Explosive decomposition of heavy-metal azides

A real-time investigation of the explosive decomposition of heavy-metal azides is reported. A multichannel instrument configuration designed specifically for the goals of the study is described; it is capable of measuring the transient conductivity and the spectral and kinetic characteristics of the luminescence and absorption of exploding samples with nanosecond time resolution. New phenomena are discovered and analyzed in detail: the predetonation conductivity and predetonation luminescence of heavy-metal azides. The conductivity of silver azide in the predetonation state is used to make an experimentally justified decision as to whether the explosion is driven by a thermal or chain mechanism, in favor of the latter. The sum-total of the new data provides the basis for the development of an experimentally justified model of predetonation luminescence and the explosive decomposition process of heavy-metal azides, including the following principal stages: hole trapping by a cation vacancy, reconstruction of the center as a result of chemical reaction with the formation of a quasi-local hole state in the valence band, hole detrapping from the reconstructed center, carrier multiplication as a result of impact ionization by hot holes, and reconstruction of a local state in the bandgap, thereby establishing conditions for repetition of the investigated chain of processes. Zh. éksp. Teor. Fiz. 116, 1676–1693 (November 1999)     

Deflagration-to-detonation transition in spiral channels

The deflagration-to-detonation transition in hydrogen–air mixtures that fill spiral channels has been studied. A spiral channel has been produced in a cylindrical detonation tube with a twisted ribbon inside. The gas mixture has been ignited by means of a spark gap switch. The predetonation distance versus the twisted ribbon configuration and molar ratio between the gas mixture components has been determined. A pulling force exerted by the detonation tube after a single event of hydrogen–air mixture burnout has been found for four configurations of the twisted ribbon. Conditions under which the use of a spiral tube can be more effective (increase the pulling force) have been formulated.     

The influence of temperature gradients on the kinetic boundary layer problem for a condensing droplet

We extend an earlier method for solving kinetic boundary layer problems to the case of particles moving in aspatially inhomogeneous background. The method is developed for a gas mixture containing a supersaturated vapor and a light carrier gas from which a small droplet condenses. The release of heat of condensation causes a temperature difference between droplet and gas in the quasistationary state; the kinetic equation describing the vapor is the stationary Klein-Kramers equation for Brownian particles diffusing in a temperature gradient. By means of an expansion in Burnett functions, this equation is transformed into a set of coupled algebrodifferential equations. By numerical integration we construct fundamental solutions of this equation that are subsequently combined linearly to fulfill appropriate mesoscopic boundary conditions for particles leaving the droplet surface. In view of the intrinsic numerical instability of the system of equations, a novel procedure is developed to remove the admixture of fast growing solutions to the solutions of interest. The procedure is tested for a few model problems and then applied to a slightly simplified condensation problem with parameters corresponding to the condensation of mercury in a background of neon. The effects of thermal gradients and thermodiffusion on the growth rate of the droplet are small (of the order of 1%), but well outside of the margin of error of the method.     

Ion temperature and hydrodynamic-energy measurements in a Z-pinch plasma at stagnation

The time history of the local ion kinetic energy in a stagnating plasma was determined from Doppler-dominated line shapes. Using independent determination of the plasma properties for the same plasma region, the data allowed for inferring the time-dependent ion temperature, and for discriminating the temperature from the total ion kinetic energy. It is found that throughout most of the stagnation period the ion thermal energy constitutes a small fraction of the total ion kinetic energy; the latter is dominated by hydrodynamic motion. Both the ion hydrodynamic and thermal energies are observed to decrease to the electron thermal energy by the end of the stagnation period. It is confirmed that the total ion kinetic energy available at the stagnating plasma and the total radiation emitted are in balance, as obtained in our previous experiment. The dissipation time of the hydrodynamic energy thus appears to determine the duration (and power) of the K emission.     

