Gasless combustion wave with extremely high excess enthalpy under heat loss conditions

A numerical experiment on investigation of a gasless combustion wave propagating at the limit due to a high excess enthalpy in the heating zone is performed. Under heat loss conditions, waves emerge on the surface of the combustion front, causing changes in the rate of chemical reactions similar to those realized on the burning surface of propellants and explosives.     

Combustion of iron-zirconium thermite in a cylindrical channel of near-critical diameter

A gasless combustion wave in a cylindrical channel of near-critical diameter under conditions of intense heat losses has a wide preheat zone with considerable excess enthalpy. Changes of the thermal wave under the action of reaction retardation and acceleration waves propagating over the front surface resemble the phenomena observed during the combustion of propellants and explosives.     

Laws governing the combustion of the Ni + Al system under a low ambient gas pressure

The ambient pressure dependences of the combustion rate and combustion-induced elongation for an initial Ni + Al mixture and Ni + Al mixtures subjected to vacuum heat treatment (VHT), mechanical activation, and additional activation in water (dispersion) have been studied. Most of the studies have been conducted for mixtures based on PNK-brand nickel (carbonyl); a separate set of experiments has been conducted for PNE-brand nickel (electrolytic). These powder brands contain different amounts of impurity gases and have different particle sizes. It has been shown that VHT, the ambient gas pressure, and the type of the used nickel have a substantial effect on the combustion rate of these mixtures. For the mixtures based on PNK nickel, the increase in the combustion rate after VHT is more significant than that after mechanical activation despite a more thorough mixing of the precursors and changes in the mixture microstructure. The qualitative change in the behavior of the dependence of the combustion rate of the dispersed mixtures on the external gas pressure has been interpreted in terms of a convective–conductive combustion model. The studies have made it possible to prove the previously made conclusion about the gasless combustion of Ni + Al mixtures (PNK nickel) under high ambient gas pressures.     

A mathematical model of hotspot condensed phase ignition in the presence of a competitive endothermic reaction

We consider the propagation of a combustion front resulting from the gasless combustion of a condensed state fuel. The propagation of the front, essentially a premixed laminar flame, is supported by an exothermic reaction subject to possible heat loss through a competitive endothermic reaction. The dynamics of the endothermic process inducing the heat loss strongly depend on the temperature and the local fuel concentration. Through an analysis based on high activation energy, the steady-state values of the final burnt temperature as well as the burning velocity are obtained, and the control parameters are identified. Using a linear perturbation method, we assess the stability of the propagating front and obtain a condition for oscillatory behaviour. The critical parameter values for the transition from steady to oscillatory burning speeds are identified. The results represent a generalization of those obtained by Matkowsky and Sivashinsky to include the effects of heat loss induced by a competitive endothermic reaction.     

低温下粗糙表面接触间隙气体热导的研究

对低温非真空环境下粗糙接触界面间隙中介质气体的传热进行了理论分析．依据克努森数的大小,建立了不同传热区域的间隙气体热导理论模型．并对影响接触界面间隙热导的克努森数、普朗特数、热导率、适应系数、压力等参数进行了分析,为实际情况下接触界面的传热提供了理论基础．而且通过实验证明了在界面接触压力较小的情况下,即使对于硬度较小导热性能好的接触固体,间隙气体的导热量仍大于通过实际接触点的导热量．     

Contribution of different mechanisms of energy transfer in the development of the thermonuclear combustion wave upon fast ignition of ICF-targets

Temporal characteristics of the thermonuclear combustion wave, critical parameters of the igniter, and the total energy yield were computed using numerical modeling of the fast ignition of the spherically symmetric inertial confinement fusion (ICF) target of the reactor type taking into account different mechanisms of energy transfer from the central igniter to the main mass of fusionable fuel of the target. The program TERA was used for mathematical modeling. Along with complete calculations (including all known mechanisms of energy transfer), model computations with consecutive disengagement of energy transfer by thermonuclear charged particles (local energy deposition approximation) and by neutrons were also carried out. Our computations showed that the main effect consists in variation of the temporal characteristics of the combustion wave. Unlike the diagnostic-type targets, in the case of the reactor targets, energy transfer by neutrons exerts the main influence, and the second in importance is nonlocality of the energy deposition by charged thermonuclear particles.     

气隙式膜蒸馏中空气隙内流动与换热研究

本文针对膜蒸馏实验中开放式的窄空气隙,研究其内流动与换热机理.研究发现,对于开放式的大宽隙比(120:1)空气隙,即使其厚度仅为 1 mm,也会形成向上流动的流场,其流动状态为层流,温度分布为直线分布.空气隙内按实验结果求得的热流密度大于按导热公式计算的结果.本文给出的数值模拟结果与实验结果吻合较好.     

