Аннотация (русский):
The main objective of this book is to acquaint the reader with the main modern problems of the multisensor data analysis and opportunities of the hyperspectral shooting being carried out in the wide range of wavelengths from ultraviolet to the infrared range, visualization of the fast combustion processes of flame propagation and flame acceleration, the limit phenomena at flame ignition and propagation. The book can be useful to students of the high courses and scientists dealing with problems of optical spectroscopy, vizualisation, digital recognizing images and gaseous combustion. The main goal of this book is to bring to the attention of the reader the main modern problems of multisensory data analysis and the possibilities of hyperspectral imaging, carried out in a broad wave-length range from ultraviolet to infrared by methods of visualizing fast combustion processes, propagation and flames acceleration, and limiting phenomena during ignition and flame propagation. The book can be useful for students of higher courses and experimental scientists dealing with problems of optical spectroscopy, visualization, pattern recognition and gas combustion.

Ключевые слова:
Remote measurements, optoelectronic methods, multisensor data analysis, hyper spectral shooting, ramjet engine, Catalytic Stabilization

It has been shown experimentally that in the case of flame penetration through an obstacle, gas-dynamic factors, for example, flame-generated turbulence, can determine the kinetics of the process, including the transition of low-temperature combustion of a hydrocarbon to a high-temperature regime. It has been established that the flame front after a single obstacle does not arise in the immediate vicinity of the obstacle. The first ignition source can be observed relatively far from the obstacle surface. It is experimentally shown that the flame does not penetrate from the side of the funnel funnel (confuser) below the limit of penetration of the flame of a diluted mixture of methane with oxygen through a flat obstacle with one hole for an obstacle in the form of a funnel. However, it penetrates from the side of the funnel nose (diffuser). Within the framework of an approximate consideration using the Navier-Stokes equations in a compressible reacting medium, the features of flame propagation through a conical obstacle with additional holes on the converging generatrix are qualitatively described. In other words, the flame does not penetrate through the central hole of the converging tube, but only penetrates through the central hole of the diffuser, even if there are holes in the generatrix of the cone. The performed modeling in small volumes suggests that the most effective two-way flame-arrester in the pipe can be a system of two confusers, the funnels of which are located on the pipe axis along the gas flow and against it. An emergency can occur before and after the obstacle. The features of the penetration of the flame front through rectangular holes in comparison with round holes were experimentally investigated using color filming and visualization of a gas flow. It is shown that the length of the “flame jump” after the hole in the obstacle is mainly determined by the time of occurrence of the laminar-turbulent transition, and not by the ignition delay period. It was found that C2 radicals in detectable quantities and the main heat release in the process are observed after the flame passes the first obstacle by using 4D spectroscopy combined with high-speed color filming, i.e. after turbulization of the gas flow. The obtained result means that the used experimental technique makes it possible to separate in time and space “cold” and “hot” flames in one experiment. Key words: flame front, obstacle, hole, flame jump, laminar, turbulent, transition, diffuser, confuser Flame propagation in pipes and channels is important for establishing criteria for safe gas pumping through pipes, ensuring explosion safety in the electric power industry, mining and petrochemical industries, and also for ensuring the completeness of gas conversion in internal combustion engines [1]. The interaction between the flame and obstacles caused by the presence of equipment parts in the reaction volumes can lead to local acceleration of the flame front (FF) [2]. The influence of obstacles on the course of explosive processes, the shape of the flame in pipes and channels were studied in laboratory conditions [3,4-6]. The authors of [5, 6] pointed out the important role of acoustic waves generated by a flame and formed waves of finite amplitude on the shape of the FF. In [6, 7], it was found that the simultaneous occurrence of intense chemical transformation, heat transfer and mass transfer during turbulent mixing caused by obstacles, in combination with momentum exchange processes, can significantly accelerate a flame, cause an explosion, a transition to a supersonic combustion mode and lead to damage to a building or highway. This influence of obstacles on flame acceleration was studied, for example, in [8-10]. The purpose of this Chapter was to establish the gas-dynamic and kinetic features of the penetration of methane-oxygen flames through obstacles of various geometries using 4D spectroscopy and high-speed filming. §1. Gas-dynamic and kinetic features of the penetration of methane-oxygen flame through single holes and fine-meshed obstacles As suggested in [9, 10], when studying obstacles with small holes, flame acceleration can be explained not only by an increase in the degree of turbulence of the gas flow expanding when passing through the obstacle, but also by the accumulation of free radicals behind the obstacle. Mixing these radicals with unreacted gas should increase the flammability of the mixture. It is shown in [11, 12] that spark-initiated flames of lean hydrogen-air mixtures at 1 atm penetrate through mesh aluminum spherical obstacles with a cell size of 0.04–0.1 mm2. The flame of a mixture of 15% H2 in the air after the obstacle is accelerated. Acoustic vibrations are observed in the reactor i.e. local pulsations of gas density. In this case, the smaller the diameter of the grid sphere, the earlier the acoustic vibrations occur. On the other hand, the FF of a stoichiometric mixture of natural gas (NG) with air does not accelerate after an obstacle. In the works, it was concluded that the active centers of combustion of methane and hydrogen, which determine the propagation of the flame, have a different chemical nature. In other words, the contribution of the termination of active radicals on the surface of the obstacle is decisive in the case of NG-air mixtures. This result, along with those presented in [9, 10], indicates the important role of active intermediate combustion products during the passage of a flame through an obstacle. This section presents experimental results on flame propagation in a cylindrical channel with obstacles. The aim was to reveal the features of the FF penetration through the simplest single obstacles with one round hole and fine-mesh obstacles, and also to evaluate the effectiveness of such obstacles for suppressing the combustion of methane. The peculiarities of the passage of the FF through a single obstacle are considered in the first part of the section. An experimental evaluation of the effectiveness of fine mesh barriers for suppressing methane combustion is described in the second part. The experiments were carried out with stoichiometric mixtures of methane with oxygen diluted with CO2 and Kr at initial pressures of 100-200 Torr and a temperature of 298 K in an evacuated horizontal cylindrical quartz reactor 70 cm long and 14 cm in diameter. The reactor was fixed with two stainless steel locks at the ends (Fig. 1) and was equipped with vacuum inlets for admitting and pumping out gas. A safety door that opened outward when the total pressure in the reactor exceeded 1 atm. Fig. 1. Experimental setup. a - mesh sphere 4 cm in diameter b (wire diameter 0.1 mm, cell size 0.15 mm2) inserted into a flat obstacle 14 cm in diameter. b - (1) quartz reactor, (2) stainless steel gateways, (3) silicone gasket, (4) safety door, (5) spark ignition electrodes, (6) power supply, (7) high-speed movie camera, (8) microphone. Two spark ignition electrodes were located at one of the ends of the reactor. The spherical obstacle consisted of two stainless steel mesh hemispheres attached to a ring. Mesh spheres with a diameter of 8 cm (wire thickness 0.3 mm, cell size 0.3 mm2), 10 cm diameter (wire thickness 0.35 mm, cell size 0.5 mm2), and 13 cm diameter (wire thickness 0.5 mm, cell size 1 mm2) were used. We also used flat stainless steel mesh obstacles with a diameter of 14 cm, equal to the reactor diameter (wire thickness 0.3 mm, cell size 0.5 mm2; or with a wire thickness of 0.5 mm and cell size 0.1 mm2). In addition, mesh spheres with a diameter of 4 cm (wire thickness d = 0.1 mm, cell size 0.15 mm2) and 5 cm (wire thickness 0.15 mm, cell size 0.15 mm2) were used, inserted into a flat obstacle with a diameter of 14 cm and overlapping the cross section of the reactor. (Fig.1a). The results obtained with fine-mesh obstacles were compared with experimental data on the passage of a flame through flat obstacles with single central holes 2.5 cm and 4 cm in diameter. The investigated combustible mixture (15.4% СН4 + 30.8% O2 + 46% CO2 + 7.8% Kr) was preliminarily prepared. CO2 was added to reduce the FF speed and improve the quality of the survey. Kr was added to reduce the gas ionization threshold. The reactor was filled with a combustible mixture to the required pressure. Then, ignition was initiated with a spark (energy 1.5 J). High-speed shooting of the dynamics of the ignition and propagation of the FF was carried out from the side of the reactor (Fig. 1) using a color high-speed digital camera Casio Exilim F1 Pro (frame rate 600 s – 1) [13]. The video file was saved in the computer's memory, then frame-by-frame processing was carried out. [14]. The change in pressure during combustion was recorded using a piezoelectric sensor synchronized with a spark discharge. Acoustic vibrations were recorded with a Ritmix microphone (frequency range up to 40 kHz). The audio recording was turned on at an arbitrary moment before the ignition was initiated. During the experiment, the level of extraneous noise was minimized. The audio file was analyzed using the Spectra Plus 5.0 software package. The gases used were of "chemically pure" grade. A typical picture of flame propagation in a combustible mixture through a single obstacle is shown in Fig. 2. The footage of flame propagation through various obstacles recorded by a high-speed video camera is shown in fig. 2: a - hole diameter 4 cm; b - mesh sphere with a diameter of 4 cm; c - mesh sphere 4 cm in diameter inserted into a flat obstacle 14 cm in diameter (Fig. 1a). Let us pay attention to the following experimental features of