Filtration Combustion Wave Research Articles

Pore-scale flame oscillations and patterns of upstream combustion wave propagation in a one-layer porous burner

Upstream propagation of filtration combustion wave in a one-layer porous burner is studied experimentally using thermal imaging and high-speed filming. The burner is the one-layer packed bed of spheres modeling porous media and providing optical access to the flame. This design allowed us to study macroscopic propagation of the combustion wave as well as non-stationary flame behavior at the pore scale. Experimental results demonstrate that upstream propagating combustion wave forms non-uniform porous medium temperature distribution in transverse direction which consists of hot areas separated by low-temperature regions. Macroscopic flame propagation can be accompanied by oscillations of some flame fragments at the pore scale. The mechanism of these pulsations is identical to the flames with repetitive extinction and ignition in narrow channels with external heating. Experimental results suggest that large scale evolution of the thermal wave affects the non-stationary behavior of the flame fragments at the pore scale and vice versa. The interplay between pore-scale and macro-scale processes manifests itself in the multiple repetitions of some patterns of the combustion wave propagation. Two most typical and common patterns are described and discussed. These patterns are step-wise transition of the flame fragment into the upstream pore and transition through the series of flame oscillations with repetitive extinction and ignition. It is shown that highly simplified one-dimensional model describes flame propagation in externally heated variable cross-section channel with heat conducting walls is capable to describe main features of these two patterns. Parametric study allows to predict the effect of problem parameters on the regions of existing of one or another pattern of upstream flame propagation.Novelty and Significance StatementThis paper presents new experimental data on temperature characteristics of upstream propagating filtration combustion wave in a one-layer porous burner. For the first time the patterns of combustion wave propagation uniting macro-scale processes of the thermal wave propagation and oscillations of the flame fragments at the pore scale are described on the basis of experimental data and numerical modeling. Results obtained for the one-layer burner can apparently be generalized to the case of combustion in three-dimensional porous media and packed beds. These results are significant because extend fundamental knowledge on porous media combustion by information on pore- and macro- scale flame dynamics as well as on the interplay of these scales. Experimental results can also be useful for validation of pore-resolved numerical simulations actively developed in the last decade.

Исследование инициирования волны фильтрационного горения газа открытым пламенем

One of the processes of filtration combustion, namely the process of filtration combustion of gas, is considered. Its importance and practical significance are emphasized. The current state of research is shown and the need to study the non-stationary processes of filtration combustion of gas is revealed. The article examines one of the non-stationary processes of filtration combustion of gas, which is of great importance: the process of formation of a wave of filtration combustion of gas, which occurs when a burning gas mixture moves through an inert porous medium towards the flow of a combustible mixture. This process occurs when devices using filtration combustion of gas are operating: devices for afterburning poor gas mixtures, radiation burners, etc. A comprehensive study of this process has been carried out, including mathematical modeling and experimental research. The necessity of using a comprehensive study is shown. A method for processing experimental data is proposed. The factors influencing the process of formation of the filtration combustion wave of gas when it is initiated by an open flame are established. The influence of the gas mixture feed rate, heat loss from the porous medium to the external environment, and the grain size of the porous medium on the ignition process and the movement of the filtration combustion wave of gas was revealed.

Features of Nonstationary Filtration Combustion Wave Propagation in a Model One-Layer Porous Medium

Features of Nonstationary Filtration Combustion Wave Propagation in a Model One-Layer Porous Medium

Thermal Structure of the Filtration Combustion Wave of CаH2 in a Flow of a SiF4 + H2 Mixture

Thermal Structure of the Filtration Combustion Wave of CаH2 in a Flow of a SiF4 + H2 Mixture

Numerical study of the dependence of the wave velocity of filtration gas combustion on the process parameters

When solving problems in the field of filtration combustion of gases, numerical methods are often used to analyze the wave structure depending on changes in the hydrogen content in the mixture and other physical and chemical parameters. This work analyzes the stationary structure of the wave of filtration combustion of incompressible gas in the low-speed mode when the parameters of the gas flow and the porous medium change, as well as when the hydrogen content in the mixture changes. The problem is solved by the numerical Runge-Kutta method of the fourth order of accuracy, tested on a system of differential equations. Reduced to a dimensionless form, the system of equations is written in vector format. Programming is performed in Delphi 7. The results presented show agreement with the known experimental data and also allow us to obtain new conclusions about the structure of the filtration combustion wave of gases.

