Condensation of an argon—monosilane mixture in a free jet

A mass–spectrometric study of the condensation of pure Ar and a 5% SiH₄ + 95% Ar mixture in a supersonic pulse free jet in a broad interval of stagnation pressures is performed. It is shown that a small content of monosilane in argon leads to the fact that condensation in the mixture begins at lower stagnation pressures than in pure argon; at high stagnation pressures, mixed argon—silane complexes are formed in the flow. The sequence of the stages of cluster formation in the mixture is determined.     

Effect of process conditions on the synthesis of carbon nanotubes by catalytic decomposition of methane

A new dual-composition catalyst based on Ni-Mo/MgO with high efficiency of producing carbon nanotubes （CNTs） from methane was reported recently. In the present article, with this type of catalyst, the impact of such experimental parameters as reaction temperature, reaction time, concentration of H2, flow rate ratio of CH4 to H2 on yield and graphitization were investigated, leading to the following optimal growth conditions： reaction time 60min, reaction temperature 900℃, CH4：H2 about 100：20mL/min, under which high-yield multi-walled CNTs bundles were synthesized. Raman measurement indicated that the as-synthesized product was well-graphitized, and the purity was estimated over 95% by TG-DSC analysis. In terms of the above results, an explanation of high-efficiency formation of CNTs bundles and the co-catalysis mechanism of Ni-Mo/MgO were suggested. 2007 Chinese Societv of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V.     

Multiaxial yield surface of transversely isotropic foams: Part I—Modeling

A new yield criterion is proposed for transversely isotropic solid foams. Its derivation is based on the hypothesis that the yielding in foams is driven by the total strain energy density, rather than a completely phenomenological approach. This allows defining the yield surface with minimal number of parameters and does not require complex experiments. The general framework used leads to the introduction of new scalar measures of stress and strain (characteristic stress and strain) for transversely isotropic foams. Furthermore, the central hypothesis that the total strain energy density drives yielding in foams ascribes to the characteristic stress an analogous role of von Mises stress in metal plasticity. Unlike the overwhelming majority of yield models in literature the proposed model recognizes the tension–compression difference in yield behavior of foams through a linear mean stress term. Predictions of the proposed yield model are in excellent agreement with the results of uniaxial, biaxial and triaxial FE analyses implemented on both isotropic and transversely isotropic Kelvin foam models.     

Effect of the Stone–Wales defect on the structure and mechanical properties of single-wall carbon nanotubes in axial stretch and twist

The effect of the Stone–Wales defect due to the rotation of a pair of neighboring atoms on the equilibrium structure and mechanical properties of single-wall carbon nanotubes in axial stretch and twist is considered. The position of carbon atoms in a test section consisting of a number of repeated units hosting a solitary Stone–Wales defect is computed by minimizing the Tersoff–Brenner potential. The energy invested in the defect is found to decrease as the radius of the nanotube becomes smaller. Numerical computations for nanotubes with zigzag and armchair chiralities show that inclined, axial, and circumferential defect orientations have a strong influence on the mechanical response in axial stretch and twist. Stretching may cause the defect energy to become negative, revealing the possibility of spontaneous defect formation leading to failure. In some cases, stretching may eliminate the defect and purify the nanotube. When the tube is twisted around its axis, a neck develops at the location of the defect, signaling possible disintegration.     

Numerical simulation of hydrogen–air reacting flows in rectangular channels with catalytic surface reactions

In this work a prediction was numerically modeled for a catalytically stabilized thermal combustion of a lean homogeneous mixture of air and hydrogen. The mixture flows in a narrow rectangular channel lined with a thin coating of platinum catalyst. The solution using an in-house code is based on the steady state partial differential continuity, momentum and energy conservation equations for the mixture and species involved in the reactions. A marching technique is used along the streamwise direction to solve the 2-D plane-symmetric laminar flow of the gas. Two chemical kinetic reaction mechanisms were included; one for the gas phase reactions consisting of 17 elementary reactions; of which 7 are forward–backward reactions while the other mechanism is for the surface reactions—which are the prime mover of the combustion under a lean mixture condition—consisting of 16 elementary reactions. The results were compared with a former congruent experimental work where temperature was measured using thermocouples, while using PLIF laser for measuring water and hydrogen mole fractions. The comparison showed good agreement. More results for the velocities, mole fractions of other species were carried out across the transverse and along the streamwise directions providing a complete picture of overall mechanism—gas and surface—and on the production, consumptions and travel of the different species. The variations of the average OH mole fraction with the streamwise direction showed a sudden increase in the region where the ignition occurred. Also the rate of reactions of the entire surface species were calculated along the streamwise direction and a surface water production flux equation was derived by calculating the law of mass action’s constants from the concentrations of hydrogen, oxygen and the rate of formation of water near the surface.     