交变热流下燃烧室外对流传热特性数值研究

采用数值计算对水对流换热边界下天然气燃烧及外部对流传热特性进行了研究。研究了相同燃烧功率不同水入口流速下燃烧及传热特性,得到各个区域交界面温度分布及燃烧室内辐射换热与对流换热占总换热量比例。对天然气高温空气燃烧技术在工业锅炉等设备上的应用具有很好的指导意义。     

The influence of the state of a compressed thermonuclear substance on the combustion of inertial fusion targets under direct ignition by a short radiation pulse

One-dimensional numerical calculations were performed to study the dependence of conditions for initiating thermonuclear combustion and of the target gain of direct-ignition inertial fusion targets ignited by a short radiation pulse on the initial temperature of a preliminarily compressed fuel and the initial heat energy distribution between plasma electrons and ions in the ignition region (igniter). The igniter parameters at which an effective thermonuclear target explosion with a G ～ 10³ target gain occurred were shown to substantially depend on the initial temperature of the major fuel fraction and the initial heat energy distribution between igniter electrons and ions. The heat energy of the igniter passed a minimum as the size of the igniter decreased. The dependences of these minimum energies on the temperature of the major fuel fraction at various initial energy distributions between igniter electrons and ions were determined. An increase in the temperature of the major fuel fraction was shown to decrease the target gain.     

On the Nature of Concentration Limits of Combustion Wave Propagation in Powdered and Pelletized Ti + C + <Emphasis Type="Italic">x</Emphasis>Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Mixtures

The applicability of the percolation theory to describe the combustion of powdered and pelletized Ti + C mixtures in the vicinity of the concentration limits of the combustion wave propagation using different methods of dilution with an inert material—fine and coarse Al₂O₃ particles—has been studied. It has been shown that the pelletized mixtures diluted with coarse inert particles by more than 50% undergo incomplete combustion; at the combustion limit, the incompleteness achieves 50%; this finding is in qualitative agreement with the percolation theory. It has been found that the obtained concentration limit of combustion (75 wt %) and the ratio of the combustion velocities of the undiluted mixture and the mixture at the propagation limit (2.6) correspond to the predictions of the percolation theory. The possibility of flame propagation at the calculated combustion temperature of the mixture below the melting point of titanium is attributed to the presence of a percolation cluster. Necessary conditions for the applicability of the percolation theory to describe the combustion processes in condensed gasless systems have been formulated.     

Propagation of Cellular Modes of Titanium-Powder Layer Combustion in Air Channels,Allowing for the Effect of Natural Convection of Gas

In this paper, we have studied the effect of impurity and inert gases on the formation and propagation of cellular-combustion regimes for their inhomogeneous distribution above the surface of the reacting metal layer. The gas-dynamic aspects of the formation and steady propagation of inhomogeneous wave structures in the combustion of a titanium powder layer in through and semi-closed inclined air canals and in channels with uneven loading are considered. The gas composition heterogeneity over the reaction zone and the gas stratification, i.e., the stratification of a gas mixture of different densiies above the reacting layer, are shown to lead to the formation of inclined non-uniform and cellular fronts under conditions of a lack of active gas in the reaction zone and the loss of stability of the planar front.     

Open cycle traveling wave thermoacoustics: mean temperature difference at the regenerator interface

In an open cycle traveling wave thermoacoustic engine, the hot heat exchanger is replaced by a steady flow of hot gas into the regenerator to provide the thermal energy input to the engine. The steady-state operation of such a device requires that a potentially large mean temperature difference exist between the incoming gas and the solid material at the regenerator's hot side, due in part to isentropic gas oscillations in the open space adjacent to the regenerator. The magnitude of this temperature difference will have a significant effect on the efficiencies of these open cycle devices. To help assess the feasibility of such thermoacoustic engines, a numerical model is developed that predicts the dependence of the mean temperature difference upon the important design and operating parameters of the open cycle thermoacoustic engine, including the acoustic pressure, mean mass flow rate, acoustic phase angles, and conductive heat loss. Using this model, it is also shown that the temperature difference at the regenerator interface is approximately proportional to the sum of the acoustic power output and the conductive heat loss at this location.     

Influence of Intake/Exhaust Channel Lateral Profiling on Thermomechanics of Pulsating Flows

The hypothesis that pipeline lateral profiling can be used to regulate pulsating flows in the air-gas systems of energy machines is tested. The results of the physical simulation of gas dynamics and local heat transfer in the intake and exhaust channels of various configurations in the piston internal combustion engine in the conditions of gas-dynamic nonstationarity are presented. We have established that the lateral profiling of intake and exhaust pipes promotes the stabilization of gas flows in the air-gas systems of the engine, and decreases the intensity of the local heat transfer by up to 30%, depending on the initial conditions.     