Thermodynamically Equilibrium Compositions of the Products Formed During the Filtration Combustion of the Metal-Containing Mixtures

Thermodynamic calculations for describing the compositions of the products formed in conditions of the filtration combustion of the metal-containing mixtures were carried out. The analysis of the equilibrium compositions of the products was carried out using the TERRA high-temperature thermochemical equilibrium calculation program. According to the results of calculations, the metals were divided into two groups. First one forms both the condensed and gaseous phases and in the second one ‒ metals that are only in the condensed phase. In case of the presence of metal compounds in the gas phase, as a rule, these are the following compounds: metals, oxides, hydroxides, hydrides, sulfides and metal sulfates. Metals of the second group cannot be subjected to mass transfer under conditions of the filtration combustion wave and will remain in solid combustion products (in ash).

The Model of the Extraction Process of Rare Metals Under Condition of Filtration Combustion Wave.

To study the mass transfer of metal compounds, a model of filtration combustion of metal-containing combustible mixtures is developed. Using cadmium-containing mixture as an example, the main characteristics of filtration combustion are determined when the gas pressure at the reactor inlet is constant. It is shown that under the conditions of a filtration combustion wave, a metal can evaporate into the gas phase and be transferred with gas through the reactor. Due to the evaporation and condensation of cadmium, it is transported and accumulated before the combustion front. The possibility of controlling the mass transfer of metal compounds under the conditions of a filtration combustion wave with the aim of concentrating them is shown. It is revealed that a 4-fold increase in the pressure difference at the open boundaries of the reactor can lead to a decrease in the maximum metal concentration by about 1.5 times. An increase in the concentration of metals due to mass transfer will subsequently make it economically feasible to extract them by traditional methods.

Thermodynamic Estimating the Mass Transfer of Compounds of Rare Metals under Conditions of a Filtration Combustion Wave

A thermodynamic calculation of the possibility of mass transfer of metal compounds in conditions of filtration combustion of metal-containing mixtures was carried out. According to the results the metals were divided into two groups: the first group includes metals that form both condensed and gaseous phases, and the second group includes metals that are only in the condensed phase. Under conditions of filtration combustion wave due to continuous evaporation and condensation, it is possible to organize mass transfer of compounds of the first group metals in order to concentrate them in a specific zone of the reactor. Metals of the second group do not participate in mass transfer under the conditions of a filtration combustion wave and will remain in solid combustion products (in ash). Their concentration is possible only if there is a large amount of a combustible component in the initial mixture. In this case, due to the burnout of the combustible part and the preservation of metal compounds in the ash residue, their concentration will occur. Concentration of metals will allow performing their further economically justified recovery by traditional methods.

Features of the thermal structure of reaction waves in the Ti - Mo – N2 system

The thermal structure of filtration combustion waves in the metallic Mo – Ti system in nitrogen is studied by dynamic color pyrometry. It’s shown that the combustion of the samples is conducted in the surface mode. This mode is characterized by the propagation of a reaction wave in a narrow near-surface layer of the sample followed by the penetration of combustion from the surface to the centre. The peculiarities of combustion observed are the presence of local reaction sources in the surface reaction wave and the complex three-dimensional structure of the temperature field in the reaction wave (local reaction source – surface reaction wave - inside reaction wave). The measurements have shown that with increasing nitrogen pressure, the maximum temperature inside the sample rises by 200 K.

Ignition of a Filtration Gas Combustion Wave by a Heated Region of a Porous Medium

The ignition of a filtration gas combustion wave by a heated region of an inert porous medium is studied by numerical simulation. The mechanism of formation of the combustion wave is described. Dependences of the time of wave formation on the velocity of the gas mixture, the temperature of the heated region of the porous medium, and its length were obtained. The existence of ignition limits of the filtration combustion wave was found.

