Effect of a Cocurrent Gas Flow on the Velocity and Concentration Limits of Combustion Wave Propagation in Granulated Ti + C + <Emphasis Type="Italic">x</Emphasis>Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Mixtures

In this paper, we studied the effects of a cocurrent gas flow on the velocity and combustion limits of a granulated Ti + C mixture diluted with inert corundum granules. In the whole range of dilutions, an increase in the combustion rate was observed for the flow of active and inert gases. This effect is several times greater than that calculated based on the filtration combustion theory. Concentration combustion limits (75 wt %), the incomplete combustion of Ti + C granules, and the ratio of the combustion rates of the undiluted mixture and the mixture at the propagation limit (from 2 to 3) are well predicted by the percolation theory. The same combustion limit of concentration in the flow of inert and active gases and without flow indicates a percolation-phase transition as the reason for the cessation of combustion.     

堆积床内非驻定过滤燃烧的一维研究

多孔介质内气体过滤燃烧不同于自由流中燃烧,燃气与多孔介质强烈换热.热波波速和燃烧波波速是燃烧过程的特征参数.以惰性堆积床内的甲烷/空气的低速过滤燃烧为例,提出一维解析模型,用摄动理论推导出燃烧波波速,用直接求解方法和格林函数方法给出充分发展后的和瞬态的燃烧温度分布,并进行计算验证.     

Characteristic analysis of low-velocity gas filtration combustion in an inert packed bed

This paper investigates the low-velocity filtration combustion of lean methane–air mixtures occurring in inert packed beds by using a modified one-temperature model, considering the axial thermal diffusion owing to the convective gas–solid heat transfer. Based on the scaling analysis of various transport terms in different conservation equations, a high-activation energy asymptotic method is applied in the flame zone and results in a set of powerful analytical solutions for combustion macrocharacteristics under the fully developed conditions. These are then combined with the eigenvalue method of the modified one-temperature model in the whole flow region to study the flame behaviour analytically and numerically. Our results have shown that the combustion wave velocity is a key characteristic parameter in the filtration combustion process. Compared with other existing theoretical results, the present analytical solutions demonstrate the intricate relationships among the combustion wave velocity, the flame speed, the peak flame temperature and the effects of the variable thermo-physical properties, and show better prediction performance for the combustion wave velocity, the flame speed and the peak flame temperature. Excellent agreements with experimental results have been observed, especially for very lean filtration combustion with stream-wise propagating combustion fronts.     

Coupling of turbulence on the ignition of multicomponent sprays

This study examines the effect of turbulence on the ignition of multicomponent surrogate fuels and its role in modifying preferential evaporation in multiphase turbulent spray environments. To this end, two zero-dimensional droplet models are considered that are representative of asymptotic conditions of diffusion limit and the distillation limit are considered. The coupling between diffusion, evaporation and combustion is first identified using a scale analysis of 0D homogeneous batch reactor simulations. Subsequently, direct numerical simulations of homogeneously dispersed multicomponent droplets are performed for both droplet models, in decaying isotropic turbulence and at quiescent conditions to examine competing time scale effects arising from evaporation, ignition and turbulence. Results related to intra-droplet transport and effects of turbulence on autoignition and overall combustion are studied using an aviation fuel surrogate. Depending on the characteristic scale, it is shown that turbulence can couple through modulation of evaporation time or defer the ignition phase as a result of droplet cooling or gas-phase homogenization. Both preferential evaporation and turbulence are found to modify the ignition delay time, up to a factor of two. More importantly, identical droplet ignition behavior in homogeneous gas phase can imply fundamentally different combustion modes in heterogeneous environments.     

Structure of a pressure driven flame in porous media

The model of a combustion wave driven by local elevation of pressure in inert porous media filled with flammable gas is investigated. The combustion wave structure is studied using the Zel'dovich approach. Analytical formulae for the combustion wave speed and for values of temperature and pressure behind the flame front are derived.     

Fast and slow modes of the propagation of the combustion front in heterogeneous systems

Experimental results are presented in favor of the existence of fast and slow modes of the propagation of the combustion front in diluted heterogeneous mixtures of reactive particles. A theoretical combustion model is proposed to explain the existence of these modes. The transition from the fast to the slow mode, which occurs in a narrow range of the degree of dilution of the mixture by inert powder, is associated with the break of a percolation cluster formed by reagent particles that are in direct contact with each other. After such a break of the cluster, the thermal energy of combusting particles is still insufficient to maintain the combustion wave. A sharp decrease in the front velocity in this case is associated with the necessity of heating inert regions inevitably appearing in its path.     

