Abstract (English):
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.

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

It was found that the temperature of the ignition limit above the palladium surface at 1.75 atm for mixtures of 30% methane + 70% hydrogen + air (=0.9, T=317 0C) and 30% propane + 70% hydrogen + air (=1, 106 0C), measured by the “bottom approach” temperature method, decreases during subsequent ignitions. The flammability limit returns to the initial value after the reactor is treated with oxygen or air, i.e. hysteresis takes place. The temperature of the ignition limit of mixtures of 30% (C2, C4, C5, C6) + 70% H2 + air (=0.6, 1.1, 1.2, 1.2, respectively) above the palladium surface is 19 ÷ 35 0C at 1.75 atm; there is no hysteresis. It is shown that the lean (=0.6) mixture of 30 ethane + 70% hydrogen + air has the lowest temperature of the ignition limit: 24 0C at 1 atm. The effective activation energy for the ignition of mixtures over palladium is estimated as ~ 2.4 ± 1 kcal / mol. It was found that the separation of the CH and Na emission bands in time during the combustion of a mixture of 30% propane + 70% H2 + air (=1) found in this work, is due to the occurrence of hydrodynamic instability of the flame when it touches the end of the cylindrical reactor. It was found that the ignition temperatures of hydrogen - oxygen and hydrogen - methane - oxygen mixtures under the pressure of heated wires of palladium, platinum, nichrome and kantal (fechral) at a total pressure of 40 Torr increase with a decrease in the hydrogen content in the mixture; only heated palladium wire exhibits a noticeable catalytic effect. A qualitative numerical calculation made it possible to reveal the role of the additional branching reaction H + HO2 → 2OH. Key words: combustion, ignition, palladium, hysteresis, sensor It is known that methane and oxygen on a heated platinum wire can release a significant amount of heat in a dark reaction [1]. Interest in catalytic oxidation processes and their mechanisms is constantly growing due to the broad prospects of using this technology in combustion in power generation systems [2-4], to reduce the concentration of methane in the air [5]; in the use of catalytic converters in vehicles to reduce emissions of harmful gases [6]. The issue of ensuring hydrogen safety at nuclear power plants using catalytic afterburners is a topical issue [7]. There is also a great deal of interest in catalytic partial oxidation leading to intermediates that are critical in the synthesis of target industrial compounds. § 1. Study of the combustion of hydrogen-air and hydrogen-methane-air mixtures over the surface of palladium metal with the combined use of a hyperspectral sensor and high-speed color filming The mechanism of the oxidation of hydrogen and hydrocarbons on noble metals has not yet been sufficiently understood. Experiments on isotope exchange [8] have shown that in case of chemisorption of methane on noble metals results in the formation of adsorbed methyl or methylene radicals. Their interaction with adsorbed oxygen can lead either to direct oxidation to carbon dioxide and water or to the formation of adsorbed formaldehyde [9]. Until now, the nature of the active surface is generally unknown. In the case of palladium, oxidation can occur on the metal itself, on the surface of Pd (II) oxide, or even on a surface partially covered with adsorbed oxygen, and at the same time. According to X-ray photoelectron spectroscopy (XPS) data [10], the smaller the size of the palladium crystallites, the more likely they are to exist in the oxide form. It should be noted that because of a certain degree of conversion of the reagent, a significant amount of heat can be released. This leads to a significant increase in temperature, thus the stability of the catalyst at high operating temperatures must be known [2]. As the temperature rises, the activation energy of methane oxidation on the Pd catalyst changes sharply. The temperature where this transition occurs is a function of the catalyst composition [11]. It should be emphasized that noble metals form oxides, which, depending on their reactivity, determine the rate and mechanism of the catalytic process. This significantly complicates the search for optimal conditions for catalysis. For example, Pd readily transforms into PdO at temperatures lower than 1100 K, but PtO2 already decomposes at temperatures above 825 K. Due to the greater stability of PdO compared to PtO2, in the case of a Pd-containing catalyst, the active phase is most likely PdO, in while in the case of a Pt catalyst, the active phase is Pt. The activity of PdO is higher than that of Pt. This allows for higher conversions when using PdO. Controlling the presence of PdO in the gas phase, which is necessary to establish the mechanism of catalysis, is complicated by the fact that the spectroscopic data on PdO are scattered and contradictory [12 - 14]. In [15], the emission spectra of PdO obtained during the pyrolysis of polymer complexes (PtCl2) upon excitation at a wavelength of 355 nm are presented. Thus, the appearance and participation of a chemically active surface in gas combustion significantly complicate the understanding of the process, not only due to the emergence of new control parameters, but also difficulties in registering catalyst molecules or particles. It is worth reminding that the hyperspectrometers used in this chapter and discussed in the previous chapters are devices that allow remote registration of reflected, scattered and emitted radiation, obtaining its spectrum in a wide wavelength