the process. FF after the obstacle does not arise in the immediate vicinity of the obstacle. The first ignition site can be observed relatively far from the surface of the obstacle. As follows from Fig. 2, the smaller the hole diameter, the farther from the obstacle the flame front appears (the difference is indicated by the arrows in the figure). Fig. 2.a - high-speed shooting of the FF propagation through a round hole 2.5 cm in diameter in a flat obstacle 14 cm in diameter, b - high-speed shooting of FF propagation through a round hole 4 cm in diameter in a flat obstacle 14 cm in diameter, c - high-speed shooting of the FF propagation through a mesh sphere with a diameter of 4 cm (wire diameter 0.1 mm, cell size 0.15 mm2) inserted into a flat obstacle with a diameter of 14 cm, Fig. 1a), combustion of a mixture of 15.4% СН4 + 30.8% O2 + 46% CO2 + 7.8% Kr at an initial pressure of 170 Torr. The frame number is counted from the moment of ignition initiation. The arrows indicate the distances of the flame front after the obstacle. As seen from Fig. 2c, the FF occurs at the greatest distance from the obstacle when an obstacle in the form of a grid sphere is used, while a "jump" of the flame through a single obstacle can be observed at initial pressures less than atmospheric. In addition, the flame "skips" in a diluted mixture much farther than in a stoichiometric mixture at 1 atm [9]. The determination of the dependence of the magnitude of the "jump" of the flame on the geometrical arrangement of a complex obstacle will be carried out below in §3. It should be noted that the accumulation of free radicals behind the obstacle was observed experimentally [9]. Mixing these radicals with an unreacted combustible mixture increases the flammability of the mixture. This means that the analysis should take into account the main features of the kinetic mechanism of combustion. In the course of the preliminary analysis, a qualitative two-dimensional numerical simulation of the passage of a flame through an obstacle was carried out using the Navier-Stokes equations with a chemical reaction in the approximation of a small Mach number (acoustic approximation). We considered the simplest single flat obstacle with a central hole. Let us recall that any comparison of the experimentally recorded motion of the FF with the result of numerical simulation can be considered reliable only in a qualitative aspect. It is beathere are no uniqueness theorems for the Navier-Stokes equations in a compressible reacting medium. Therefore, the agreement between the calculated and experimental values is not an argument in favor of the chosen kinetic reaction mechanism, since there may be other sets of control parameters describing the same experimental profiles (the uniqueness of the solution has not been proven). It is possible to analyze reliably only the qualitative change in the speed of movement of the boundary of the front of a chemical reaction, as well as the shape of this boundary, the degree of its disturbance. Consideration of the detailed kinetic mechanism of combustion introduces additional uncertainty in the simulation results. The overwhelming majority of kinetic parameters are not known with sufficient accuracy. The question of the completeness of the kinetic mechanism is always open, so that adequate conclusions can be drawn based on numerical simulations. Thus, it is desirable to estimate qualitative calculations that allow tracing the trends in the development of processes in the conditions of the experiments being performed. A qualitative consideration of the transition of flame propagation from spherical to cylindrical mode was carried out using the example of a two-dimensional plane problem in the side view projection in order to compare the results of the qualitative calculation with experimental ones and to establish further directions for modifying the calculation. As is known from the publication[15], the relationship between the main factors causing the instability of hydrodynamic and acoustic flames can be taken into account when considering the Navier-Stokes equations for a compressible medium in the acoustic approximation (which corresponds to substantially subsonic flames). The system of dimensionless Navier-Stokes equations in the approximation of a small Mach number [see. 13, 16-21], which describes the propagation of a flame in a two-dimensional channel, showed qualitative agreement with experiments [13] and is discussed in detail in Chapter 4. The initial values and dimensionless parameters were chosen the same as in [13]. In a number of calculations, the reaction rate was specified not by the Arrhenius equation, but using the simplest chain mechanism С → 2n w0 n + C → 3n + products W where C is the dimensionless concentration of the starting substance, n is the dimensionless concentration of the active intermediate, w0 and W are the rates of nucleation and branching of reaction chains, respectively. In further calculations, the nucleation reaction rate w0 for the flame propagation process is assumed to be small according to [22]. The Arrhenius law describes the temperature dependence of the reaction rate n + C → 3n + products. In this case, equations (f) and (g) of system (1), (see Chapter 4) were replaced by the following equations: , , The initial condition for the concentration of the starting substance is changed to С0=1. The initiation condition is T = 10 at the right boundary of the channel (the initial dimensionless temperature is T = 1). The channel contains a single vertical obstruction with a centrally located hole or a spherical mesh obstruction. There are also boundary conditions (including obstruction) , as well as convective heat transfer . The calculation results are shown in Fig. 3. As can be seen from Fig. 3, the analysis of the Navier-Stokes equations in the approximation of a small Mach number makes it possible to qualitatively describe the experimental features of the penetration of the FF through a single obstacle (see Fig. 2). In other words, the appearance of a flame front not in the immediate vicinity of the obstacle, but at some distance behind him. Thus, it is sufficient to analyze the simplest model of a single plane obstacle with a hole using the first-order Arrhenius reaction to describe this important experimental regularity. As also follows from Fig. 3, taking into account the chain transformation mechanism makes it possible to describe the movement of the reaction zone back to the obstacle after the flame breakthrough (Fig. 3c, frame d) in comparison with the representation of the reaction by a simple Arrhenius dependence. Fig. 3. Simulation results of flame propagation through a hole (a-c) and an z-spherical mesh obstacle. The time is counted from the moment of initiation. a - Change in the dimensionless concentration n when the flame propagates through the hole. First-order Arrhenius reaction. The scale of the change in the degree of conversion of the reaction n is shown on the right; b- Change in dimensionless temperature T when the flame propagates through the hole. First-order Arrhenius reaction. On the right is the scale of the change in the reaction temperature T; c - Dimensionless concentration of the active intermediate product (n) for two moments of time (frame numbers 1 and 2) and dimensionless temperature (frame number 3), when a chemical reaction is represented by a simple chain mechanism. The scales for n and T are shown on the right. d - Change in dimensionless temperature during flame propagation through the mesh sphere. First-order Arrhenius reaction. The scale of the change in the dimensionless reaction temperature T is shown on the right. Thus, there is a qualitative difference in the results of calculations using the simplest chain mechanism as compared to modeling taking into account only one reaction obeying the Arrhenius law (Fig. 3 a, b). Consequently, although the appearance of a flame "jump" is mainly determined by the gas-dynamic features of the penetration of a combustible gas through an obstacle, the kinetic mechanism of combustion also noticeably affects the process. The laws governing the passage of the flame front through the spherical mesh also qualitatively (note that the mesh was modeled very illustratively, as well as the reaction mechanism) coincide with the experiment. When using an obstacle in the form of a grid sphere in qualitative agreement with Fig. 2c FF occurs at the greatest distance from the obstacle. This means, however, that we managed to take into account the main features of the passage of the flame through the obstacle. These features, as can be seen from the simulation, are mainly determined by the gas dynamics of the combustion process. The next part of the paragraph is devoted to assessing the effectiveness of obstacles to suppress flame propagation during methane combustion. Video footage of the FF propagation in a gas mixture is indicated in fig. 4 (I): a - in the absence of obstacles; b - dependence of the amplitude of acoustic vibrations on time. A video footage of the FF propagation in a gas mixture is shown in fig. 4 (II): a - in the presence of three mesh obstacles (8, 10, 13 cm in diameter) nested into each other, b - the dependence of the amplitude of acoustic vibrations on time. As can be seen from the above figures, the introduction of obstacles in the form of nested mesh spheres leads to a noticeable suppression of the flame after the obstacle, which is indicated by a change in the maximum level of the acoustic signal, which in the presence of obstacles is approximately 20 times less than in an empty reactor. To determine the dependence of the maximum intensity of the acoustic signal during the propagation of the combustion wave on the number of obstacles was of interest. We point out that the influence of obstacles is expressed in a dual way. On the one hand, the interaction of the FF with an obstacle can cause the development of flame instability, contributing to the acceleration of the FF. On the other hand, the contact of the FF with the surface of the obstacle can lead to an increase in the contribution of heterogeneous reactions, especially the termination of reaction chains [12], as well as to an increase in heat losses. The dependence of the maximum intensity of the acoustic signal on the number of obstacles is shown in fig. 4 (III). Let's pay attention to the following experimental features. Experience has shown the acceleration of the flame in the presence of a single obstacle in comparison with combustion without an obstacle. In these cases, the reactor safety door opened outward, i.e. the pressure in the reactor exceeded 1 atm (the established limit) (according to the readings of the pressure sensor, 1.5 atm was recorded), it did not matter whether the obstacle was flat or spherical. As seen from Fig. 4, two or more obstacles (both nested spherical and flat) greatly suppress flame