Characteristic regimes of premixed gas combustion in high-porosity micro-fibrous porous media

Dynamical behaviour of the premixed flame propagating in the inert high-porosity micro-fibrous porous media has been studied numerically. Effects of mixture filtration velocity, equivalence ratio and burner transverse size on the flame structure have been investigated and the regions of existence of different combustion regimes have been determined. It was found that the influence of the hydrodynamic instability on the flame dynamics is significant in the case of the moderate and high filtration velocities and this effect is negligible at the low velocities. At the moderate filtration velocities the effect of hydrodynamic instability manifests in the flame front deformation and in particular in the flame inclination. It was found that the flame can be stabilized within the whole interval of the filtration gas velocity, whereas in the ordinary porous media the standing wave is settled only at fixed value of gas filtration velocity. This finding is in line with recent experimental results on combustion in micro-fibrous porous media (Yang et al., Combust. Sci. Tech. 181 (2009), 1–16). Possible physical interpretation of the flame anchoring effect may be given on the base of present numerical analysis. At the high filtration velocities the hydrodynamic instability manifests itself in periodical appearance of the moving wrinkles on the flame front surface which forms non stationary high temperature trailing spots behind the leading part of the flame front. Such dynamics may be associated with splitting wave structures which were revealed in previous experiments (Yang et al., Combust. Sci. Tech. 181 (2009), 1–16).     

Numerical simulation of modes of combustion and detonation of hydrogen-air mixtures in porous medium in the framework of the mechanics of two-phase reaction mediums

Numerical simulation is carried out for combustion and detonation waves propagating through a motionless gas mixture in a porous inert charge. Computations are performed in a one-dimensional approximation by means of an EFAE computer program that was developed in the framework of the mechanics of multiphase reaction mediums. The chemical conversion of gas is modeled by a one-stage reaction of the Arrhenius type with constants selected based on existing experimental data on the ignition lags behind the reflected shock waves. Computations are performed for hydrogen-air mixtures with 35 and 15% hydrogen and compared with literature experimental data in which the initial pressure and the diameter of charged particles are varied. All three combustion modes (slow, fast, and supersonic) observed in the experiment and combustion failure under conditions lower than threshold are followed by numerical simulation. In addition, the computations qualitatively reproduced experimental data on the change of the combustion mode in the case of transfer from stoichiometric to a lean mixture and data on the combustion wave velocity and limiting conditions of combustion mode transition and failure of flame as a function of the initial pressure and the charged particle size. It is shown that supersonic waves propagating with a velocity of lower than 1100 m/s do not have a Chapman-Jouguet surface in the end of the reaction zone and it is evident that they can be related to detonation, as in the cited literature.     

Numerical model of two-dimensional heterogeneous combustion in porous media under natural convection or forced filtration

A novel mathematical model and original numerical method for investigating the two-dimensional waves of heterogeneous combustion in porous media are proposed and described in detail. The mathematical model is constructed within the framework of the model of interacting interpenetrating continua and includes equations of state, continuity, momentum conservation and energy for solid and gas phases. Combustion, considered in the paper, is due to the exothermic reaction between fuel in the porous solid medium and oxidiser contained in the gas flowing through the porous object. The original numerical method is based on a combination of explicit and implicit finite-difference schemes. A distinctive feature of the proposed model is that the gas velocity at the open boundaries (inlet and outlet) of the porous object is unknown and has to be found from the solution of the problem, i.e. the flow rate of the gas regulates itself. This approach allows processes to be modelled not only under forced filtration, but also under free convection, when there is no forced gas input in porous objects, which is typical for many natural or anthropogenic disasters (burning of peatlands, coal dumps, landfills, grain elevators). Some two-dimensional time-dependent problems of heterogeneous combustion in porous objects have been solved using the proposed numerical method. It is shown that two-dimensional waves of heterogeneous combustion in porous media can propagate in two modes with different characteristics, as in the case of one-dimensional combustion, but the combustion front can move in a complex manner, and gas dynamics within the porous objects can be complicated. When natural convection takes place, self-sustaining combustion waves can go through the all parts of the object regardless of where an ignition zone was located, so the all combustible material in each part of the object is burned out, in contrast to forced filtration.     

Theory of a waveguide laser

The role of diffusion of the components of the gas mixture is the generation mechanism of a waveguide CO₂ laser is analyzed. A method is proposed for reducing the multicomponent problem of linear diffusion with dissipation to a one-component problem. The superposition of relaxation modes for a cylindrical waveguide is considered. The diffusion contribution to the loss of electromagnetic energy is estimated for the case of the field hybrid mode EH₁₁ of a dielectric waveguide. The rapid increase in the loss for a SiO₂ waveguide tube and the observation of the filtration properties of the laser in conditions when the diffusion is taken into account are noted.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 17–21, July, 1975.     

惰性多孔介质内过滤燃烧特性分析

以惰性堆积床内甲烷/空气低速过滤燃烧为例,因次分析系统特征尺度,基于修正的单温度模型,提出一种封闭的耦合解析方法,对燃烧过程进行理论分析.研究了燃烧波波速、火焰传播速率、最高燃烧温度等燃烧特性参数,将计算结果与实验结果以及前人的理论结果进行对比.     

Two-phase filtration of multicomponent mixtures with a retrograde region of the phase diagram

The results of mathematical and physical modeling of an isothermal filtration of multicomponent mixtures with a retrograde region of the phase diagram are presented. By the example of mixtures of hydrocarbons of the methane line, the conditions at which the hydrocarbon-two-phase fluid?porous space system is a self-oscillation system are determined. The data obtained make it possible to explain certain regularities of the filtration of gas-condensate mixtures and can be used to increase the extraction of gas-condensate deposits.     