A structural mechanics approach for predicting the mechanical properties of carbon nanotubes

Based on molecular mechanics, a structural mechanics model of carbon nanotubes (CNTs) was developed with special consideration given to the bending stiffness of the graphite layer. The potentials associated with the atomic interactions within a CNT were evaluated by the strain energies of beam elements which serve as structural substitutions of covalent bonds in a CNT. In contrast to the original model developed by Li and Chou (Int. J. Solids Struct. 40(10):2487–2499, 2003), in the current model the out-of-plane deformation (inversion) of the bond was distinguished from the in-plane deformation by considering a rectangular cross-section for the beam element. Consequently, the model is able to study problems where the effect of local bending of the graphite layer in a carbon nanotube is significant. A closed-form solution of the sectional properties of the beam element was derived analytically. The model was verified through the analysis of rolling a graphite sheet into a carbon nanotube. Using the present model, the buckling behavior of nanotubes under bending is simulated. The predicted critical bending angle agrees well with molecular dynamics simulations.     

Finite differences study of ignition in a methane–air mixture flow

A detailed simulation of the ignition process and premixed flames propagation, taking into account molecular transport, chemical reaction, thermodynamics and convection, is built by making use of the implicit finite difference scheme with the help of the Tridiagonal Matrix Algorithm. The velocity of chemical reaction is expressed by means of Arrhenius form of first order in both fuel and oxygen. The main objective of this work is to define numerically in two cases, u=0.1 m/s and u=0.4 m/s, the ignition temperature of the methane–air mixture with different air excess coefficients in the mixture. In addition, the effect of the thickness of the region ignition and of ignition location on the transient behavior of the flame was studied.     

Waves on the free surface of a two‐phase medium

A boundaryvalue problem is posed to determine the wave motion caused by propagation of a gravity wave on the free surface of a layer of a twophase medium. The problem is solved analytically in the linear approximation. The shape of the free surface, the phase velocity, and the frequency and damping factor of the wave are determined. An example of the solution of the problem is given.     

Exact solution of the capillary diffusion equation for a three‐component mixture

The paper gives an exact solution of the steady system of equations for stable threecomponent diffusion in the entire range of concentrations for a long capillary under a controlled capillary pressure differential. The solution allows one to calculate the distributions of component concentrations and mixture density along the capillary. It is shown that if the diffusion coefficients are markedly different, an extremum of mixture density can arise inside the capillary. In particular, if the density of the mixture in the upper flask is higher than that in the lower flask and the stratification of the system is generally stable, a region with a reverse density gradient that is unstable against gravity convection can appear inside the capillary. A comparison with experimental results shows that the resistance to gravity convection is disturbed when an extremum of mixture density arises in the channel during steady diffusion.     

Global nonlinear stability in the Bénard problem for a mixture near the bifurcation point

The nonlinear stability of the motionless state of a binary fluid mixture heated and salted from below, in the Oberbeck-Boussinesq scheme, for stress-free and rigid-rigid boundary conditions and Schmidt numbers P_C greater than Prandtl numbers P_T, is studied in the region around the bifurcation point of linear instability. An improvement of the results in Mulone [11] is found for small values of p = P _C /P _T and P_T. For p sufficiently large the critical nonlinear Rayleigh number is very close to the linear one (with relative difference less than in the sea water case) Received December 12, 2002 / Published online April 23, 2003 RID="a" ID="a" e-mail: mbasurto@dmi.unict.it RID="b" ID="b" e-mail: lombardo@dmi.unict.it ID="Communicated by Brian Straugham, Durham"     

Modeling of the mean Poincaré map on a class of random impact oscillators

In this paper, a class of random vibro-impact systems is studied. For this class of random systems, the general discrete-time model of systems described by mean of impact Poincarè map have been derived. Two engineering examples: a marine engine resiliently mounted under shock excitation and a stochastic rattling system have been investigated. The calculated results show that those models can reveal complex nonlinear behaviors. The bifurcation diagrams exhibit the routes to random chaos.     