propagation. We point out that in these cases the reactor safety door did not open, i.e. the pressure in the reactor did not exceed 1 atm (<500 Torr according to the readings of the pressure sensor). Thus, Fig. 4 illustrates the existence of two modes: flame acceleration after an obstacle and its suppression after an obstacle. In this case, one obstacle in the conditions of our experiment leads to the acceleration of the flame, and a larger number of obstacles to the suppression of the flame. In this case, the suppression of the flame is due to both open circuits and heat losses on the surface of the obstacle. Let us generalize the results obtained in this section. It is shown that the specific features of the penetration of a flame of a methane-oxygen mixture diluted with an inert gas through single holes and spherical obstacles are mainly determined by gas-dynamic factors. In this case, the kinetic mechanism of combustion also affects the process of flame penetration through the obstacle. Fig. 4. Dependence of the efficiency of flame suppression on the number of obstacles. A mixture of 15.4% PG + 30.8% O2 + 46% CO2 + 7.8% Kr at an initial pressure of 170 Torr. The frame number is counted from the frame with the moment of ignition initiation. I - a - high-speed shooting of FF propagation in a gas mixture in a quartz reactor without obstacles, spark discharge (1.5 J), shooting speed 600 frames/s; b - dependence of the amplitude of acoustic vibrations on time. Several frames from (a) are shown to compare the times for high-speed video filming and acoustic oscillogram; II - a - high-speed shooting of FF propagation in a gas mixture in a quartz reactor in the presence of three nested mesh obstacles (8, 10, 13 cm in diameter), spark discharge (1.5 J), shooting speed 600 frames/s; b - time dependence of the acoustic vibration amplitude. Also shown are some frames from a. III - dependence of the maximum intensity of acoustic vibrations on the number of obstacles. 15.4% СН4 + 30.8% O2 + 46% CO2 + 7.8% Kr at an initial pressure of 170 Torr: 1 - spherical mesh nested obstacles, 2 - flat mesh obstacles with a diameter of 14 cm (wire diameter 0.5 mm and cell size 0.1 mm2); It has been established that the flame front after a single obstacle does not arise in the immediate vicinity of the obstacle. The first ignition site can be observed relatively far from the obstacle surface. It is shown that the use of a grid sphere as an obstacle leads to an increase in the length of the flame "jump" behind the obstacle in comparison with a round hole. It is shown that two or more obstacles, both spherical and flat, under experimental conditions noticeably suppress the propagation of the flame. The results obtained in visualizing the development of flame front instability are important for solving the problems of explosion safety for volumes with complex geometrical arrangement. § 2. Peculiarities of penetration of flames of dilute mixtures of methane with oxygen through a single hole in a flat obstacle, diffuser, confuser and combined obstacles. In the event of a hypothetical industrial accident, a significant amount of highly flammable gas could be released. After mixing with atmospheric air, the resulting explosive mixture, if ignited, can endanger the integrity of the room in which the accident occurred. Due to the complexity of both the physical and chemical processes of combustion and the geometrical arrangement of the reactor, the features of the propagation of the flame and the load on separate fragments of the reactor of complex geometry cannot be calculated with sufficient accuracy by now. It should be noted that the complete system of Navier-Stokes equations for a compressible reacting medium can be simplified and solved for non-isothermal flows only if the approximation of a small Mach number (acoustic approximation) is used. In order to describe slow processes of turbulent combustion, the Navier-Stokes equations are a good basis for modeling in a reacting medium in the approximation of a small Mach number [14-16, 18]. When the laminar flame moves into the area of unburnt premixed combustible gases, the combustion wave propagates due to the appearance of a heating zone (thermal conductivity) and diffusion of active combustion centers into the unreacted mixture. The structure of the flame determines how much energy is transferred to the unreacted gas. In accordance with how the gradients of temperature and concentrations of active centers change, the flame can either accelerate or go out. The gas-dynamic characteristics of the flow also affect the structure of the flame. This section describes the features of flame propagation through a flat obstacle and a conical sink with a round hole. To know the diameter of the hole through which the flame front does not penetrate under these conditions is of practical interest for solving the problems of explosion safety [9]. In the publication, the penetration of a flame through an obstacle with a single circular hole (which is characterized by a blocking ratio , where d and D denote the hole diameter and the internal diameter of the reactor, respectively) is discussed, for example, in [27-30]. In [30], it was proposed to use the Karlovitz number K to estimate the probability of the penetration of an isotropic turbulent flame through a single hole: , where - kinematic viscosity of the gas mixture, – velocity of the laminar flame, - local flow rate directly behind the obstacle, - diameter of the hole. It was postulated that flame extinguishing occurs at a critical value . According to [29], flame extinguishing takes place when the product , where - Lewis number. However, the determination of the quantity is associated with significant difficulties, which, along with attempts at determination are described in [7]. This semi-empirical approach takes into account the role of the kinetic mechanism of combustion when the flame penetrates through the hole only in the magnitude of the laminar flame velocity . This section describes an experimental study of flame penetration through a single hole in a flat obstacle and a conical funnel for diluted methane-oxygen mixtures with inert additives. The ways of developing numerical models of combustion are also discussed, which can be used to calculate the process of flame propagation through obstacles of various geometrical arrangements. The experimental results can be used to improve numerical models of flame propagation. The experiments were carried out with stoichiometric mixtures of methane with oxygen diluted with CO2 and Kr at initial pressures of 100-200 mm Hg and an initial temperature of 298 K. The setup shown in Fig. 1, namely, a horizontally disposed cylindrical quartz reactor 70 cm long and 14 cm in diameter (reactor 1), into which either a funnel or flat obstacles with holes or a combination thereof were inserted. Evacuated steel cylindrical reactor (reactor 2), 12 cm in diameter and 25 cm long with an optical quartz window at the end was used (as described in [13]) in Chapter 3 [31, 32]. Two electrodes for initiating the flame with a spark discharge were located at the end of reactor 1 and in the center of reactor 2. Reactor 1 was vacuum-sealed in two stainless steel sluices equipped with inlets for gas admission and evacuation. One of the locks was equipped with a safety door that opened outward when the total pressure in the reactor exceeded 1 atm. Flat plastic obstacles D = 14 cm with round holes = 0.993, = 0.993, BR = 0.968 and = 0.918 were placed in the reactor. A plastic conical funnel with a diameter of 14 cm was also used as a barrier (the nose of the funnel was 1 cm long and 1 cm in diameter ( = 0.995)). In a number of experiments, complex obstacles were used. They consisted of a confuser (= 0.99) and a flat obstacle 14 cm in diameter with a 4 cm hole covered with a net (obstacle A), or a mesh sphere 4 cm in diameter (obstacle B) (wire diameter 0.1 mm, mesh size 0.15 mm2) inserted into a flat obstacle with a diameter of 14 cm, placed directly behind the confuser. Complex obstacles consisting of a flat obstacle with a diameter of 14 cm with a single hole 25 mm in diameter and a second flat obstacle with a single hole 25 mm in diameter (obstacle C, Fig. 5, 1), a second flat obstacle with a single hole 25 mm in diameter, closed a flat mesh (obstacle D, Fig. 5, 2) and a second flat obstacle with a single hole 40 mm in diameter, into which a mesh sphere was inserted (obstacle E, Fig. 5, 3) were also used. The second obstacle was located at a distance of the magnitude of the “flame jump” after a flat obstacle with a single hole 25 mm in diameter. This distance, measured empirically, was 12 cm. Fig. 5. Complex obstacles, consisting of a flat obstacle with a diameter of 14 cm with a single hole 25 mm in diameter and a second flat obstacle with a single hole of 25 mm in diameter (1), a second flat obstacle with a single hole 25 mm in diameter, covered with a flat mesh (2) and a second a flat obstacle with a single hole 40 mm in diameter, into which a mesh sphere was inserted (3). The obstacles were located in such a way that the combustion wave could move them, but could penetrate the obstacle only through the central hole. The combustible mixture (15.4% CH4 + 30.8% O2 + 46% CO2 + 7.8% Kr) was previously prepared. CO2 was added to reduce the speed of the flame front and thus improve the quality of the survey. Kr was added to reduce the breakdown energy of the gas mixture discharge. The reactor was filled with a combustible mixture to the required pressure. Then, a spark discharge was carried out (the discharge energy was 1.5 J). Filming of the dynamics of ignition and propagation of the combustion fountain was carried out from the side of the reactor 1 and from the end of the reactor 2 through an optical window using a color high-speed digital camera Casio Exilim F1 Pro (frame rate 600 s– 1) [31]. The video file was saved in the computer memory and then its frame-by-frame processing was carried out [32]. The change in pressure during combustion was recorded by a piezoelectric sensor synchronized with a spark discharge. We used chemically grade gases. The typical results of experiments on high-speed shooting of the flame front propagation in a mixture of 15.4% CH4 + 30.8% O2 + 46% CO2 + 7.8% Kr at an initial pressure of 170 mm Hg through round holes a) = 0.993, b) = 0.968 and c) = 0.918 in a flat obstacle with a diameter of 14 cm was demonstrated in fig. 6. After ignition, a laminar flame spread is observed. When the flame reaches the hole, the flame is extinguished at = 0.993 i.e. the disappearance of the flame behind the hole, and at lower values = 0.968 and = 0.918, the flame front penetrates through the hole. This indicates the existence of a critical hole diameter for flame penetration, in accordance with [7]. Since it was not possible to verify the criterion (see above) due to the difficulty of experimental determination [7] under our conditions, then an attempt was made to identify the control parameters that determine the critical condition for flame attenuation when passing through the hole. In the next series of experiments (see Fig. 7 a, b), high-speed filming of the propagation of the flame front in a mixture of 15.4% CH4 + 30.8% O2 + 46% CO2 + 7.8% Kr at an initial pressure of 170 Torr was carried out through a conical funnel 14 cm in diameter described in the experimental part. Here a - from the side of the funnel spout (diffuser); b - from the side of the funnel socket (confuser). As can be seen from Fig. 7, under our conditions, the flame front penetrates through the funnel from the side of its nose (diffuser), but if the flame front approaches from the side of the funnel inlet (confuser), then the flame is extinguished. It is noteworthy that in the case of a funnel as an obstacle, the use of BR to characterize an obstacle becomes ambiguous, since at = 0.995 (which is much lower than the penetration limit (see Fig. 2.2) for a flat obstacle), the flame does not really penetrate from the funnel mouth (confuser). However, it easily penetrates from the side of the funnel spout (diffuser). Fig. 6. Results of high-speed survey of flame propagation through a round hole a) - = 0.993, b) = 0.968 and c) - = 0.918. 