range [16,17]. The hyperspectrometer makes it possible to study the temporal characteristics of the processes occurring on a narrow strip of the investigated surface. Those. The 4D dimension is formed by the x coordinate, the spectral coordinate - by the wavelength , the intensity of the spectral line I and the time t. The work [18] demonstrated the possibility of studying combustion and explosion processes using remote hyperspectral sensing. High-speed filming is also used in this book as a remote study of various processes. In [19, 20], the method of high-speed color photography was used to study spark-initiated ignition of hydrogen-air and pentane-air mixtures. This method makes it possible to visualize the propagation of the flame front and reveal the features of combustion processes, in particular, the transition of a smooth flame front into a cellular structure. This section is devoted to identifying the regularities of the combustion of hydrogen and mixtures of hydrogen with methane over metallic palladium, including using an optoelectronic complex based on a hyperspectral sensor and high-speed color filming. Experimental part The experiments were carried out in a heated horizontal cylindrical stainless steel reactor 25 cm long in length and 12 cm in diameter, equipped with a tangential gas inlet, collapsible covers, and an optical quartz window in one of the coatings (Fig. 1). In experiments in which it was required to avoid gas circulation due to the presence of a tangential inlet (Fig. 1), an aluminum ring with an outer diameter of 11.2 cm and an inner diameter of 11 cm was introduced into the reactor perpendicular to the gas flow (see also paragraph 5 of Chapter 3). Fig. 1. Block diagram of the experimental setup, end view (a) and side view (b). The line along which the 4D spectral survey was carried out is indicated in red. The width of this line is about 1 mm. (1) reactor, (2) solenoid valve, (3) buffer volume, (4) gas cylinder, (5) hyperspectrometer, (6) digital video camera, (7) folding mirror, (8) internal asbestos insulation, (9) heater, (10) external asbestos insulation, (11) optical window, (12) pressure sensor, (13) ADC converter and computer for receiving and accumulating data, (14) millivoltmeter for taking thermocouple readings, (15) aluminum ring to prevent gas circulation, (16) Pd spiral, (17) Wheatstone bridge. The node for tangential gas injection into the reactor is highlighted in the blue circle. The temperature measurement accuracy was 0.3 K. The evacuated and heated to the required temperature reactor was quickly filled with a gas mixture from the high-pressure buffer volume to the required pressure. An electromagnetic valve was used to open and close the gas lines. Due to the sharp pressure drop in the buffer volume and in the reactor, a gas vortex arises in the reactor, leading to a reduction in the time for establishing a uniform temperature distribution. To prevent gas circulation, an aluminum ring was introduced into the reactor perpendicular to the gas flow, as described in the last paragraph of Chapter 3. The pressure during the filling and combustion process was recorded using a "Karat-DI" tensoresistive sensor, a signal from which was fed through an ADC to a computer. Pd wire (in a number of experiments, Pt wire) 80 mm long and 0.3 mm in diameter in the form of a spiral was placed into the reactor. This wire was used to initiate the ignition of the combustible mixture. Also, the wire was connected as a shoulder of the Wheatstone bridge, which allowed to control its average temperature. Before each experiment, the reactor was evacuated to 0.1 Torr. The total pressure in the reactor and the pressure in the buffer volume were monitored with a pressure gauge. We used chemically pure gases and 99.85% Pd. The registration of the FF ignition and propagation was carried out through the optical window with VID-IK3 hyperspectrometers (see Chapters 2, 4), BIK, as well as a color high-speed film camera Casio Exilim F1 Pro (frame rate -1200 s-1 at a resolution of 336x96 pixels, 600 frames per second at a resolution of 432x192 pixels or 300 frames per second at a resolution of 512x384 pixels) or PHANTOM (frame rate - 4000 s-1 at a resolution of 1300x800 pixels). The obtained data were written into the computer memory, and then they were processed. We used hyperspectrometers of both the visible and near infrared range (400-970 nm VID-IK3 [5]) and the NIK hyperspectrometer in the wavelength range 970-1700 nm. Optical schemes of VID-IK3 and NIK hyperspectrometers are given in Chapters 2 and 4. It is worth reminding that the optical system of the NIK hyperspectrometer is similar to the optical system of the VID-IK3 hyperspectrometer, only a diffraction grating is used as a spectral splitter. A detailed description of the structure of the hyperspectrometers used and methods for studying combustion and explosion processes, together with the use of high-speed color filming, can be found in Chapter 2. Results and discussion The typical results of simultaneous recording of the pressure change (a) and the change in the resistance Pd of the wire (b, c) upon ignition of a mixture of 40% H2 air at 128 °C at P0 = 1 atm is shown in fig. 2a. As seen from Fig. 2a, the total pressure in the reactor reaches 1 atm before ignition, i.e. ignition occurs after the completion of the gas injection. Since palladium wire does not heat up uniformly due to heat dissipation at the soldering points, the dependence of resistance on time is somewhat more inertial than the pressure curve. The vertical segment of this dependence corresponds