Modeling the mechanics of HMX detonation using a Taylor–Galerkin scheme

Design of energetic materials is an exciting area in mechanics and materials science. Energetic composite materials are used as propellants, explosives, and fuel cell components. Energy release in these materials are accompanied by extreme events: shock waves travel at typical speeds of several thousand meters per second and the peak pressures can reach hundreds of gigapascals. In this paper, we develop a reactive dynamics code for modeling detonation wave features in one such material. The key contribution in this paper is an integrated algorithm to incorporate equations of state, Arrhenius kinetics, and mixing rules for particle detonation in a Taylor–Galerkin finite element simulation. We show that the scheme captures the distinct features of detonation waves, and the detonation velocity compares well with experiments reported in literature.     

Visualization of deflagration-to-detonation transitions in a channel with repeated obstacles using a hydrogen–oxygen mixture

The deflagration-to-detonation transition in a 100 mm square cross-section channel was investigated for a highly reactive stoichiometric hydrogen oxygen mixture at 70 kPa. Obstacles of 5 mm width and 5, 10, and 15 mm heights were equally spaced 60 mm apart at the bottom of the channel. The phenomenon was investigated primarily by time-resolved schlieren visualization from two orthogonal directions using a high-speed video camera. The detonation transition occurred over a remarkably short distance within only three or four repeated obstacles. The global flame speed just before the detonation transition was well below the sound speed of the combustion products and did not reach the sound speed of the initial unreacted gas for tests with an obstacle height of 5 and 10 mm. These results indicate that a detonation transition does not always require global flame acceleration beyond the speed of sound for highly reactive combustible mixtures. A possible mechanism for this detonation initiation was the mixing of the unreacted and reacted gas in the vicinity of the flame front convoluted by the vortex present behind each obstacle, and the formation of a hot spot by the shock wave. The final onset of the detonation originated from the unreacted gas pocket, which was surrounded by the obstacle downstream face and the channel wall.     

Collaborative optimization of screening efficiency and screen surface load — Part I: Modeling and evaluation

The screen surface load (SSL) caused by granular materials is an important factor affecting the structural performance of vibrating screen. Based on virtual experiment, a multi-objective collaborative optimization method is proposed to control the SSL under high screening efficiency (SE) in this work. Firstly, a DEM model was established to study the influence of process parameters on SE and SSL. Secondly, the NSGA-II (Non-dominated Sorting Genetic Algorithm) was employed to optimize the screening parameters with both SE and SSL as targets. The optimization method proves to be effective implementing on a linear vibrating screening. With SE equals to 98.5%, the SSL optimizable range is 39.2%. While compromising the SE to 88.7%, the SSL optimizable range improves to 48.6%. The result shows that the collaborative optimization could effectively control the SSL while maintaining a high SE, which is of great significance to improve the service life of screen surface and screen body.     

Kinematics of Bulkley–Herschel fluid flow with a free surface during the filling of a channel

The non-Newtonian fluid flow with a free surface occurring during the filling of a plane channel in the gravity field is modeled. The mathematical formulation of the problem using the rheological Bulkley–Herschel model is presented. A numerical finite-difference algorithm for solving this problem is developed. A parametric investigation of the main characteristics of the process as functions of the control parameters is performed. The effect of the rheological parameters of the fluid on the distribution of the quasisolid motion zones is demonstrated.     

Effect of shock–wave loading on the properties of cryogenic TiNi — a shape–memory alloy

Results of an experimental study of the shock–wave deformation of TiNi and its effect on the crystallographic structure and temperature of austenite–martensite transformations are given. It is found that, for pressures of up to 2 GPa, shock–wave loading changes the defect structure and parameters of the lattice; however, this does not lead to a noticeable change in the temperature of the austenite–martensite transformation and the manifestation of the shapeNdash;memory effect.