15.4% CH4 + 30.8% O2 + 46% CO2 + 7.8% Kr at an initial pressure of 170 Torr and a capture rate of 600 frames/s. The number in each frame corresponds to the frame number after ignition is initiated. Results of numerical calculation of the process of flame propagation through the hole. d) - change in the degree of transformation for a simple chain mechanism; e) - change in the degree of conversion of the reaction for a first-order reaction; f) - change in the degree of conversion of the reaction for a first-order reaction for a narrower channel. The scale of the degree of conversion of the reaction is shown on the right. Fig. 7. The results of high-speed shooting of flame propagation through a funnel 14 cm in diameter with an opening angle of 900 0: a) diffuser, b) confuser. A mixture of 15.4% CH4 + 30.8% O2 + 46% CO2 + 7.8% Kr at an initial pressure of 170 Torr, a shooting speed of 600 frames/s. The number in each frame corresponds to the frame number after ignition is initiated. Results of numerical calculation of the process of flame propagation through the funnel. c) - change in the dimensionless flame temperature from the side of the funnel funnel for the first order reaction, funnel opening angle 900 0; d) - change in the dimensionless flame temperature from the side of the funnel funnel for a first-order reaction, funnel opening angle 1500 0; e) - change in the dimensionless flame temperature from the side of the funnel funnel for a first-order reaction, funnel opening angle 1500 0, larger opening; f) - change in the dimensionless flame temperature on the side of the funnel bell for a first-order reaction, funnel opening angle 1500 0, smaller opening; The scale of the dimensionless temperature change is shown on the left. To reveal the features of flame propagation through a confuser and a diffuser with additional holes on the generatrix of the cone was of interest. A plastic funnel d = 14 cm with a central hole and two holes (each 17 mm in diameter) on the generatrix of the cone (the opening angles of the cones were 450 0, 550 0 and 830 0 (Fig. 8)) was oriented as a diffuser or as a confuser and placed in the center reactor. The obstacle was fixed in the reactor with a polyurethane foam ring. Fig. 8. Conical obstacle with three holes (opening angle 830). Acoustic vibrations were recorded by using a Ritmix microphone (up to 40 kHz). The audio file was analyzed by using the Spectra Plus 5.0 software package. Typical sequences of video frames of high-speed shooting of the flame front propagation in a combustible mixture at an initial pressure of 165 Torr through a conical obstacle oriented as a confuser a), c), d) and as a diffuser b), e), f) are shown in Fig. 9 for the angles of disclosure of the cone 5500 and 8300. As can be seen from fig. 9, in our conditions the flame always penetrates only through the central opening of the diffuser. If we talk about a confuser, the flame penetrates only through its side holes at a cone opening angle of 5500 (Fig. 9a, frames 21, 22, Fig. 9c, frame 19). The penetration of the flame through the confuser is accompanied with a sharp and loud sound, while the safety door opens. We point out that the propagation of the flame through the diffuser is not accompanied by a sharp sound effect, the safety door does not open. The above-said dependences of the acoustic amplitude on time when the flame penetrates through an obstacle in the form of a confuser (Fig.10a) and a diffuser (Fig.10b) is illustrated in fig. 10. Several video frames from Fig. 9a and fig. 9b are shown in Fig. 10. The center of each video frame corresponds to the current time. From the results obtained above, it can be concluded that a full-scale experiment with a confuser with an opening angle of less than 8300 on a large scale at atmospheric pressure is dangerous for the integrity of the installation and for the life of personnel, i.e. its implementation is impractical. Fig. 9. Video footage of high-speed filming of the flame front propagation through a conical obstacle with a central hole and two holes on the generatrix of the cone; a) - confuser (opening angle of the cone 5500), b) - diffuser (opening angle of the cone 5500), c) - confuser (opening angle of the cone 8300), d) - confuser (opening angle of the cone 8300), an interference filter of 430 nm is placed in front of the video camera; e) - diffuser (opening angle of the cone 8300), f) - diffuser (opening angle of the cone 8300), an interference filter of 430 nm is placed in front of the video camera. The initial pressure is 165 Torr. The number on the frame corresponds to the sequence number of the video frame after the moment of initiation. Fig. 10. Dependences of the amplitudes of acoustic vibrations on time during the propagation of a flame in a gas mixture at an initial pressure of 165 Torr. a) confuser, several frames from Fig. 2a, the center of each frame corresponds to the current time. b) diffuser, several frames from Fig. 2b, the center of each frame corresponds to the current time. The flame begins to penetrate through the central opening of the diffuser (Fig.9c, frames 19, 20) with an increase in the opening angle. It should be noted that the diameters of the holes in the conical obstacle are much less than the minimum diameter of flame penetration through a flat obstacle with a single central hole (20 mm [13]). Therefore, the value of the minimum size of a single hole should not be used when assessing the fire safety of a room with several openings. It is because with an increase in the number of holes, the diameter of the hole sufficient for flame penetration decreases. It should be noted also that in the case of a flat obstacle with three holes (the opening angle is obviously 18000), the flame penetrates through each of these three holes. In our case, reflected acoustic waves appear in the conical cavity, which is accompanied by the appearance of stagnant zones and the interaction of these waves with the initial combustion front, which generated these waves. The maximum pressure in this case is recorded at some distance from the top of the cone. We also point out that the flame does not penetrate through the central opening, regardless of the existence of additional holes on the generatrix of the cone with a decrease in the opening angle in a confuser with a central hole (opening angle less than 4500). For obstacles of this type, numerical modeling was also carried out using the Navier-Stokes equations for a compressible reacting medium in the approximation of a small Mach number. The initiation condition and boundary conditions were taken the same as in the analysis of the above problem of flame penetration through a cone with a central hole. The chemical reaction was represented by a single first-order Arrhenius reaction. The qualitative results of numerical simulation of the process of flame penetration through a conical obstacle in the form of a confuser and a diffuser are shown in Fig. 11. As can be seen, the results of the calculations are in qualitative agreement with the experiments shown in Fig. 9, i.e. the flame penetrates through the diffuser (Fig. 11c). The flame does not penetrate through the central opening of the confuser with an opening angle of 10000 (Fig. 11a). At a larger aperture angle (15000, Fig. 11b), the flame penetrates through all three holes in the confuser in qualitative agreement with experiment (Fig. 9). It should be noted that in the case of a flat obstacle with three holes (one of them is central, the opening angle is 18000), the flame penetrates through each of these three holes. Obviously, qualitative consideration (for example, a single reaction instead of a complete chemical mechanism, two-dimensional modeling, etc.) does not allow one to obtain an exact value of the opening angle at which the flame begins to penetrate through the central opening of the confuser. Fig. 11. Results of numerical calculation of the process of flame penetration through a conical obstacle. a) - change in dimensionless temperature when the flame penetrates through the confuser, opening angle 10000; b) - change in dimensionless temperature during flame propagation through the confuser, opening angle 15000; c) - change in dimensionless temperature during flame propagation through the diffuser, opening angle 10000. The dimensionless temperature scale is shown on the right. In addition, such a qualitative difference from the process of flame penetration through a flat obstacle with a central hole indicates a significant role of the interaction of acoustic vibrations of a reactor containing a conical cavity with a propagating combustion front [13] even in the case of a subsonic flame. The simulation carried out in small volumes allows us to assume that in the event of an emergency situation, the flame will not penetrate through the open valve located in the center of the confuser located in the pipe. In this case, the most effective double-sided flame arrester in the pipe can be a system of two confusers, the funnels of which are located on the pipe axis along the gas flow and against it. It is because an emergency can occur before and after the obstacle. A hole or valve can be located in the middle. The sequences of frames of high-speed shooting of the FF propagation in the combustible mixture at an initial pressure of 180 Torr through complex obstacles A and B are shown in Fig. 12a and 13a, respectively. As seen from Fig. 12a and 13a, the first ignition site is observed at a noticeable distance from the surface of the obstacle, especially in the case of obstacle B. Also from fig. 12a and 13a, it follows that the