Effect of the composition of the combustible mixture on the development of flame front instability

Numerical simulations are used to study the effect of the chemical composition of the combustible mixture on the development of the hydrodynamic instability of the flame front, its acceleration, and the possibility of transition to the detonation regime. The combustion of hydrogen-containing mixtures in confined spaces (channels) was considered. Calculations were performed within the framework of a two-dimensional hydrodynamic model for the combustion of premixed mixtures with account of viscosity, heat conduction, multicomponent diffusion, and chemical kinetics. It was demonstrated that the presence of an inert component and the deviation of the mixture composition from stoichiometry caused not only a quantitative but also a qualitative change in the character of burning of gaseous combustible mixtures.     

Cellular wave mode of the infiltration combustion of porous media

The paper summarizes the results of experimental studies of the structuring of the infiltration combustion front in a burning titanium powder layer. The unsteady processes of formation and propagation of cellular and nonuniform combustion waves in heterogeneous media in straight-through and semi-closed channels in conditions of restricted gas exchange and presence of inert gaseous impurities are analyzed. The structural characteristics of cellular combustion waves in a metal powder layer under conditions of natural gas infiltration are reported.     

Modeling of heterogeneous combustion in porous media under free convection

A mathematical model and a numerical method, based on the combination of explicit and implicit finite difference schemes, have been developed for investigating the unsteady gas flows in porous objects with zones of heterogeneous combustion when gas pressure at object boundaries is known. Used approach enables to solve problems of filtration combustion for both forced filtration and free convection, so it can be efficiently applied for modeling the combustion zones in porous media, which may arise from natural or man-caused disasters. One-dimensional unsteady processes of heterogeneous combustion in porous object under free convection have been investigated using numerical experiment. Two regimes of combustion wave propagation have been revealed – wave movement up and down in the object – and it is shown that these regimes are significantly different in degree of burn of solid combustible material, the temperature in the combustion zone and propagation velocity of combustion wave.     

堆积床内甲烷/空气预混燃烧的理论分析

在多孔介质内组织预混燃烧,燃气与多孔介质有强烈的换热作用,燃烧过程伴随着化学反应和热输运的强烈耦合。本文以惰性氧化铝球堆积床内的甲烷/空气预混燃烧为例,提出解析模型,对燃烧过程进行理论分析,给出温度分布的解析解,发现了超绝热火焰温度燃烧现象。     

Filtration combustion of charcoal-polyethylene systems

The characteristics of the filtration combustion of a carbon-polyethylene-inert material mixture with superadiabatic heating are experimentally studied. It is demonstrated that the composition of the products and the nature of the thermal degradation of the polymer strongly influence the burning rate and thermal structure of the wave. In the filtration combustion wave, a part of the polymer can be pyrolyzed and carried away by the gas flow, with the rest of it being oxidized in the combustion zone.     

Two-dimensional gas flows under heterogeneous combustion of solid porous media

Two-dimensional unsteady gas flows in porous media with heterogeneous-combustion centers are investigated under forced filtration and free convection. With the use of numerical methods, it is shown that complex gas flows including vortex ones can arise under the combustion of solid porous media. In the case of forced filtration, the gas tends to flow around the heated portion of an object preferring to flow along cold regions. Under natural convection, the vortex gas flows, which can exist for a reasonably long time and strongly affect the oxidizer inflow into the reaction zone, arise at the initial moment of the process in the combustion zone and in its vicinities.     

Lower concentration limit of carbon filtration combustion

For mixtures of carbon materials and an inert filler, dependences of the characteristics of the filtration combustion wave on the gaseous oxidizer supply rate at a fuel content in the mixture of less than 7 wt % were obtained. The existence of a lower concentration limit for a steady-state filtration combustion wave was established. It was demonstrated that at a given intensity of heat loss, the concentration limits are determined by the reactivity of the carbon material and the oxidizer supply rate. At the effective coefficient of heat loss α = 8 W/(m² K), effective conductivity of mixture material λ = 2 W/(m K), and air supply rate G = 0.1 m/s, the lowest fraction of carbon in the mixture at which combustion is still possible was 4.5 wt % for carbon-carbon composite, 2.5 wt % for activated birch coal, and 2.0 wt % for birch coal, the most reactive kind of carbon.     

Experimental observations on the thermal degradation of a porous bed of tires

In this paper, an experimental study on the forward combustion of a bed of tires and refractory briquettes is presented. Temperature measurements within the reactor were obtained as a function of time as well as the evolution of the fuel bed. The products of combustion were cooled down, and usable liquid fuel was recovered and measured. The reaction was found to become unstable for fuel concentrations lower than 50%. The results show that the airflow and tire concentration define different modes of combustion while the reaction remains oxygen limited. Oil production is maximized when an increase in airflow leads to a transition from a rate-limited reaction to a heat transfer-limited propagation. Variation of the tire concentration shows the importance of the inert in achieving high conversion rates.