to the change in resistance at the moment of ignition. Temperature calibration was performed by changing the temperature of the reactor. However, the temperature measured by means of a Pd wire is the lower limit of the actual temperature of the ignition site [21]. The main result of the experiment is that the temperature of the reactor upon ignition of a mixture of 40% H2 - air above Pd (108 °C, 1 atm) is at least ~ 160°C less than above the surface of Pt (260 °C, 1 atm, 40% H2 - air) [21]. It should be noted that the heating recorded on a Pd wire (360 °C, Fig. 2a) is insufficient for thermal initiation of the ignition of a 40% H2 - air mixture [21]. Thus, the contribution of surface catalytic reactions to the direct initiation of hydrogen combustion over palladium, in contrast to platinum is very noticeable. It is worth reminding that the role of catalytic processes on a Pt surface is only to heat the surface to the ignition temperature. The role of the emission of active centers from the surface is insignificant. The spatial development of ignition and flame propagation in a 40% mixture of H2 - air and (80% H2 + 20% CH4)stoichiome + air was studied over a Pd wire (Fig. 2b, c). In the same way as in the case of Pt, Pd is heated before and after ignition due to catalytic reactions on the Pd surface. It can be seen that in the presence of a Pd wire, the cellular structure of the flame front is not observed in comparison with the results obtained on the Pt surface [22]. This is due to the greater stability of PdO in comparison with PtO2, which decomposes even at 500 °C and is a very unstable compound [22]. Fig. 2. a) Simultaneous registration of the heating and average temperature of the Pd coil upon initiation of the ignition of a mixture of 40% H2 with air with palladium. T0 = 128 0C, P0 = 1.08 atm; b) a sequence of video frames of the initiated ignition of a mixture of 40% H2 with air. T0 = 120 0C, P0 = 1.25 atm, 600 frames/s; c) a sequence of video frames of initiated ignition of a stoichiometric mixture (80% H2 + 20% CH4) stoichiome + air. T0 = 190 0C, P0 = 1.17 atm, 600 frames/s. The temperature dependence of the hydrogen concentration at the flammable limit was determined experimentally to reveal the contribution of surface reactions (including those responsible for heating the Pd wire). The ignition limits of stoichiometric mixtures 6-40% H2 + air (indicated by crosses) and (20-60% H2 + 80-40% CH4) stoichiome + air are shown in Fig. 3a. As you can see, the Pd wire ignites a mixture of 40% H2 - air in the reactor, which is only heated to 70 °C. For comparison, ignition of the same Pt mixture with a wire requires heating to 260 °C [21]. In addition, as can be seen from the figure, the minimum H2 concentration at the limit is approximately 5%, which is very close to the concentration limit of H2 ignition at atmospheric pressure when initiated by a spark [23, 24]. This means that CH4 in H2 - CH4 - air mixtures reacts only in the gas phase and not on the Pd surface. It should be emphasized that also that Pt wire of the same size does not ignite any of the stoichiome + air mixtures (20-60% H2 + 80-40% CH4) at reactor temperatures up to 450 °C. On the contrary, the Pd wire ignites mixtures (H2 30-60% + 70-40% CH4)stoichiome + air (circles in Fig. 3a). However, the mixture (20% H2 + 80% CH4)stoichiome + air at temperatures up to 450 °C failed to ignite Pd with a wire, probably because the H2 concentration in the mixture (2.2%) turned out to be lower than the concentration limit of hydrogen ignition [23, 24]. The temperature dependence of the H2 fraction in combustible mixtures in Arrhenius coordinates is shown in Fig. 3b. As can be seen from the figure, this dependence can be approximated by a straight line (correlation coefficient 0.98). The data were processed using the Statistica 9 software package (Statsoft). We can conclude that from fig. 3b, the dependence for H2 - CH4 - air mixtures is determined only by the H2 fraction in the mixture. We limited ourselves to 40% H2 in the mixture, because after a further increase in the H2 content, the hydrogen oxidation reaction slows down [24]. For this reason, the value of the effective activation energy obtained below is only an estimate. Let's consider the nature of the resulting dependence. For a stoichiometric mixture 2H2 + O2, the lower flammable limit (marked with the subscript lim) at low pressures: 2k2 (O2)lim = k4 i.e. (O2)lim = 1/2 (H2) lim = k4/k2, where k4 – the rate constant of heterogeneous termination of active combustion centers (weakly dependent on temperature) and k2 is the activated rate constant of branching (16.7 kcal/mol [25]). Fig. 3. a) Experimental dependence of the ignition temperature at the concentration limit on the hydrogen content in the mixture, crosses refer to the hydrogen-air mixture; b) Dependence a) in Arrhenius coordinates. Thus, we obtain the Arrhenius dependence ln (H2) lim on 1/T with a positive slope. Obviously, the heterogeneous nature of the process on Pd significantly complicates the analysis. However, it can be assumed that in the catalytic oxidation of H2, the reaction rate depends mainly on the H2 concentration, which can be expressed for the steady state as the ratio of some two effective constants [25]. The experimental value of the effective activation energy of the process is E = 3.5 ± 1 kcal/mol, which is typical for surface processes [25]. It should be noted that the value of the effective activation energy is close to the activation energy of adsorption - desorption of hydrogen on Pd [26]. However, in order to ensure ignition, a cycle of