K. G. Shkadinskii

Cellular filtration combustion of porous layers

The cellular filtration combustion of porous energetic compositions that interact with an active gas reagent and form condensed reaction products is studied for titanium powder combustion as an example. A simple and visually accessible quasi-experimental analysis of the nonlinear propagation of the combustion front with adjustable filtration transport of gas is proposed. Under conditions of a deficiency of the gas reagent and instability, the flat front of filtration combustion is divided into isolated cells of exothermic chemical reaction propagating in a pulsating mode. The results obtained are interpreted within a model of thermal filtration stability of the filtration combustion front.

Nonstationary Regimes of Transformation of Multilayered Heterogeneous Systems

A nonstationary mathematical model of thermal propagation of flame in a layered heterogeneous system is proposed. The structure and dynamics of the frontal exothermal transformation in quasihomogeneous, transitional, and relay-race regimes are studied. Averaged characteristics of the front and dynamics of transformation of individual elements of a “discrete” combustion wave are analyzed using the model proposed. A correlation is established between the combustion of a model medium and real heterogeneous compositions. It is shown that the maximum combustion velocity is reached at an intermediate level of medium dispersion in a transitional parametric region. Key words: combustion waves, heterogeneous systems, multilayered nonstationary regimes, modeling.

Critical phenomena in the cellular mode of filtration combustion in the presence of heat loss

Critical conditions for combustion failure due to heat loss to the environment are examined. The process of filtration combustion is considered under conditions where a cellular structure of the front is realized, because the planar combustion front loses its stability and splits into separate cells of exothermic chemical conversion, which propagate in self-sustained mode. The size and structure of the cells of chemical interaction depend nonlinearly on the governing parameters, including the rate of heat loss to the environment. Within the framework of a mathematical model of filtration combustion, the steady-state dynamics of the combustion process and the structure of the cell of exothermic chemical reaction of a powder mixture with a gaseous reagent with the formation of solid products are simulated. The specifics of the evolution of the cell before combustion failure as a function of the heat loss rate are studied.

Dynamics of Thermal Explosion in the Postinduction Period

The dynamics of thermal self-acceleration of the reaction and heating is studied for gasless condensed compositions in the induction and postinduction periods up to complete transformation of the substance. It is shown that the propagation of the reaction over the sample is of a distinct frontal character in a broad range of values of the Biot criterion Bi and within the framework of the macrokinetics of “weak” deceleration.” Two qualitatively different mechanisms of front propagation are revealed. This is the normal propagation of the combustion front initiated by the ignition zone for large values of Bi. For small values of Bi, propagation of the front is the apparent effect, which is due to subsequent adiabatic self-ignition of separate portions of the substance uniformly heated during the induction period. In the second case, by the propagation velocity, we mean the “phase” velocity.

Experimental study of the heterogeneous filtration combustion mode

External manifestations of the heterogeneous combustion front are characterized. Gasdynamic aspects of the phenomenon are considered, and experimental data on the change of combustion modes during natural filtering of an air mixture are analyzed. Results of an investigation of the bifrontal structure of the combustion zone in a horizontal plane layer during convective gas transfer are presented.

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.

Combustion conditions of volatile materials decomposing in condensed and gas phases

A classical model of combustion conditions is presented in which conversion occurs only in the gas phase. This paper presents an approximate analytical approach to such a model of combustion, allowing investigations to be undertaken over a wide range of parameter variation and the combustion characteristics to be estimated sufficiently and simply, which is important for a multiparameter process. The conceptual aspect of the method of determing the combustion characteristics is described; the theoretical pressure dependences of the combustion characteristics are shown.

Quasiisobaric approximation in combustion theory

Based on the laminar combustion of gases and the filtration combustion of porous condensed compounds, some general approaches to the study of their mathematical models are demonstrated that simplify their quantitative analysis. Processes in a wide circle of phenomena proceed under conditions of a weakly disturbed pressure field, though providing macroscopic convective transfer. Under conditions of the substantially changed temperature field, in contrast to condensed media, gaseous media change their specific volume and increase mobility, which should be considered in combustion processes. The suggested quasi-stationary consideration of reacting medium flows simplifies their computation and analysis.

Pulsating cellular regimes of infiltration combustion of porous media

The study examines the modes of propagation of cellular infiltration combustion waves through a high-porosity layer of an energetic composition yielding condensed reaction products. Under conditions of instability, a planar infiltration combustion front breaks into individual exothermic reaction cells, which propagate through the heterogeneous mixture in the pulsation mode. Based on video records and theoretical analysis of the process, the dynamics of propagation of a pulsating cellular front is investigated. The dependences and mechanisms of transformation of cellular combustion modes are established. The evolution of a cellular frontal structure within a separate pulsation period is investigated.

Postinduction Processes During Thermal Explosion in the Systems “Porous Material–Reactive Gas–Solid Product”

A complete solution of the unsteady‐state filtration problem of thermal explosion incorporating the postinduction period is given for the first time. The paper describes a study of the temperature‐field dynamics, pore‐gas pressure, and the degree of condensed‐phase conversion versus the reactive‐gas deficiency in a reactive porous material. Focus is on the formation and propagation of frontal regimes of exothermic chemical reactions (their number, direction, and velocity of propagation, degree of condensed‐phase conversion at the front). The study revealed “double self‐ignition” phenomena and combustion‐wave propagation regimes with incomplete conversion at the front. A surface regime of thermal explosion limited by gas filtration from the outside was considered. The regularities in the dynamics of the exothermic chemical reaction found in the present study allow one to qualitatively control high‐temperature synthesis under thermal explosion conditions.