Cyclic deflagration-to-detonation transition in the flow-type combustion chamber of a pulse-detonation burner

The possibility of realization of a rapid cyclic deflagration-to-detonation transition (DDT) with a frequency of up to 2 Hz under conditions of high-velocity flow (～10 m/s) and separate supply of the combustible mixture components (methane and air) in a tube, 5.5 m in length and 150 mm in diameter, with an open end at a low ignition energy (～1 J) is for the first time demonstrated. It is shown that such a tube with turbulizing obstacles of special shape and placement can ensure reliable DDT at a distance of 3–4 m from the ignition source within Δτ_DDT ≤ 20 ms after ignition. The results will be used in the development of a new type of industrial burner—a pulse-detonation burner for high-rate heating and fragmentation, combining thermal and shock-wave (mechanical) impacts on the treated object.     

Direct numerical simulations and analysis of three-dimensional n-heptane spray flames in a model swirl combustor

Three-dimensional n-heptane spray flames in a swirl combustor are investigated by means of direct numerical simulation (DNS) to provide insight into realistic spray evaporation and combustion as well as relevant modeling issues. The variable-density, low-Mach number Navier–Stokes equations are solved using a fully conservative and kinetic energy conserving finite difference scheme in cylindrical coordinates. Dispersed droplets are tracked in a Lagrangian framework. Droplet evaporation is described by an equilibrium model. Gas combustion is represented using an adaptive one-step irreversible reaction. Two different cases are studied: a lean case that resembles a lean direct injection combustion, and a rich case that represents the primary combustion region of a rich-burn/quick-quench/lean-burn combustor. The results suggest that premixed combustion contribute more than 70% to the total heat release rate, although diffusion flame have volumetrically a higher contribution. The conditional mean scalar dissipation rate is shown to be strongly influenced, especially in the rich case. The conditional mean evaporation rate increases almost linearly with mixture fraction in the lean case, but shows a more complex behavior in the rich case. The probability density functions (PDF) of mixture fraction in spray combustion are shown to be quite complex. To model this behavior, the formulation of the PDF in a transformed mixture fraction space is proposed and demonstrated to predict the DNS data reasonably well.     

Inhomogeneity of local temperature in small clusters in microcanonical equilibrium

Overall homogeneity of temperature is a condition for thermal equilibrium, but, as is demonstrated by classical molecular dynamics simulations, the local temperatures of atoms in small, isolated crystalline clusters in microcanonical equilibrium are not uniform. The effective temperature determined from individual atomic velocity decreases with distance from the cluster center. It is argued that these effects are due to the conservation of angular and translational momentum. A general microcanonical expression is derived for the spatial dependence of the statistics of the kinetic energies of individual atoms; this fits the numerical observations well.     

Deflagration-to-detonation transition in narrow channels: hydraulic resistance versus flame folding

This paper is concerned with the identification of the key interactions controlling the deflagration-to-detonation transition in narrow smooth-walled channels. Two agencies contributing to the transition are discussed: hydraulic resistance and flame folding. Depending on the kinetics and parameters of the system, nucleation of detonation may occur near the channel wall and/or in the channel interior. A possibly unexpected outcome of the resistance-folding interplay is the non-monotonicity of the dependence between the predetonation run up time/distance and the channel width.     

Explosive luminescence of silver azide

Spectroscopic-kinetic investigations of the luminescence accompanying the explosive decomposition of silver azide are performed. A new phenomenon is observed: predetonation luminescence. A comparison of the predetonation luminescence spectrum with the band structure is in agreement with a model in which the exothermic reaction 2N ₃ ⁰ →N₆ provides the energy for the explosion. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 2, 101–103 (25 July 1997)     

Soret效应对具有自由表面的圆柱形浅池内双组分溶液热对流影响的实验研究

为了探究Soret效应对具有自由表面的圆柱形浅液池内双组分溶液热对流过程的影响, 通过实验观察了质量分数为50%的正癸烷/正己烷混合溶液在不同深宽比的液池内流动失稳后的自由表面耗散结构及液池内的温度波动. 结果表明, 双组分溶液流动失稳的临界热毛细Reynolds数小于纯工质的值, 且其随液层深宽比的变化规律与纯工质相同. 当深宽比小于0.0848时, 流动失稳后在自由表面观察到热流体波, 监测点处温度波动主频随热毛细Reynolds数增大而增加; 当深宽比大于0.0848时, 随热毛细Reynolds数的增大, 流动失稳后自由表面依次呈现轮辐状、花苞状、分离-合并-分离交替变化的条纹状结构.     