reactions must occur in which branching occurs [24]. The activated (E = 16.7 kcal/mol [25]) homogeneous branching reaction H + O2 → O + OH is the slowest elementary reaction of the cycle. Therefore, the activation energy of branching should determine the temperature dependence of the overall process, as it happens for experiments with metallic Pt ([21], see also Chapter 5). This means that in the case of Pd, branching can be of a heterogeneous nature, because the effective activation energy is close to ~ 3.5 kcal/mol. The obtained approximate value of E along with the results presented in Fig. 3a, b, can be used in practical application to assess the flammability of mixtures H2 - CH4 - air in the presence of metallic palladium. It can be seen from fig. 2b and 2c that, upon initiation of Pd by a wire, combustion is accompanied by an orange glow for both hydrogen and hydrogen-methane mixtures. While this glow propagates for the latter mixture nonuniformly and independently on the spherical flame front. An attempt was made to establish the nature of this glow using the hyperspectral method. The optical and IR emission spectra of a flame of hydrogen and a mixture (80% H2 + 20% CH4) stoichiome + air, experiments with metallic Pt [21] is shown in fig. 4a, b, c. This means that in the case of Pd, branching can be of a heterogeneous nature, because the effective activation energy is close to ~ 3.5 kcal/mol. It can be seen from fig. 3b, c that, upon initiation of Pd by a wire, combustion is accompanied by an orange glow for both hydrogen and hydrogen-methane mixtures, while for the latter mixture, this glow propagates nonuniformly and independently on the spherical flame front. An attempt was made to establish the nature of this glow using the hyperspectral method. The optical and IR spectra of radiation from a flame of hydrogen and a mixture (80% H2 + 20% CH4) stoichiome + air, recorded along a vertical line along the diameter of the optical window (red line, Fig.1a) is shown in fig. 4a, b, c. Let us preliminarily point out that a hydrogen flame at low pressures is practically invisible, since its radiation is mainly due to the radiation of hydroxyl radicals ОН А2–X2 in the ultraviolet region at 306 nm [24]. The combustion spectrum of a stoichiometric mixture of pentane with air + 10% CO2 is shown. This spectrum contains intense lines of atoms of alkali metals sodium (581 nm) and potassium (755 nm), inherent in all hot flames [24] and water vapor bands of water vapor bands in the range 900-970 nm [27, 28]. In the IR spectrum, broad bands of water are observed between λ = 1300 nm and 1600 nm. A relatively narrow band of the OH* radical is recorded at about 1400 nm [29]. It should be noted that the spectra presented in [28] and in this work were recorded in the same reactor using the same hyperspectral technique. It follows from Figs 4a, b that the main features of the spectra of the flame of 40% H2 + air, initiated by palladium in the visible region, in comparison with the optical spectra of the emission of a hydrogen flame initiated by a platinum wire and a spark discharge ([28], Fig. 13 a) are: a) the absence of a system of emission bands in the range of 570 - 650 nm, referred to in [30] as H2O*. This may be because we used in this work an optical window made of leucosapphire rather than quartz, as in [28]. Leucosapphire, unlike quartz, does not contain active surface OH groups. Thus, the emergence of water bands in the region of 570 - 650 nm may be due to the adsorboluminescence of water on quartz. Such a process is impossible on leucosapphire. b) increased intensity of water bands in the region 900 - 970 nm in comparison with the intensities of lines of alkali metals. This indicates the emergence of an additional source of excited H2O molecules. Earlier, we showed [31] that the catalytic activity of the palladium surface in the hydrogen combustion reaction is higher than that of the platinum surface. In other words, there is a fast catalytic reaction of hydrogen oxidation on the hot Pd surface along with the initiation of gas combustion. This reaction can lead to the formation of an additional amount of excited water molecules. We summarize the results obtained in this section. It has been shown experimentally that the ignition temperature of a 40% H2 - air mixture over metallic Pd (70 °C, 1 atm) is ~ 200 °C lower than over the Pt surface (260 °C, 1 atm). In addition, the Pd wire initiates the ignition of mixtures (H2 for 30-60% + 70-40% CH4) with stoichiom + air; Pt wires of the same size cannot ignite these mixtures up to 450 °C. This means that Pd wire is more efficient than Pt wire. It was shown that the cellular structure of the flame front during ignition on a Pd wire is not observed in comparison with the results obtained on the Pt surface. Therefore, Pd is more applicable for hydrogen recombiners in nuclear power plants, since catalytic particles do not appear in the gas phase, as is the case when using Pt. The experimental value of the effective activation energy of the process is estimated as 3.5 ± 1 kcal/mol, which is typical for the surface process. This indicates a significant role of the dark reaction of the consumption of H2 and O2 on Pd, which is observed directly at low pressures. The presence of this reaction reduces the likelihood of an accidental explosion compared to Pt. It was found that in the presence of leucosapphire the system of H2O* emission bands in the region of 570 - 650 nm is absent. A possible explanation of this phenomenon is given. An explanation is proposed for the appearance of an additional source of excited