Hydrodynamic and thermal mechanisms of filtration combustion inclinational instability based on non-uniform distribution of initial preheating temperature

Filtration combustion (FC) is one style of porous media combustion with inert matrix, in which the combustion wave front propagates, only downstream or reciprocally. In this paper, we investigate the FC flame front inclinational instability of lean methane/air mixtures flowing through a packed bed as a combustion wave front perturbation of the initial preheating temperature non-uniformity is assumed. The predicted results show that the growth rate of the flame front inclinational angle is proportional to the magnitude of the initial preheating temperature difference. Additionally, depending on gas inlet gas velocity and equivalence ratio, it is demonstrated that increase of gas inlet gas velocity accelerates the FC wave front deformation, and the inclinational instability evolves faster at lower equivalence ratio. The development of the flame front inclinational angle may be regarded as a two-staged evolution, which includes rapid increase, and approaching maximum value of inclinational angle due to the quasi-steady condition of the combustion system. The hydrodynamic and thermal mechanisms of the FC inclinational instability are analyzed. Consequently, the local propagation velocity of the FC wave front is non-uniform to result in the development of inclinational angle at the first stage of rapid increase.

The rate of stationary wave of filtration combustion of gases in the same fields of temperature and concentration

Combustion of hydrogen-methane-air mixtures of gases in an inert porous medium is considered when the temperature fields of the medium end the concentration of the missing component of the mixture are similar. The relationship between the functions of temperature and concentration, as well as the equation for the numerical calculation of the temperature distribution, are obtained. Numerical calculations were made for different compositions of the hydrogen-methane-air mixture of gases and their effects on the wave velocity were determined. Dependences of the wave velocity, the equilibrium temperature, the characteristic size of the combustion zone, and the diffusion coefficient of the missing component on the gas injection rate are studied.

Numerical simulation of the hybrid filtration combustion of biomass

Hybrid filtration combustion waves in a porous media were numerically analyzed for biomass and methane-air mixtures. Temperature and products of the combustion waves were studied in the range of equivalence ratio from 0.2 to 1.1. A model, based on a two-temperature approximation and combustion chemistry is developed to analyze temperature profiles and species of the phenomena. The numerical solutions predictions for temperatures and chemical products are in good agreement with experimental data for some biomasses contrasted. It was found that with ultra-lean methane mixtures, the combustion temperature presents essentially higher values than stoichiometric mixtures. Experimental validation suggests that the chemistry of the process is very sensitive to CO formation. For low values of the biomass fraction in the porous medium, it is found that homogenous reactions are dominant whereas for high values (over 40%) the heterogeneous reactions are more influent. Thus, for ultra-lean methane-air mixtures, hydrogen production is enhanced when the biomass fraction in the medium is increased.

Effect of thermo-hydrodynamic instability on structure and characteristics of filtration combustion wave of solid fuel

Numerical modelling of flame front stability for the inverse wave (with trailing combustion front) of filtration combustion of solid fuel is performed. The problem is treated in terms of dimensionless variables and parameters. It is found that propagation of a plane combustion front becomes unstable under certain conditions. In this case the front spontaneously inclines. The thermo-hydrodynamic mechanism is supposed to be responsible for instability developing. Anisotropic effective mass diffusivity (dispersion) is also taken into account. It turns out that anisotropic diffusivity affects structure and conversion distribution of the inclined combustion front. It is shown that the key parameters determining stability of combustion wave are dimensionless gas flow rate and width of reactor. The range of these parameters corresponding to the stable plane front is determined. It is shown that stability occurs either for small reactor widths (dimensionless values <1), or low gas flow rate (below 0.2). The optimised values of considered dimensionless parameters for maximal productivity are determined.