Influence of water mist on propagation and suppression of laminar premixed flame

The combustion of premixed gas mixtures containing micro droplets of water was studied using one-dimensional approximation. The dependencies of the burning velocity and flammability limits on the initial conditions and on the properties of liquid droplets were analyzed. Effects of droplet size and concentration of added liquid were studied. It was demonstrated that the droplets with smaller diameters are more effective in reducing the flame velocity. For droplets vaporizing in the reaction zone, the burning velocity is independent of droplet size, and it depends only on the concentration of added liquid. With further increase of the droplet diameter the droplets are passing through the reaction zone with completion of vaporization in the combustion products. It was demonstrated that for droplets above a certain size there are two stable stationary modes of flame propagation with transition of hysteresis type. The critical conditions of the transition are due to the appearance of the temperature maximum at the flame front and the temperature gradient with heat losses from the reaction zone to the products, as a result of droplet vaporization passing through the reaction zone. The critical conditions are similar to the critical conditions of the classical flammability limits of flame with the thermal mechanism of flame propagation. The maximum decrease in the burning velocity and decrease in the combustion temperature at the critical turning point corresponds to predictions of the classical theories of flammability limits of Zel'dovich and Spalding. The stability analysis of stationary modes of flame propagation in the presence of water mist showed the lack of oscillatory processes in the frames of the assumed model.     

多孔陶瓷板中城市燃气预混燃烧的试验研究与数值计算

符号表尸可燃混合气体的密度。可燃混合气体的流速。轴向距离。Stefan－Boltzmann常数Tg混合气体的温度R通用气体常数ho多孔陶瓷材料的导热系数DO单元体的内径J单元体的壁厚Tin燃气入口端面的环境温度几单元体的壁温P混合气体压力Te。烟气出口端面的环境温度HO对流换热系数l单元体轴向全长。’积分变量A板面面积。板面孔的数量1引言多孔陶瓷板中的燃气预混燃烧具有热效率高，污染物排放量低的特点，符合九十年代人们对燃烧的要求，因此对多孔陶瓷板中的可燃气体的预混燃烧的研究逐渐成为了国际上的一个热点。目前国内有些厂家己在生产…     

Spinning combustion regimes in gasless systems containing one melting component

The propagation of a gasless combustion wave in a continuous cylindrical sample pressed from a mixture of solid reactants has been numerically simulated with allowance made for the melting of one of the components. The melting point of the component has been assumed to be equal to the reaction rate “cutoff” temperature. Symmetric and asymmetric combustion regimes can take place in the sample, depending on the sample radius. The reaction rate cutoff temperature is an additional perturbation factor in the propagation of the gasless combustion wave.     

Effect of some mechanical activation parameters on the SHS characteristics

The effect of some mechanical activation parameters (duration, ball acceleration, atmosphere) on the self-propagating high-temperature synthesis (SHS) has been investigated by the example of the Ni + Al and Ti + Al systems. The main regularities of SHS wave propagation (gasless combustion) in mixtures subjected to preliminary mechanical activation significantly differ from the SHS regularities in traditional powder systems. This fact is explained primarily by the change in the reaction mixture microstructure and the formation of composite particles during mechanical activation.     

Enhancement of gas phase heat transfer by acoustic field application

This study discusses a possibility for enhancement of heat transfer between solids and ambient gas by application of powerful acoustic fields. Experiments are carried out by using preheated Pt wires (length 0.1-0.15 m, diameter 50 and 100 micro m) positioned at the velocity antinode of a standing wave (frequency range 216-1031 Hz) or in the path of a travelling wave (frequency range 6.9-17.2 kHz). A number of experiments were conducted under conditions of gas flowing across the wire surface. Effects of sound frequency, sound strength, gas flow velocity and wire preheating temperature on the Nusselt number are examined with and without sound application. The gas phase heat transfer rate is enhanced with acoustic field strength. Higher temperatures result in a vigorous radiation from the wire surface and attenuate the effect of sound. The larger the gas flow velocity, the smaller is the effect of sound wave on heat transfer enhancement.     

Wavespeed analysis: Approximating Arrhenius kinetics with step-function kinetics

The accuracy of using step-function approximations to the Arrhenius exponential in computing the wavespeed in combustion wave propagation is investigated. Gaseous and gasless combustion, and first- and second-order reactions are included in the study. The theoretical analysis is based on Melnikov theory from dynamical systems. The error is shown to be small in most instances. The analytical results are supported with numerical simulations.     

Filtration combustion of a porous structure in a multicomponent gas environment

A mathematical model for describing the quasi-isobaric filtration combustion of porous materials with the formation of condensed reaction products in a multicomponent gas is developed. Two-stage combustion waves (control modes) at the counter filtration of gas mixture are examined. The effect of inert gas component on the structure of a two-stage filtration combustion wave is studied, and the critical conditions of the changeover between filtration combustion modes caused by inert gas concentration variation are determined. It is demonstrated the characteristics of the two-stage combustion front propagating in the control mode in a multicomponent gas flow depends on the porosity of the heterogeneous system.