water molecules emitting in the range of 900-970 nm. Fig. 4. a) - The emission spectrum of combustion of a mixture of 40% H2 + air, hyperspectrometer VID-IK-3, 30 frames/s; b) - Time dependence of the intensity of the emission spectra of the combustion of the mixture (80% H2 + 20% CH4)stoichiome + air, hyperspectrometer VID-IK-3, 300 frames/s; c) - Time dependence of the intensity of the emission spectra of the combustion of a mixture of 40% H2 + air, NIK hyperspectrometer, 300 frames/s. The emission spectra of combustion of a 40% H2 + air mixture in this spectral region do not differ qualitatively. As the spectrum number increases, time increases. §2. Ignition of mixtures hydrogen - hydrocarbon (C1-C6) - air above the surface of palladium at pressures of 1 ÷ 2 atm Hydrogen-hydrocarbon blended fuels are gaining attention as alternative fuels for energy production for two main reasons. The first reason relates to the addition of hydrogen to methane in order to improve performance, expand the range of use and reduce pollutant emissions when using lean mixtures in stationary [32] and mobile [33] systems. The second reason is associated with the prospect of using hydrogen in fuel cells and devices using combustion [34] in the development of hydrogen power engineering. It is clear that the use of premixed combustion is one of the promising methods to meet the stringent requirements for limiting unwanted NOx emissions from energy production incl. in internal combustion engines [35, 36]. The reduction in combustion temperature that is achieved using lean mixtures can significantly reduce NOx emissions, but more research is needed to address issues that are hindering widespread adoption of this technology. For example, lower combustion temperatures of premixed mixtures can lead to suppression of oxidation reactions, an increase in unwanted CO emissions and a deterioration in the stability of combustion chambers [37]. Natural gases, which are primarily methane, can contain from several percent to 18% of other gases, depending on the field [33, 37]. These impurities are usually C2 and C3 hydrocarbons - ethane and propane. Changes in the composition of natural gas can cause changes in combustion chemistry and NOx emissions. Catalytic ignition can show its main advantages when using lean fuels [38], since the use of a catalyst can initiate combustion of leaner mixtures than a conventional spark plug. At the same time, there are no extinguishing effects during electrical breakdown, as when using spark plug electrodes, the ignition site can be placed in an arbitrary place in the combustion chamber. Catalytic ignition does not require electrodes and an ignition system. Therefore, electrode erosion cannot take place, therefore the operating time of the catalytic ignition system will be significantly longer than for a device using a spark plug. There is a need to develop catalysts that provide oxidation at low temperatures (<300 °C) for a new generation of highly efficient internal combustion engines [39]. The problems of catalyst stability were studied, for example, in [40]. Palladium alumina catalyst is unstable during methane conversion. The addition of platinum to these catalysts provides significantly higher stability. On the other hand, Pd-Pt catalysts are reversibly poisoned by water vapor, i.e., after the water is removed, their activity is restored. At the same time, a decrease in the activity of Pd/Al2O3 is not observed for all fuels, as a result of the combustion of which water vapor is formed. Hydrogen is very stably oxidized to Pd/Al2O3. In this case, the degree of conversion of ethane decreases slightly with time, but not to the extent that is observed during the conversion of methane. The noble metals Pt and Pd affect the flammability of fuels based on methane and hydrogen in different ways. It was shown that the ignition temperature of a mixture of 40% H2 - air on palladium (70 °C, 1 atm) is ~ 200 °C lower than on the Pt surface (260 °C, 1 atm) [31, 41]. In addition, Pd ignites stoichiometric mixtures (30 ÷ 60% H2 + 70 ÷ 40% CH4) + air ( = 1,  is the fraction of fuel mixed with air: H2 + 0.5 (O2 + 3.76N2)). Metallic Pt does not ignite these mixtures up to 450 °C, i.e. metallic palladium is more efficient than Pt. It was also shown that the cellular structure of the flame front during ignition on the palladium surface is not observed in comparison with the results obtained on the Pt surface. Thus, palladium appears to be more suitable for use in hydrogen recombiners at nuclear power plants, because catalytic particles as ignition centers arising from the thermal decomposition of labile oxide (PtO2) cannot appear in the gas phase [42]. The experimental value of the effective activation energy for the ignition of hydrogen-air mixtures over the surface of palladium metal is estimated as 3.5 ± 1 kcal/mol, which is typical for surface processes. This indicates a significant role of the dark reaction of hydrogen oxidation observed above the palladium surface at low pressures [31]. Obviously, this reaction behavior reduces the probability of an accidental explosion in comparison with Pt. The paragraph is devoted to establishing the combustion features of fuels containing hydrogen - hydrocarbon mixtures (C1 - C6, namely CH4, C2H6, C3H8, C4H10, C5H12, C6H14) with  = 0.6 ÷ 1.2 above the palladium surface at a total pressure of 1 ÷ 2 atm. The purpose of this section was both to establish the features of the propagation of the flame front in mixtures and the temperature dependence of the ignition limit over the palladium surface. Experimental technique