Kinetic theory of transport coefficients of a relativistic binary mixture

Within the framework of relativistic kinetic theory expressions are derived for the diffusion and thermal diffusion coefficient of a binary mixture.     

Numerical Method and Composition at and out of Chemical Equilibrium in a Multitemperature Plasma. Application to a Pure Nitrogen Plasma

The disequilibrium between the temperatures (excitation, rotation, vibration, translation) in plasmas lead us to specific chemical potentials. Then, they are used to develop a mass action law in a multitemperature plasma. This mass action law enables us to determine the composition of a pure nitrogen mixture out of thermal equilibrium. The composition evolution versus time is determined from a kinetic method modified to take into account different temperature hypotheses. The composition and different thermodynamic properties of a pure nitrogen mixture are given at the chemical equilibrium, then versus time for various temperature hypotheses and thermal disequilibriums.     

Transport properties of a binary mixture of CO2ben N2 from the pair potential energy functions based on a semi-empirical inversion method

The potential energy surface of a CO₂-N₂ mixture is determined by using an inversion method, together with a new collision integral correlation [J. Phys. Chem. Ref. Data 19 1179 (1990)]. With the new invert potential, the transport properties of CO₂-N₂ mixture are presented in a temperature range from 273.15 K to 3273.15 K at low density by employing the Chapman-Enskog scheme and the Wang Chang-Uhlenbeck-de Boer theory, consisting of a viscosity coefficient, a thermal conductivity coefficient, a binary diffusion coefficient, and a thermal diffusion factor. The accuracy of the predicted results is estimated to be 2% for viscosity, 5% for thermal conductivity, and 10% for binary diffusion coefficient.     

Intensification of shock-induced combustion by electric-discharge-excited oxygen molecules: numerical study

Mechanisms of combustion enhancement in a supersonic H₂–O₂ reactive flow behind an oblique shock wave front are investigated when vibrational and electronic states of O₂ molecule are excited by an electric discharge. The analysis is carried out on the base of updated thermally nonequilibrium kinetic model for the H₂–O₂ mixture combustion. The presence of vibrationally and electronically excited O₂ molecules in the discharge-activated oxygen flow allows to intensify the chain mechanism and to shorten significantly the induction zone length at shock-induced combustion. It makes possible, for example, to ignite the atmospheric pressure H₂–O₂ mixture at the distance shorter than 1 m behind the weak oblique shock wave at a small energy E_s = 1.2 × 10^–2 J · cm^–3 input to O₂ molecules. At higher pressure it is needed to put greater specific energy into the gas in order to ignite the mixture at appropriate distances. It is shown that excitation of O₂ molecules by electric discharge is much more effective for accelerating the hydrogen–oxygen mixture combustion than mere heating the gas.     

Thermal conductance in a quantum waveguide modulated with quantum dots

We investigate the thermal conductance in a quantum waveguide modulated with quantum dots at low temperatures. It is found that the thermal conductance sensitively depends on the geometrical parameters of the structure and boundary conditions. When the stress-free boundary conditions are applied in the structure, the universal quantum of thermal conductance can be found regardless of the geometry details in the limit T→0. For an uniform quantum waveguide, a thermal conductance plateau can be observed at very low temperatures; while for the quantum waveguide modulated with quantum dots, the plateau disappears, instead a decrease of the thermal conductance can be observed as the temperature goes up in the low temperature region, and its magnitude can be adjusted by the radius of the quantum dot. Moreover, it is found that the quantum waveguide with two coupling quantum dots exhibits oscillatory decaying thermal conductance behavior with the distance between two quantum dots. However, when the hard-wall boundary conditions are applied, the thermal conductance displays different behaviors.     

