high-pressure zones in the vicinity of the bubble interface could induce a leftward velocity, which can promote the formation of jets. In addition, the time for the bubble evolution before the impingement of reflected shock wave in case 1 is larger than that in case 2, so the distorted bubble is getting more flat, which would induce the weakening of the influence of reflected shock in the bubble.

Development of instability in the region of mixing of fast-moving gases of different densities is a topical problem for energy-commuting facilities. Conventionally, the mixing layer is considered as a surface of the continuity break, i.e., as the contact break. The interaction of the shock wave with a perturbed contact discontinuity generates the Richtmeyer‐Meshkov instability. At the final stage, in the region of the initial contact break, the region of turbulent mixing separating the flows of compressed gases is formed. Numerous works on numerical modeling of the development of the Richtmeyer‐Meshkov instability based on the Euler equations did not take into account the influence of processes of mutual penetration of gases. Therefore, it is of interest to investigate this problem based on the equations of the double-speed two-temperature gas mixture, where each component has its own velocity and temperature. This approach allows one to describe the development of instability at various stages allowing for mutual penetration of gases. The necessity of applying models of multicomponent systems to describe the destruction of the contact boundary and formation of the region of the mixture was, e.g., pointed out in [1]. In [2], a semiempirical model of turbulent mixing of a multicomponent medium based on the use of its own velocity for each component is constructed. In this model, it is assumed that turbulent mixing appears instantly. Below, based on the equations of the double-speed two-temperature gas dynamics of the mixtures, we investigate the processes at the initial stages of mixing. We previously [4] investigated the interaction of the mixing layer with the incident shock wave and compression wave [5]. In this work, on the basis of equations of the doublespeed two-temperature gas dynamics of the mixtures [3], we investigate the processes of development of the Richtmeyer‐Meshkov instability during the interaction of the incident shock wave with the perturbed mixing layer allowing for the further repeated interaction of the layer with the waves reflected from the end. The initial sinusoidally perturbed diffusion layer of mixing was formed on the basis of the solution suggested in [4] with the perturbation amplitude a 0 , the perturbation wavelength λ , and the initial layer width δ 0 . The interaction of the mixing layer with the incident shock wave, which transfers from the light gas into the heavy gas and from the heavy gas into the light gas, is considered. The parameters of the mixture in the layer are described by the equations of the double-speed two-temperature gas dynamics of the mixtures [3], which transform to the Euler equations for pure gas out of the layer. The method of calculation of the starting set of equations is described in [4, 5]. The shock wave moved from upward to downward. At the upper boundary, the condition that derivatives were equal to zero was given, and the lower boundary is a solid wall. Symmetry conditions were given at the side boundaries. We considered the incidence of the shock wave on the mixing layer with the reflection angle in a onedimensional approximation in the absence of perturbations in the initial mixing layer. The results of these investigations are in complete agreement with the conclusions of [6].

Reflection of a plane shock wave from a wall has been simulated on a microscopic scale using a direct simulation Monte Carlo technique of the type developed by Bird. A monatomic gas model representing argon was used to describe the fluid medium and a simple one‐parameter accommodation coefficient model was used to describe the gas‐surface interaction. The influence of surface accommodation was studied parametrically by varying the accomodation coefficient from zero to one. Results are presented showing the temporal variations of flow field density, and mass, momentum, and energy fluxes to the wall during the shock wave reflection process. The energy flux was used to determine the wall temperature history. Comparisons with experiment are found to be satisfactory where data are available.

Detonation onset following shock wave focusing