magnitude of the “flame jump” (the distance of the appearance of the flame behind the obstacle) is much greater in the presence of a mesh sphere compared to an obstacle containing a flat mesh. It should be noted that in agreement with the results presented in Fig. 7b (diffuser), the flame under our conditions does not pass through a complex obstacle containing a diffuser instead of a confuser. Fig. 12. a) High-speed imaging of FF propagation through a complex obstacle consisting of a confuser 14 cm in diameter and a flat mesh with a hole 4 cm in diameter with a mixture of 15.4% PG + 30.8% O2 + 46% CO2 + 7.8% Kr at an initial pressure of 180 Torr. The number on the frame corresponds to the sequence number of the frame after the moment of ignition initiation. b) Numerical modeling of the process of flame propagation through a complex obstacle. As seen from Fig. 14, ignition after these combined obstacles occurs in our conditions already in the immediate vicinity of the second obstacle, the first ignition source is observed directly at the surface of the second obstacle for all three combinations. This means that the magnitude of the “flame jump” is mainly determined by the nature of the combination of obstacles, i.e. gas dynamic factors. Fig. 13. a) High-speed imaging of the process of FF propagation through a complex obstacle consisting of a confuser 14 cm in diameter and a grid sphere of 4 cm in diameter 15.4% PG mixture + 30.8% O2 + 46% CO2 + 7.8% Kr at an initial pressure of 180 Torr. The number on the frame corresponds to the sequence number of the frame after the moment of initiation. b) The results of calculating the process of flame propagation through a complex obstacle. The sequences of frames of high-speed shooting of the FF propagation in a combustible mixture at an initial pressure of 180 Torr through complex obstacles C, D, and E are shown in Fig. 14. Numerical simulations using dimensionless Navier-Stokes equations for a reacting compressible medium in the low Mach number approximation (see Chapter 4, system (I)) describing flame propagation in a two-dimensional channel showed qualitative agreement with experiments. In this work, the initial values and dimensionless parameters were chosen the same as in Chapter 4. The problem was solved by the finite element method using the FlexPDE 6.08 software package, 1996-2008 PDE Solutions Inc. [15]. The initial condition was T = 10 at the right boundary of the channel; there was a vertically located obstacle in the channel with a hole in the center. The boundary conditions (including the hole) were , as well as convective heat transfer on the wall . Fig. 14. High-speed survey of FF propagation through combined obstacles, consisting of: a - a flat obstacle with a diameter of 14 cm with a single hole 25 mm in diameter and a second flat obstacle with a single hole with a diameter of 25 mm (obstacle C) b - a flat obstacle with a diameter of 14 cm with a single hole 25 mm in diameter and a second flat obstacle with a single hole with a diameter of 25 mm, covered with a flat mesh (obstacle D); c - a flat obstacle with a diameter of 14 cm with a single hole 25 mm in diameter and a second flat obstacle with a single hole with a diameter of 40 mm, into which a mesh sphere was inserted (obstacle E); A mixture of 15.4% PG + 30.8% O2 + 46% CO2 + 7.8% Kr at an initial pressure of 180 Torr. The number on the frame corresponds to the sequence number of the frame after the moment of initiation. The results of calculations carried out near the limit of flame penetration through the obstacle are shown in Fig. 6 d, e, f. As can be seen from the figure, the calculation results are in qualitative agreement with the experimental data presented in Fig. 6 a, b. The performed calculation also makes it possible to qualitatively reveal both the role of active combustion centers (Fig. 6d) and heat losses (Fig. 6f) when the flame penetrates through the obstacle. Under the conditions of penetration of the flame front through the hole (Fig. 6e) for a single first-order reaction obeying the Arrhenius law, taking into account either heat losses (the channel width in Fig. 6f is equal to 0.6 of the channel width in Fig. 6e) or the simplest chain mechanism (instead of one reaction) leads to a limit for the penetration of the flame through the hole. In this latter case, the losses are provided by taking into account the breakage of the active centers of the circuits ( ).It should be noted that in Fig. 6 d, e, f all other parameters in the calculation are the same. The results of numerical simulation of flame penetration through a conical funnel are shown in Fig. 7 c-e. As can be seen from the figure, the simulation results are in qualitative agreement with the experimental results shown in Fig. 7 a, b. Indeed, in accordance with the experiment, the flame penetrates through the funnel from the side of its nose (Fig.7a, f), and when the flame propagates from the side of the funnel funnel, no flame penetration is observed (Fig.7b, c, d), the flame is extinguished. This qualitative difference from the process of flame penetration through a flat obstacle with a central hole indicates a noticeable role of the interaction of acoustic vibrations in a reactor containing an obstacle with a propagating combustion front. Thus, a numerical experiment shows that with a wider opening of the funnel, the flame does not penetrate through the obstacle on the side of the funnel mouth. In other words, there is a critical hole diameter. Peculiarities of FF penetration through complex obstacles also qualitatively coincide with experiment. In qualitative agreement with Fig. 8a, 9a, in the presence of a grid sphere as an obstacle, the length of the "flame jump" is much larger than with a flat grid (Figs. 8b, 9b). Therefore, (even taking into account the qualitative nature of the calculations and the very formal modeling of the mesh sphere), we were able to illustrate the main features of the FF propagation through complex obstacles. The same applies to the peculiarities of flame penetration through the combined obstacles C, D and E (fig. 12-15). At the end of the section, let us stop on the possibilities of analysis based on the Navier-Stokes equations in the acoustic approximation. Figure 11.IIb of Chapter 3 experimentally demonstrates the appearance of streams of hot glowing gas from a volume bounded by a spherical mesh obstacle that appears after the flame reaches the walls of the reactor. Determining whether it is possible to interpret this phenomenon using the acoustic approximation of the Navier-Stokes equations (Chapter 4, system of equations (II)) was of interest. The problem was solved by the finite element method using the FlexPDE 6.08 software package, 1996-2008 PDE Solutions Inc. [15]. Fig. 15. Results of numerical modeling of the flame propagation process through a complex obstacle: a - obstacle C; b - obstacle D; c - obstacle E. The dimensionless temperature scale T is shown on the right. The initial condition was T = 10 on the left channel segment at y = 0.5. There was a mesh obstacle in the channel (Fig. 16). The boundary conditions (including the hole) were on the reactor wall and n = 0 on the grid surface, where - dimensionless coordinate (x, y), as well as the condition for convective heat transfer . The chemical reaction was represented by the simplest chain mechanism (Chapter 4, system of equations (II)). From the calculation results presented in Fig. 16, it can be seen that density fluctuations are actually observed in the area of the mesh obstacle. The physical interpretation of this phenomenon requires further research. Modeling based on the Navier-Stokes equations for a compressible reacting medium also allows you to analyze the influence of scale on the course of the process and establish the possibility of manifestation of scale effects, which were discussed in paragraph 2 of Chapter 3. Fig. 16. The emergence of gas flows in the area of a circular mesh obstacle after initiated ignition at successive times in 0.05 s (from left to right). Calculation based on the Navier-Stokes equations for a compressible reacting medium in the acoustic approximation. In this section, experimental data are presented that can be an argument in favor of the existence of a scale effect when hydrodynamic instability occurs. Let us first note that combustion processes in large volumes have attracted great interest in recent decades. Tests in large volumes are performed to characterize the explosion and detonation characteristics of hydrogen-air and hydrogen-oxygen mixtures. Most of the existing experiments use shock tubes for these types of explosions. A small part of these works is devoted to experiments in spherical geometry [33-36]. At the same time, one can conclude from the existing publication that modeling, and, accordingly, understanding, ignition and combustion in large volumes in the presence of mesh obstacles are still at an early stage. In particular, this is due to the lack of experimental data because of the cost and danger of creating large-scale experimental stands. The problem was solved by the finite element method [15]. The initial condition was T = 10 on a circle enclosed in a circular mesh obstacle (Fig. 17). The boundary conditions were at the reactor wall and n = 0 at the grid surface for Fig. 17, where - dimensionless coordinate (x, y), as well as the condition for convective heat transfer . The chemical reaction was represented by a single Arrhenius reaction (Chapter 4, system of equations (II)). Fig. 17. Calculation of flame penetration through the mesh obstacle, model scale a) is 5 times smaller than model b). Figure a) shows the propagation of the front with respect to the concentration of the initial substance. The dependences of the path X, which the flame front passes (estimated by the largest value of the y coordinate at the temperature front) after the mesh obstacle, on time are illustrated in fig. 18. It is seen from fig. 2.14 (dependences 1 and 3) that under the condition n = 0 on the surface of the mesh (dependence 3), that is, the mesh is chemically inert, the flame begins to propagate outside the mesh earlier than under the condition n = 0 on the surface of the mesh, that is, when the mesh is chemically active. In other words, it provides effective destruction of active intermediate combustion products (dependence 1). The result obtained is in qualitative agreement with the experimental dependences shown in Fig. 11 IIb and 13b Chapters 4. Fig. 18. Dependences of the path X, which the flame front passes (estimated by the largest value of the y coordinate at the temperature front) after the mesh obstacle, on time. 1 - fig. 16 a); 2 - fig. 16 b), all X values are divided by a scale factor of 5; 3 - fig. 16 a), but n = 0 on the mesh surface. It also follows from fig. 18 that the propagation velocities of the combustion front, taking into account the scale factor, are