Kinetic parameters,collision rates,energy exchanges and transport coefficients of non-thermal electrons in premixed flames at sub-breakdown electric field strengths

The effects of an electric field on the collision rates, energy exchanges and transport properties of electrons in premixed flames are investigated via solutions to the Boltzmann kinetic equation. The case of high electric field strength, which results in high-energy, non-thermal electrons, is analysed in detail at sub-breakdown conditions. The rates of inelastic collisions and the energy exchange between electrons and neutrals in the reaction zone of the flame are characterised quantitatively. The analysis includes attachment, ionisation, impact dissociation, and vibrational and electronic excitation processes. Our results suggest that Townsend breakdown occurs for E/N = 140 Td. Vibrational excitation is the dominant process up to breakdown, despite important rates of electronic excitation of CO, CO₂ and N₂ as well as impact dissociation of O₂ being apparent from 50 Td onwards. Ohmic heating in the reaction zone is found to be negligible (less than 2% of peak heat release rate) up to breakdown field strengths for realistic electron densities equal to 10¹⁰ cm^?3. The observed trends are largely independent of equivalence ratio. In the non-thermal regime, electron transport coefficients are insensitive to mixture composition and approximately constant across the flame, but are highly dependent on the electric field strength. In the thermal limit, kinetic parameters and transport coefficients vary substantially across the flame due to the spatially inhomogeneous concentration of water vapour. A practical approach for identifying the plasma regime (thermal versus non-thermal) in studies of electric field effects on flames is proposed.     

Decaying turbulence in the presence of a shearless uniform kinetic energy gradient

The decay of turbulent kinetic energy in nearly isotropic grid turbulence has been studied extensively as a fundamental point of reference for turbulence theories and numerical simulations. Most studies have focused on nearly homogeneous turbulence characterised by power-law decay. Other studies have focused on so-called shearless mixing layers, in which two regions with the same mean velocity but distinctly different kinetic energy levels slowly diffuse into each other downstream thus providing information about spatial transport of turbulence. Here, we introduce and study another type of shearless turbulent flow. It has initially a nearly uniform spatial gradient of kinetic energy of the form k ～ β(y ? y₀), where y is the spanwise position. In the experiments, this gradient is generated with the use of an active grid and screens mounted upstream of the wind-tunnel’s test section, iteratively designed to produce a uniform gradient of turbulent kinetic energy without mean velocity shear. Data are acquired using X-wire thermal anemometry at different spanwise and downstream locations. Profile measurements are used to quantify the constancy of the mean velocity and the linearity of the initial profile of kinetic energy. Measurements show that at all spanwise locations, the decay in the streamwise direction follows a power-law but with exponents n(y) that depend upon the spanwise location. The results are consistent with a decay of the form k/?u?² = β(x/x_ref)^?n(y)(y ? y₀)/M. Results for the development of integral length scale, and for velocity skewness and flatness factors are also presented. Significant deviations from Gaussianity are observed especially for the spanwise velocity component in the lower kinetic energy region. Future experiments will be needed including measurements of the dissipation rate ? at sufficient accuracy, in order to unambiguously partition the energy decay into dissipation and spatial diffusion.     

Kinetic energy of a trapped Fermi gas interacting with a Bose-Einstein condensate

We study a confined mixture of bosons and fermions in the regime of quantal degeneracy, with particular attention to the effects of the interactions on the kinetic energy of the fermionic component. We are able to explore a wide region of system parameters by identifying two scaling variables which completely determine its state at low temperature. These are the ratio of the boson-fermion and boson-boson interaction strengths and the ratio of the radii of the two clouds. We find that the effect of the interactions can be sizeable for reasonable choices of the parameters and that its experimental study can be used to infer the sign of the boson-fermion scattering length. The interplay between interactions and thermal effects in the fermionic kinetic energy is also discussed. Received 13 September 1999 and Received in final form 22 February 2000     

Thermal diffusion effects in hydrogen-air and methane-air flames

The influence of thermal diffusion on the structure of hydrogen-air and methane-air flames is investigated numerically using complex chemistry and detailed transport models. All the transport coefficients in the mixture, including thermal diffusion coefficients, are evaluated using new algorithms which provide, at moderate computational costs, accurate approximations derived rigorously from the kinetic theory of gases. Our numerical results show that thermal diffusion is important for an accurate prediction of flame structure.