Modelling of Inclined Wave of Filtration Solid Fuel Combustion Using CFD-DEM Technique

We investigate flame front for the inverse wave (with trailing combustion front) of filtration combustion of solid fuel. This problem naturally demands coupled Euler-Lagrange approach, as it contains phenomena that are inherent for gaseous or disperse phase.It is found that propagation of plane combustion front becomes unstable under certain conditions; as a result, the front spontaneously inclines. The thermo-hydrodynamic mechanism (originating from temperature dependence of gas permeability) is responsible for inclination developing.

Simulation of the hydrodynamic instability of a filtration combustion wave in a porous medium

This paper presents a two-dimensional model of the propagation of a filtration combustion wave in a flat channel with cocurrent flow of a gas containing an oxidizer. It is shown that the increase in the permeability of the porous medium with fuel burnup leads to instability of the flat front and the formation of a structure called a finger. The reasons for the occurrence of the finger are explained, and the dependences of its most important characteristics on the permeability ratio of the initial fuel and combustion products, the specific heat of the injected gas and the width of the channel in which the filtration combustion occurs are determined.

Modeling of filtration combustion of gases in a cylindrical porous burner with allowance for radiative heat transfer

The problem of stabilization of the wave of filtration combustion of a premixed mixture of gases in a porous cylindrical burner is solved numerically. Two formulations of the problem are considered. In the first case, a one-dimensional steady problem of gas combustion in a porous cylindrical burner with allowance for radiative heat transfer inside the porous skeleton and emission of radiation from the burner surface is solved. In the second case, the problem is solved within the framework of the conventional one-dimensional steady diffusion-thermal model of filtration combustion of gases with allowance for radiative heat losses only from the external surface of the burner. The results obtained by two models are compared. It is shown that radiative heat transfer exerts a significant effect on the process of filtration combustion in a porous body, in particular, on the temperature distributions in the gas and the solid, as well as on the radius of stabilization of the combustion front. Heat losses in the form of emission into the ambient medium are found to be caused to a large extent by emission from internal layers of the porous body owing to radiative heat transfer, not only by emission from the external surface of the burner.

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.

Hydrodynamic instability of the coflow filtration combustion: Numerical simulation

The stability of the front to infinitely small distor� tions was analyzed under the assumption that the reac� tor channel width is infinitely large in comparison with the combustion zone thickness (5). It was concluded that the plane filtration combustion wave may be unstable. The experimental investigation of the filtra� tion combustion wave stability confirmed (6) that the propagation of a plane filtration combustion front may be unstable and may be accompanied by the formation of structures similar to Saffman-Taylor fingers, which were observed as a highviscosity liquid was displaced by a lowviscosity liquid (7). In this work, for the first time, we made mathemat� ical modeling of twodimensional unsteady filtration combustion during the forced oxidant filtration in the combustion wave direction. The purposes of the study was to determine the conditions for the occurrence of filtration combustion with a stable plane front in chan� nels of finite width and investigate the filtration com� bustion wave propagation modes in the range of insta� bility of the plane reaction front. The physical formulation of the problem is the fol� lowing. A gas mixture with oxidant concentration a0 is fed to a reactor filled with a porous mixture containing a fuel component. The gas flow rate at the inlet (x = 0) is maintained constant (G0). Reaction initiation with a hightemperature source leads to reaction front prop� agation in the х direction at rate u. As the fuel is burned out, the structure of the porous medium changes, because of which the porosity, thermal diffusivity, and permeability of the combustion products may differ from the respective characteristics of the initial reac� tion mixture. The problem is considered in the plane geometry under the assumption that the reactor is shaped as a thin slitlike channel. If the reaction front propagation velocities are low, which is characteristic of filtration combustion waves, the difference between the temperatures of the parti� cles and the gas can be ignored and the porous medium can be considered locally uniform in temper� ature and concentration. Under this assumption, the dimensionless system of equations describing filtra� tion combustion has the form (1, 2)

Limits of the propagation of filtration combustion waves at high pressures of a gaseous oxidant

Limits of the propagation of filtration combustion waves at high pressures of a gaseous oxidant