practically the same. This means that within the framework of the acoustic approximation of the Navier-Stokes equations for a compressible reacting medium with one reaction in the form of Arrhenius, one should not expect the manifestation of scale effects. Let us pay attention to the fact that in order to describe the quantitative regularities of the penetration of the flame front through a single hole, it is necessary to analyze the three-dimensional model. At the same time, the results of two-dimensional modeling are in qualitative agreement with the experimentally observed regularities. We briefly summarize the results obtained in this section. It has been shown experimentally that below the limit of penetration of the flame of a dilute methane-oxygen mixture through a flat obstacle with a single hole, the flame does not penetrate through the confuser, but penetrates through the diffuser. Such a qualitative difference from the process of flame penetration through a plane obstacle with a central hole according to numerical modeling using the dimensionless Navier-Stokes equations for a reacting compressible medium in the low Mach number approximation indicates a noticeable role of the interaction of acoustic vibrations in a reactor containing an obstacle with a propagating combustion front. In addition, the simulation performed in small volumes suggests that in the event of an emergency, the flame will not penetrate through the open valve located in the center of the confuser located in the pipe. In this case, the most effective double-sided flame-arrester in the pipe can be a system of two confusers, the funnels of which are located on the pipe axis along the gas flow and against it. It is because an emergency can occur before and after the obstacle. A hole or valve can be located in the middle. It should be noted that the results obtained when visualizing the penetration of a flame through obstacles of various shapes are important for solving explosion safety problems in volumes with complex geometrical arrangements. § 3. Factors determining the length of the flame jump after penetration through a small hole. One of the oldest unsolved problems in fluid mechanics is the theoretical description of the occurrence and growth of disturbances in laminar flow, which lead to turbulent flow. This extremely complex process is not fully understood at this time. Despite many systematic experimental and theoretical studies, the reason for the violation of the laminarity of the gas flow. The appearance of turbulence, for example, in the presence of obstacles in round pipes, remains unclear [37, 38]. In the event of ignition following an emergency in the gas infrastructure in an industrial or civil facility, pressure buildup due to flame acceleration because of the flame front turbulence can endanger the integrity of the building and the life of personnel [2,8, 28, 39]. Although the main features of flame acceleration have been investigated by various authors [6,7,9,40], the array of reliable experimental data obtained by high-resolution measurement methods that determine such process variables as density, temperature, velocity, and concentration of active particles is still relatively small. In particular, this is because the required resolution in time and space cannot be easily achieved. Only at the present time the use of inertialess optical methods has become widespread for the study of fast transient processes [2]. It should be noted that in [9, 40], the study of combustion of lean mixtures of hydrogen with air was carried out in a cylindrical tube separated by an obstacle with a small hole in the center. Studies have shown that the attenuation of a hydrogen flame when propagating through a small hole can occur at concentrations much higher than the concentration limit of flammability of the mixture. In addition, it was experimentally shown [14] that an acoustic resonator (Helmholtz resonator) connected to a cylindrical reactor can provide significant flame acceleration upon initiated ignition of a lean (15%) mixture of hydrogen and oxygen near the detonation limit. These data are important both for solving explosion safety problems and for developing calculation methods for simulating and establishing the causes of the accidents described above. It was shown above that ignition of a diluted stoichiometric methane-oxygen mixture (total pressure up to 200 Torr) after a single obstacle with a small round hole is observed at some distance after the obstacle, and not immediately after it. The mesh sphere as an obstacle leads to an increase in the magnitude of the "jump" of the flame through the obstacle in comparison with the round hole. The symmetry of the described obstacles made it possible to use 2D modeling for high-quality calculations, however, modeling the penetration of flame through rectangular holes involves 3D analysis. For this purpose, it is necessary to obtain reliable experimental data on the penetration of the flame front (FF) through a narrow rectangular opening. It was shown in [12, 41] that the active centers of combustion of methane and hydrogen, which determine the propagation of the flame, have a different chemical nature. In other words, the rate of death of active intermediate combustion centers on the surface of the obstacle makes a significant contribution to the penetration of the FF through the obstacle in the case of air mixtures and natural gas. And it is insignificant in the propagation of a hydrogen flame. The results of experiments on flame propagation through obstacles with spherical and rectangular holes placed in a cylindrical channel are shown below. The purpose of this section was to establish the peculiarities of FF penetration through rectangular versus round holes using high-speed color flame filming and gas flow visualization when a fine inert powder is illuminated in the flow with a flat laser beam. In this section, the phenomenon of acceleration of an initially slow flame of stoichiometric methane-air mixtures diluted with inert additives by single obstacles of various geometric shapes is investigated. The flame propagation in stoichiometric mixtures of methane with oxygen, diluted with CO2 and Kr at initial pressures in the range of 100-200 Torr and a temperature of 298 K was investigated in an evacuated horizontally located cylindrical quartz reactor 70 cm long and 14 cm in diameter. The reactor was fixed in two stainless steel locks equipped with holes for gas evacuation and a safety lock that opened outward when the total pressure in the reactor exceeded 1 atm [13]. A pair of spark discharge electrodes were located at the left end of the reactor (Fig. 19). Thin obstacles 14 cm in diameter with rectangular holes (7 and 10 mm wide and 65 mm long) and spherical 20 and 25 mm in diameter were placed vertically in the center of the reactor. In some experiments, a rectangular hole 10 mm wide and 65 mm long was covered with an iron mesh with cells 0.5 mm in size (wire diameter 0.25 mm). The possibility of registering the gas flow was provided by illumination with a flat laser beam of ultrafine MgCO3 particles carried away from the shelf (14) by the gas flow as the flame propagated from left to right (Fig. 19). The combustible mixture (15.4% CH4 + 30.8% O2 + 46% CO2 + 7.8% Kr) was preliminarily prepared. CO2 was added to reduce the FF rate and thereby improve the quality of the survey. Kr was added to reduce the discharge threshold. The reactor was filled with a combustible mixture to the required pressure. Then, the flame travel was initiated by a spark discharge (J = 1.5 J). The dynamics of ignition and FF propagation was recorded on the side of the reactor (Fig. 3.1) with a Casio Exilim F1 Pro color high-speed digital video camera (frame rate 600 s–1) [13, 14]. The video file was saved in the computer memory, and then it was digitally processed [32]. In the experiments, reagents of the chemically pure grade were used. . Fig. 19. Experimental setup. (1) quartz reactor 14 cm in diameter and 70 cm long, (2) stainless steel airlock, (3) Viton gasket, (4) stainless steel door, (5) spark ignition electrodes, (6) power supply, (7) high-speed video camera, (8) rail, (9) optical window, (10) swing mirror, (11) flat laser beam, (12) short-focus-lens, (13) semiconductor laser (534 nm), (14) reservoir with ultrafine powder MgCO3, (15) an obstacle with a hole. The pressure sensor is located on the door (4). The sequences of video frames of the penetration of the combustible mixture flame through round and rectangular holes are shown in Fig. 20. Fig. 20. a - high-speed filming of FF propagation through a rectangular hole 7 mm wide and 65 mm long, b - high-speed shooting of the FF propagation through a round hole 25 mm in diameter, c - high-speed shooting of FF propagation through a rectangular hole 10 mm wide and 65 mm long (the slit is located vertically), d - high-speed shooting of FF propagation through a rectangular hole 10 mm wide and 65 mm long (the slit is located horizontally) e - high-speed shooting of the FF propagation through a rectangular hole 10 mm wide and 65 mm long, covered with a mesh (the slot is placed vertically), The initial pressure is 170 Torr. The number on the frame corresponds to the sequence number of the frame after spark ignition. (frame rate 600 s-1.). A video sequences of flame propagation through a 7 mm wide rectangular hole is shown in fig. 20a. As can be seen from the figure, the FF does not penetrate the obstacle, which indicates the existence of a limit for flame penetration along the width of the slit, since the FF penetrates through a slit 10 mm wide (Fig. 20c). Ignition after the passage of the FF through the obstacle does not occur immediately after the obstacle. The first ignition site is observed in the immediate vicinity of the obstacle surface, in contrast to the penetration of the FF through a round hole (Fig. 20b), see also [13], when a noticeable “flame jump” is observed after the obstacle (“flame jump” is the distance at which there is a flame front behind the obstacle, see the previous paragraph). As can be seen from Fig. 20e (frames 24, 25), in the presence of a grid on a rectangular obstacle 10 mm wide and 65 mm long, a second “flame jump” is observed. It shoul be noted that the accumulation of free radicals behind the obstacle is observed experimentally [7-9]. Mixing these radicals with unreacted gas obviously increases the explosiveness of the mixture. This indicates the need to take into account the main features of the combustion chain mechanism in 3D numerical modeling, as was done during the analysis of the two-dimensional problem in [13]. The results of registration of the gas flow by illuminating the MgCO3 particles blown out by the gas flow from reservoir 14 (Fig. 19) with a flat laser beam after round and rectangular holes are shown in Fig. 21. As seen from Fig. 21a, b, d, the flux density after the obstacle