The experiments were performed with gas mixtures of 30% hydrocarbon (C1 - C6) + 70% H2 + air at  = 0.6–1.2, and a pressure of 1–2 atm. In the experiments, we used a heated cylindrical stainless steel reactor 25 cm long and 14 cm in diameter, equipped with an optical sapphire window in one of the ends (Fig. 5). Fig. 5. Diagram of the experimental setup: 1 - stainless steel reactor, 2 - electric heater, 3 - thermal insulation, 4 - valves, 5 - high pressure buffer volume, 6 - optical window, 7 - digital movie camera, 8 - palladium coil, 9 - manometer, 10 - registration system, 11 - digital millivoltmeter, 12 - Wheatstone bridge, 13 - hyperspectrometer 400-1000 nm, 14 - hyperspectrometer 1000-1700 nm. The temperature measurement accuracy was 0.3 K. Ignition registration and flame propagation was carried out using a high-speed color Casio Exilim F1 Pro camera (frame rate 600 s-1). The video file was saved in the computer memory, and then it was processed frame by frame [42]. The evacuated and heated reactor was quickly filled with the test gas mixture from the buffer volume to the required pressure. The flammability limit was determined as the average of two close temperatures at a given pressure. At a higher temperature, ignition occurred, at a lower temperature, there was no ignition. An electromagnetic valve was used to quickly open and close gas lines. A capacitive pressure gauge was used to record the pressure during gas inlet and combustion. A palladium coil made of wire 80 mm long and 0.3 mm in diameter was placed in the reactor. This coil was used both to initiate the ignition of the combustible mixture and to estimate the heating value of the wire as a shoulder of the bridge circuit. Before each experiment, the reactor was evacuated to 0.01 Torr. After each ignition, the reactor was evacuated for 1.5 hours to remove most of the water vapor. The total pressure in the reactor was recorded with a vacuum gauge, the pressure in the buffer volume was monitored with a reference manometer. There were used chemically pure gases and Pd 99.85%. Results and discussion Typical sequences of video footage of the spatial development of ignition initiated by a palladium wire and flame propagation of preliminarily prepared mixtures of 30% CH4 + 70% H2 + air and 30% C2H6 + 70% H2 + air at  = 0.6 ÷ 0.9, and a pressure of 1.75 atm are presented in fig. 6 a, b. In the same way as in the case of Pt [31, 41, 42], palladium wire is heated before and after ignition due to catalytic reactions on the surface of palladium metal. As can be seen from the figure, in lean mixtures, a cellular structure of the flame front is observed. The thermal diffusion instability of a fuel-poor flame leads to the appearance of cellular structures [43, 44]. Fig. 6 a) High-speed registration of initiation of combustion with a palladium coil and flame propagation in mixtures a) 70% CH4 % + 30% H2 + air,  = 0.7, P = 1.75 atm, 270 0C, 600 s-1; b) 30% C2H6 + 70% H2 + air,  = 0.6, P = 1.75 atm, 390 0C, 300 s-1. The numbers on each frame correspond to the sequential number of the video image during the ignition. The results of simultaneous recording of pressure changes and changes in the resistance of a palladium wire (proportional to self-heating) during ignition at P = 1.75 atm of mixtures a) 30% C2H6 + 70% H2 + air,  = 0.6, 39 0C and b) 30 % C6H14 + 70% H2 + air,  = 1.2, 36 0C in fig. 7. Since the Pd foil is heated nonuniformly [42], the time dependence of the resistance, which represents the relative temperature, is somewhat “delayed” in comparison with filming. A kink in this dependence corresponds to a change in resistance at the moment of ignition. Obviously, the temperature value measured with the palladium resistance is the lower limit of the true temperature of the center of ignition, which initiates the combustion of the gas, since it takes a certain time to heat the entire palladium coil. The dashed curve in Fig. 7 shows the change in the resistance of the palladium coil in when a mixture of 30% + 70% H2 + air is injected into the reactor. Thus, the first maximum in the dependence of the resistance on time during combustion refers not to the ignition process, but to the interaction of hydrogen with the palladium surface. Fig. 7 Simultaneous recording of pressure and resistance changes of a palladium coil during ignition a) 30% C2H6 + 70% H2 + air, = 0.6, P = 1.75 atm, 39 0C b) 30% C6H14 + 70% H2 + air,  = 1.2, P = 1.75 atm, 36 0C. The ignition delay is indicated in the figure. The dotted line in Fig. 7b - change in the resistance of the palladium coil when a mixture of 30% Ar + 70% H2 + air is injected into the reactor up to 1.75 atm. As can be seen from Fig. 7, the total pressure in the reactor reaches 1.75 atm before the moment of ignition, i.e. ignition occurs after completion of the gas injection in cases a) and b). The ignition delay period  for a mixture of 30% C2H6 + 70% H2 + air is ~ 2 s;  is 8 s at 24 0C and P = 1 atm. Thus, the combustion of this fuel can be initiated by a palladium surface at room temperature, without external physical stimulation. In this case, the mixture of 30% C2H6 + 70% H2 + air with  = 0.6 has the lowest temperature of the ignition limit: 24 0C at 1 atm. It has been shown that mixtures of 30% CH4 + 70% H2 + air and 30% C3H8 + 70% H2 + air exhibit two temperature limit of ignition. A higher value can be achieved with a “bottom” approach in temperature, a lower one is achieved when the reactor is treated with ignitions. The foregoing is illustrated by the dependences of the flammability of mixtures 70% CH4 + 30% H2 + air (Fig.8a) and 30% C3H8 + 70% H2 + air (Fig.8b) on the number of successive ignitions at P = 1.75 atm. As can be seen from the figure, the ignition temperature in the "fresh" reactor (approach from below: there were no ignitions in the reactor before) is ~ 315 0C at  = 0.9. At this temperature, mixtures with  <0.9 at the same pressure in the "fresh" reactor do not ignite. However, during treatment with flames, the flammable limit temperature decreases markedly and amounts to 274 0C at = 0.7 after 7 fires. It was shown that the process is reversible: after treating the reactor with oxygen (1 atm О2 for 2 min), the ignition limit returns to its initial value of ~ 315 0C. Similar dependences were also observed in the case of ignition of a mixture of 30% C3H8 + 70% H2 + air (Fig.8b). The ignition limit temperature in the "fresh" reactor is ~ 108 0C at  = 1. During subsequent ignitions of the same compound, the temperature limit is reduced to 30 °C after 7 fires. The process is also reversible: after the reactor is treated with oxygen (1 atm O2 for 2 min), the ignition limit returns to its initial value of ~ 108 0C. Fig. 8. Dependence of the flammability of mixtures a) 30% CH4 + 70% H2 + air, b) 30% C3H8 + 70% H2 + air  = 1, from the number of consecutive ignitions. P = 1.75 atm. Filled circles are ignition, empty circles are non-flammable. Thus, the observed phenomenon is hysteresis; it can be caused by reversible changes in the palladium surface and, consequently, in the activity of the catalyst. It should be noted that reversible changes in the palladium surface are observed only for fuels H2 - methane and H2 - propane. For the other mixtures studied, the hysteresis effect is absent. This means that the flammability limit over palladium is also determined by the peculiarities of the kinetic mechanism of hydrocarbon oxidation. The temperatures at the ignition level for these mixtures at a total pressure of 1.75 atm are presented in Table 1. Table 1 Flammable limits for mixtures 70% H2 + 30% (C2, C4-C6) at 1.75 atm Fuel 30%C2H6 +70%H2  30%C4H10 +70%H2  30%C5H12 +70%H2  30%C6H14 +70%H2  Temperature at the flammable limit, 0C 20 28 24 36 To estimate the effective activation energy of the gross reaction for mixtures that do not exhibit features associated with reversible changes in the activity of the catalyst, the temperature dependences of the ignition delay times were obtained. The experimental temperature dependences of the ignition delay periods in Arrhenius coordinates for the ignition of mixtures of 30% (C2H6, C4H10, C5H12, C6H14) + 70% H2 + air are shown in Fig. 9. As seen from Fig. 9, these dependences can be approximated by a straight line (correlation coefficient 0.98). The data were processed using the Statistica 9 software package (Statsoft). The experimental value of the effective activation energy of the gross processes obtained from fig. 9 is E = 2.4 ± 1 kcal/mol, which is characteristic of the surface process [25]. Fig. 9. Experimental temperature dependences of ignition delay periods in Arrhenius coordinates at P = 1.75. Solid circles - 30% C2H6 + 70% H2 + air,  = 0.6; Empty circles - 30% C4H10 + 70% H2 + air,  = 1.1; Triangles - 30% C5H12 + 70% H2 + air,  = 1.2. This value is very close to that obtained in the previous paragraph from the dependence of the H2 content in mixtures H2 - air and H2 + CH4 + air on temperature (3.5 ± 1 kcal/mol). Hence, we can conclude that the temperature dependence for mixtures of 30% (C2, C4, C5, C6) + 70% H2 + air is determined only by the content of H2 in the mixture, as shown for a mixture of H2 - air and H2 - CH4 - air in [41]. This may mean that the obtained estimates of the effective activation energy refer to the same process, probably the branching reaction [41]. This means that the branching reaction is of a heterogeneous nature. In addition, the lowest temperature of the ignition limit of a hydrogen - air mixture on the surface of palladium is ~ 700 0C for a mixture of 40% H2 + 60% air. Since the ignition limit temperature for mixtures of 70% hydrogen + 30% hydrocarbon (C2-C6) + air is ~ 400 lower (see Table 1), this indicates an important role of reactions involving hydrocarbon molecules on the palladium surface. The results of frame-by-frame processing of filming of Pd-initiated ignition of a mixture of 70% H2 + 30% C3H8 + air ( = 1, P0 = 1.73 atm) are shown in fig. 10. It can be seen from fig. 10 that when Pd is initiated by the wire until the flame touches the reactor walls (frames 10 - 19). In addition, a spatially inhomogeneous combustion of the combustible mixture occurs, then when the flame touches the rear wall of the reactor (which is closer to the Pd spiral). There is a sharp increase in the combustion intensity (frames 23 -26). It can be seen that after the completion of the combustion process in the volume, the afterburning of the combustible mixture on the palladium coil continues. An attempt was made to establish the nature of this glow using the hyperspectral method. The optical and IR emission spectra of the mixture 60% H2 + 40% C3H8 + air ( = 1), recorded along the vertical line along the diameter of the optical window are demonstrated in fig. 11 a, b, c. This corresponds approximately to the red line in Fig. 1a. Fig. 10. High-speed registration of the process of initiation of combustion with a palladium coil and flame propagation in a mixture of 70% C3H8 % + 30% H2 + air,  = 1, P = 1.75 atm, 35 0C, 600 s-1. The numbers on each frame correspond to the sequential number of the video image during the ignition. Fig. 11. a) Dependence of the intensity of the emission spectra of the combustion