has two maxima: the first maximum is near the obstacle, the second maximum is observed noticeably farther from the obstacle surface. It can be seen that the distance of the appearance of the second density maximum correlates with the distance of the appearance of the “flame jump” through the corresponding obstacle. The results of filming a gas flow through a rectangular hole 10 mm wide and 65 mm long and through the same hole, but closed with a metal mesh, are shown in Fig. 21c and Fig. 21d respectively. Fig. 21. High-speed filming with gas flow visualization: a - the spread of the FF through a round hole 25 mm in diameter; b - distribution of FF through a rectangular hole 10 mm wide and 65 mm long (the slit is located vertically); c - FF propagation through a rectangular hole 10 mm wide and 65 mm long (the slit is located horizontally); d - FF propagation through a rectangular hole 10 mm wide and 65 mm long, closed by a mesh (the slit is placed horizontally). The initial pressure is 170 Torr. The number on the frame corresponds to the sequence number of the frame after spark ignition (frame rate - 300 s-1). As in the case of FF penetration through a round hole, the flux density after the obstacle shows two maxima. The first maximum is near the obstacle, the second maximum is observed noticeably farther from the obstacle surface (Fig.21c frame 12, Fig.21d frames 11, 12). As can be seen, in the presence of a grid, the second “flame jump” is observed at a greater distance. Consequently, a correlation between the position of the second density maximum and the magnitude of the “flame jump” also exists, i.e. and after rectangular holes compared to the magnitude of the “flame jump” through the hole without mesh. To identify the main factors affecting the length of the "flame jump" when the FF penetrates through small holes, let us compare the features of the FF penetration through round and rectangular holes. It can be seen from fig. 21 that long before the FF contact with the obstacle, small particles illuminated by the laser beam already begin to move (Fig.21a, frame 8, Fig.21b, frame 7, Fig.21c, frame 8, Fig.21d, frame 7). In other words, an initially undisturbed submerged jet is formed in the gas behind the obstacle (Fig. 21c, frames 8-10, Fig. 21d, frames 7-9). After the contact of the FF with the obstacle, the primary ignition sites (local volumes containing gas heated to the combustion temperature and active combustion centers [7-9]) arise in this submerged jet. It can be assumed that the primary foci move in the submerged jet during the ignition delay period (induction period), and then ignition occurs. However, the above mentioned does not agree with [9], where it is stated that when the flame front passes through the hole, a high degree of flow turbulence leads to the suppression of the flame behind the hole in the region before combustion occurs. Let us estimate approximately the time t of motion of the primary source in a submerged jet in the approximation of an incompressible medium [42]. For the axial component of the velocity in a parallel flow in a parallel flow ; in an axisymmetric flow R/0 = 0.96/(0.07x/R – 0.29). Here - is the flame velocity at the moment of contact between the FF and the obstacle, x is the coordinate, L is the width of the rectangular slot, R is the radius of the round hole, and the numerical values are empirical parameters from [42]. Then and, accordingly, the ratio [42] for the same values of the upper limit of integration x = 10 is ~ 4; for x = 3, this ratio is ~ 2. This means that in the case of a parallel flow, the primary site moves by a distance x in a time that is much less than in the case of an axisymmetric flow. Therefore, during the ignition delay period, the primary site will move farther from the obstacle than in the case of an axisymmetric flow. On the other hand, the experiment shows (cf. Figs. 20b and 20d) that ignition occurs earlier in the case of the penetration of the FF through a rectangular hole rather than a round one. This means that if the “flame jump” were determined by the ignition delay period, then the length of the “flame jump” would be shorter for a round hole, which contradicts the experiment. Another explanation could be as follows. Lemanov et al. In [43] determined the coordinate of the laminar-turbulent transition in a submerged jet for various values of the Reynolds number by means of visualization and anemometric measurements. They showed that the length of the laminar section in a plane flow is much shorter (by a factor of 2–5) than in an axisymmetric one. This suggests that the magnitude of the “flame jump” in the submerged jet formed after the hole is determined by the time of occurrence of the transition from laminar to turbulent flow, and not by the delay time of the ignition of the combustible mixture. The relatively weak influence of the velocitybof combustion reaction on the magnitude of the flame jump is another reason in favor of the above explanation. The comparative contribution of gas-dynamic and chemical factors to the magnitude of the flame jump was approximately estimated using numerical modeling based on the analysis of the Navier-Stokes equations for a compressible reacting medium in the approximation of a small Mach number. These equations were proposed in [15-19] and they describe the propagation of a flame in a two-dimensional channel. The solution of these equations showed qualitative agreement between the calculations and experiments [13, 26]. The penetration of flame through an axisymmetric obstacle in three channels of different widths was analyzed. The system of Navier-Stokes equations in a compressible reacting medium in the acoustic approximation was solved by the finite element method using a package (FlexPDE 6.08, 1996-2008 Inc. [24]). The simplest chain mechanism was used [16, 23, see also Chapter 4]. The results of numerical simulation of flame penetration through the hole are shown in Fig. 22. The boundary conditions (including the hole) were (Fig. 22, I) n = 0 (Fig. 22 II), and the condition of convective heat transfer where is the dimensionless coordinate (x, y). The ignition initiation condition was T = 10 at the right border of the channel. There was a vertically located obstacle in the channel with an axisymmetric hole. As can be seen from the calculations, the results of calculations for (the calculation is shown in Fig. 22 I) and (the calculation is shown in Fig. 22 II) show that the smaller the channel width, the shorter the “flame jump”. The results of numerical simulation of flame penetration through the hole are shown in Fig. 22. The boundary conditions (including the hole) were (Fig. 22, I) n = 0 (Fig. 22 II), and the condition of convective heat transfer where - dimensionless coordinate (x, y). The ignition initiation condition was T = 10 at the right border of the channel; there was a vertically located obstacle in the channel with an axisymmetric hole. As can be seen from the calculations, the results of calculations for (the calculation is shown in Fig. 22 I) and (the calculation is shown in 22 II) show that the smaller the channel width, the shorter the “flame jump”. Fig. 22. Results of calculating the process of flame propagation through the hole, a, b, c - different values of the channel width (1, 0.8, 0.6 in dimensionless units, respectively) I - the change in the dimensionless temperature of flame propagation through the hole for nξ = 0; II - change in the dimensionless temperature of flame propagation through the hole for n = 0; The dimensionless temperature scale T is shown on the right. In addition, in the case (Fig. 22 II), the death of active combustion centers occurs at each collision of an intermediate particle with a wall (the so-called diffusion region of chain termination [22]); the rate of death of active centers is higher than for the (the so-called kinetic region of chain termination [22], Fig. 22 I). Therefore, the value of the ignition delay period for should be greater than for [22], and the value of the "flame jump" should be correspondingly larger for . This contradicts the calculations. On the other hand, experimental data [44] demonstrate an increase in the time of occurrence of a laminar-turbulent transition in pipes with an increase in the pipe diameter. As follows from Fig. 22, the results of the numerical experiment agree precisely with the experimental data [44], which also testifies in favor of the main contribution of gas-dynamic factors to the magnitude of the “flame jump”. § 4. Spectral features of the emission of methane-oxygen flames in conditions of penetration through obstacles. Possibilities of the 4D spectroscopy method. It can be seen that after ignition is initiated by a spark discharge at the left end of the pipe from the above sequences of video frames (see Fig. 7a, frame 23, as well as the results of works [12, 45-47]), the "blue" FF propagates from left to right and reaches the obstacle. Then a secondary "yellow" FF occurs. This means that the degree of conversion in the blue FF and active intermediate products of methane combustion after the flame passes through the obstacle initiate the propagation of the "yellow" FF. The results obtained correlate with the results of [47], where it was shown that the FF in a hydrocarbon-air mixture in a heated cylindrical reactor is always yellow in color ("hot" flame, Fig. 1b [47]), although the flame at the initial room temperature in the same mixture and in the same blue reactor ("cold" flame, Fig. 1a, b [47]). It should be noted that the blue color of the flame is due to the emission of CH (431 nm) and, possibly, CH2O (470 nm) radicals. The yellow color of the hot flame is caused by the emission of excited Na atoms or a lack of an oxidizing agent, i.e. the formation of soot [12]. A block of reactions of hydrocarbon oxidation to CO is considered [22] to be realized in a “blue”, “cold” flame, and in a “yellow”, “hot” flame, the next block of reactions of CO oxidation to CO2 is realized. This means that it is possible to separate these two macrokinetic processes in space in experiments on the passage of flames through obstacles. The complex obstacles described in Section 2 and consisted of a flat obstacle with a diameter of 14 cm (reactor diameter) with a single hole 25 mm in diameter and a second flat obstacle with a diameter of 14 cm with a single hole 25 mm in diameter (obstacle C §2) or a second flat obstacle with a diameter of 14 cm with a single hole with a diameter of 25 mm, covered with a flat mesh (obstacle D §2 of this Chapter). The second obstacle was located at a distance of the magnitude of the “flame jump” after a flat obstacle 14 cm in diameter with a single hole 25 mm in diameter. According to experimental data, this distance was 12 cm. In the experiments described in this section, along with high-speed filming, the combustion process was recorded by the 4D spectroscopy method (1-time, 2-wavelength, 3-spectrum intensity at a given wavelength, 4-coordinate of the emitting fragment of the light source) through the side surface of the quartz reactor fig. 23). Fig. 23. Using hyperspectrometers to study flames: a) - (1) quartz reactor, (2) stainless steel airlocks, (3) silicone gasket, (4) safety door, (5) spark ignition electrodes, (6) power supply, (7) ) high-speed movie camera, (8) microphone, (9) - rotating mirror, (10) - hyperspectrometers unit, (11) - obstacles, b) and c) - photographs of the setup. In fig. a) and b) the line along which the 4D spectral survey was carried out is marked in red. The width of this line is about 1 mm. In the experiments described in this section, video recording of the combustion was also carried out with one color high-speed video camera Casio Exilim F1 Pro (frame rate - 300 - 1200 s-1) simultaneously with registration with hyperspectrometers, or simultaneously with two Casio Exilim F1 Pro film cameras from the side of the reactor, as described above. (The resulting video files were recorded in the computer memory and then frame-by-frame processing was carried out) (Fig. 24). Fig. 24. Experimental setup. a - (1) quartz reactor, (2) stainless steel gateways, (3) silicone gasket, (4) safety door, (5) spark ignition electrodes, (6) power supply, (7) high-speed movie camera, (8) microphone, (9) - obstacle, (10) - interference filter, b - photograph of the experimental setup. The design and use of the VID-IK3 hyperspectrometer (push broom type) [45, 46] and its modified version are described in detail in Chapter 2 and Chapter 4. The characteristics of the hyperspectrometers are given in Table 1 of Chapter 4. The appearance of both instruments installed on the rotating device, is shown in Fig. 23, and the construction (the same for both units) is shown in Chapter 2. The spectrum (a) and the time-base sweep of the spectrum of the methane-oxygen flame (b) is shown in fig. 25 a, b. Bands in the region of 600 nm were observed in a hydrogen flame in [48]. Table 4 of Chapter 4 from [48] shows the assignment of the bands in Fig. 25 to water vapor, which is a product of the oxidation reaction of hydrogen and hydrocarbons. The spectrum shown in Fig. 25, agrees with the literature data [22] and contains СН (A1Δ–X2 Π) bands in the 431 nm region, C2 (A3Pg –X3Pu) (1-0, 0-0, 0-1 transitions) in the 470 - 570 nm region [48] and emission bands of water vapor (for example, (1, 2, 0), (3, 0, 0) [49]). It should be noted that the CH and C2 bands refer to the contribution of the zone of intense chemical transformation (FF zone) [22] to the total spectrum, and the emission bands of water vapor - to the emission region of combustion reaction products. Thus, the features of the flame spectrum in the visible region, namely the system of radiative bands of water in the region of 570 - 650 nm, along with the doublet of sodium (581 nm) and potassium (755 nm) atoms inherent in all hot flames [22], are related in this case with radiation from an area filled with combustion products. It can be seen from fig. 25b that the intensities of all spectral lines from the spectrum with the x = 234 coordinate to the spectrum with the x = 1 coordinate change symbatically. There is no situation when the intensity of the bands in one region of the spectrum increases in space and decreases in the other region of the spectrum. This is because the observed spectral lines belong only to the reaction products or appear in the zone of the reaction products (Na, K). Thus, it was observed by 4D spectroscopy that after the obstacle (see Fig. 4.1) a high-temperature mechanism of methane combustion is realized, accompanied by the excitation of atomic lines of alkali metals. The establishment of the regularities of the formation of intermediate particles in the low-temperature region of combustion (before the obstacle) was also carried out using two high-speed movie cameras Casio Exilim F1 Pro, equipped with interference filters (Fig. 23, a, b). This made it possible not only to increase significantly the sensitivity of the measuring equipment, but also to reveal the spatial features of the formation of intermediate combustion particles. Fig. 25. Emission spectra of the flame of a mixture of 15.4% PG + 30.8% O2 + 46% CO2 + 7.8% Kr after the first obstacle at an initial pressure of 180 Torr. a - x = 125, y = 30; b - x = 30, y = 80, 125, 160, 190, 230. The registration rate is 300 spectra/s. We used simultaneously two interference filters 435 nm СН (A1 Δ–X2 Π) (transmission at a maximum of 40%, ± 16 nm) and 520 nm C2 (A3Pg –X3Pu) (transition 0-0) (transmission at a maximum of 40%, ± 15 nm) to establish the features of the appearance in time and space of active intermediate particles CH and C2. Combinations of glass filters were also used for 435 nm (transmission at maximum 70%, ± 35 nm), 520 nm (transmission at maximum 35%, ± 60 nm), and 590 nm for the line of Na atoms (transmission at maximum 70%, ± 25 nm ) in order to establish the features of the appearance in time and space of active intermediate particles and self-heating. This is because the radiation of Na atoms is caused by their thermal excitation [22], which occurs at a flame temperature above 1200 °C [50]. The results of high-speed video filming of the flame penetration of a combustible mixture of 15.4% PG + 30.8% O2 + 46% CO2 + 7.8% Kr at an initial pressure of 180 Torr through a flat obstacle 14 cm in diameter with a single hole 25 mm in diameter are demonstrated in fig. 26. There is also a second flat obstacle with a single hole 25 mm in diameter mm, covered with a flat mesh (obstacle D §2 of this Chapter), recorded without light filters at a rate of 300 frames/s in comparison with the registration of the СН band (A1 Δ – X2 Π) b) and the C2 band (A3Pg –X3Pu) (transition 0- 0) c), which are registered at the same speed. It can be seen from fig. 26 that before the obstacle there is a blue glow in the reactor due to the emission of CH radicals, C2 radicals in recorded quantities are observed only after the first obstacle. It is also clearly seen from fig. 27 with the results of recording the radiation of a propagating flame using glass filters in the wavelength range of 435 nm, 520 nm, and 590 nm, that both the appearance of C2 radicals in recorded quantities and the main heat release in the process are observed after the first obstacle, i.e. after turbulization of the gas flow. The obtained result means that the used experimental technique makes it possible to separate in time and space “cold” and “hot” flames in one experiment. This result is also important for the verification of numerical models of methane combustion. Fig. 26. High-speed survey of the propagation of the FF of the combustion of a mixture of 15.4% GHG + 30.8% O2 + 46% CO2 + 7.8% Kr at an initial pressure of 180 Torr through a combined obstacle consisting of a flat obstacle with a diameter of 14 cm with a single hole 25 mm in diameter. There is a second flat obstacle with a single hole with a diameter of 25 mm, covered with a flat mesh (obstacle D); a) - without filters, b) - interference filter 435 nm, c) - interference filter 520 nm (shooting speed 300 frames/s). The number on the frame corresponds to the sequence number of the frame after the moment of ignition initiation; other position of obstacles after penetration of flame in fig. a) due to the fact that these are different experiences; fig. b) and c) were obtained in the same experiment. Fig. 27. High-speed survey of the propagation of the FF of the combustion of a mixture of 15.4% GHG + 30.8% O2 + 46% CO2 + 7.8% Kr at an initial pressure of 180 Torr through a combined obstacle consisting of a flat obstacle with a diameter of 14 cm with a single hole 25 mm in diameter. There is a second flat obstacle with a single hole with a diameter of 25 mm, covered with a flat mesh (obstacle D); glass filters are used, a) - glass filter in the region of 435 nm (transmission at a maximum of 70%, ± 35 nm), b) - a glass filter in the region of 520 nm (transmission at a maximum of 35%, ± 60 nm), c) - combined glass filter in the region of 590 nm (transmission at a maximum of 70%, ± 25 nm). The shooting speed is 300 frames/s. The number on the frame corresponds to the sequence number of the frame after the moment of initiation. Conclusions for Chapter 6 It has been shown experimentally that in the case of flame penetration through an obstacle considered in this Chapter, gas-dynamic factors, for example, flame turbulization, can determine the kinetics of the process, including the transition of low-temperature combustion of hydrocarbons to a high-temperature regime. It has been established that the flame front after a single obstacle does not arise in the immediate vicinity of the obstacle. The first ignition site can be observed relatively far from the obstacle surface. It is shown that the use of a grid sphere as an obstacle leads to an increase in the length of the flame "jump" behind the obstacle in comparison with a round hole. It is shown that two or more obstacles, both spherical and flat, noticeably suppress the propagation of the flame, which can be associated with both heat losses from the flame front and with heterogeneous termination of reaction chains at the obstacle. It has been experimentally shown that below the limit of the flame penetration of a dilute methane-oxygen mixture through a flat obstacle with one hole, for an obstacle in the form of a funnel, the flame does not penetrate from the side of the funnel mouth, but penetrates from the side of the funnel nose. Numerical modeling of the Navier-Stokes equations for a compressible medium in the approximation of a small Mach number with the representation of a chemical process as a single reaction and the simplest chain mechanism made it possible to qualitatively describe the experimental laws. The features of the penetration of the flame front through rectangular holes in comparison with round holes were experimentally investigated by using color filming and visualization of a gas flow. It is shown that the length of the “flame jump” after the hole in the obstacle is mainly determined by the time of occurrence of the laminar-turbulent transition, and not by the ignition delay period. It was found that C2 radicals in detectable quantities and the main heat release in the process are observed by using 4D spectroscopy combined with high-speed color filming after the flame passes the first obstacle, i.e. after turbulization of the gas flow. The result obtained means that the technique used allows one to separate in time and space the “cold” and “hot” flames in one experiment. The result obtained is also important for the verification of numerical models of methane combustion. In addition, the results obtained are important for solving explosion safety problems for volumes with complex geometrical arrangements.

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