of a mixture of 60% H2 + 40% C3H8 + air ( = 1, P0 = 1.9 atm), hyperspectrometer VID-IK3, 70 frames/s; b) Dependence of the intensity of the emission spectra of the combustion of a mixture of 60% H2 + 40% C3H8 + air on time ( = 1, P0 = 1.9 atm), VID-IK3 hyperspectrometer, blue spectrum region, 70 frames/s, black vertical line limits the distortion region spectrum, located on the right and associated with a sharp increase in the sensitivity of the device; c) - Time dependence of the intensity of the emission spectra of the combustion of a mixture of 40% H2 + air, BIK hyperspectrometer, 300 frames/s. As the spectrum number increases, time increases. The spectrum in Fig. 11a, there are intense lines of atoms of alkali metals sodium (581 nm) and potassium (755 nm) inherent in all hot flames [24] and water vapor bands of the water vapor band in the range 900–970 nm [27, 28]. In the IR spectrum, broad bands of water are observed between λ = 1300 nm and 1600 nm. A relatively narrow band of the OH* radical is recorded at about 1400 nm [29] (Fig. 11c). The task was to establish the features of the appearance in time and space of active intermediate CH particles (431 nm [24],) and 590 nm for the line of Na atoms, 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 [24], which is carried out at a flame temperature not lower than 1200 0C [45]. It is seen from fig. 11b that at the beginning of the process (spectrum 25), a blue CH emission is recorded (the spectral band at 431 nm is not resolved due to collisional broadening at an initial pressure of 1.9 atm [24]). The maximum of the Na line is recorded much later (spectrum 27). Let us pay attention to the fact that the observed separation of the CH and Na radiation in time is consistent with the results obtained in [46] when a methane-air flame passes through a small hole in a flat obstacle. When the gas flow is turbulized. It can be seen from fig. 11 (corresponding to Fig. 5 from [46]) 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. When registering the radiation of a propagating flame using glass filters in the wavelength range of 435 nm, 520 nm, and 590 nm, it is clearly seen that both C2 radicals in recorded quantities and the main heat release in the process of Na luminescence are observed after the first obstacle, i.e. after turbulization of the gas flow (see Chapter 6). We point out that earlier [47] we have experimentally and theoretically established that when the methane-air mixture flame touches the end of a cylindrical reactor (see Chapter 3), there is turbulence of the combustion front caused by the occurrence of hydrodynamic instability. Thus, the time separation of the CH and Na emission bands observed in this work is due to the hydrodynamic instability of the flame when it touches the end of the cylindrical reactor. This result means that the used experimental technique makes it possible to separate the “cold” and “hot” flames in time and space in one experiment. Let us point out the increased intensity of the water bands in the region 900 - 970 nm in comparison with the intensities of the lines of alkali metals, while, according to Fig. 11a, water bands are observed at the end of the combustion process, when the sodium line is practically not observed in the spectrum (Fig. 11a, spectrum 5). As indicated in the previous paragraph, this radiation can be associated with the catalytic oxidation of unburned hydrogen and, possibly, propane on the hot surface of Pd. We briefly summarize the results obtained in this section. It has been experimentally shown that the temperature of the ignition limit above the palladium surface at P = 1.75 atm, measured by approaching from the bottom up in temperature, for mixtures of 30% methane + 70% hydrogen + air ( = 0.9, T = 317 °C) and 30% propane + 70 % H2 + air ( = 1, T = 106 °C) noticeably decreases after subsequent ignitions to T = 270 °C for H2 - CH4 - air and to T = 32 °C for a mixture H2 - C3H8 - air. The flammability limit returns to the initial value after the reactor is treated with oxygen or air, i.e. there is a hysteresis phenomenon. The flammability limit of mixtures 30% (C2, C4, C5, C6) + 70% H2 + air ( = 0.6, 1.1, 1.2, 1.2, respectively) above the surface of metallic palladium is 25 ÷ 35 0C at P = 1.75; there is no hysteresis effect. It was found that a lean mixture of 30% C2H6 + 70% H2 + air ( = 0.6) has the lowest ignition limit temperature: 24 °C at 1 atm. The estimate of the effective activation energy for the ignition of mixtures over Pd is ~ 2.4 ± 1 kcal/mol, which is typical for the surface process. It is shown that the use of Pd makes it possible to ignite combustible 30% hydrocarbon + 70% H2 at 1–2 atm at the initial room temperature without using external energy sources. It was found that the separation of the CH and Na emission bands in time during the combustion of a mixture of 30% propane + 70% H2 + air ( = 1), found in this work, is due to the occurrence of hydrodynamic instability of the flame when it touches the end of the cylindrical reactor. §3. Ignition of hydrogen-oxygen and hydrogen-methane-oxygen mixtures with heated wires at low pressure Hydrogen is a renewable energy source for the future, combustion products of which do not pollute the environment. However, before the widespread use of hydrogen, the issues of explosion safety of production, transportation and storage of hydrogen must be resolved. Accidental ignition is one of the biggest hazards, as hydrogen has much wider flammability limits than conventional fuels [48]. By analyzing the risk of accidental ignition in the event of an uncontrolled hydrogen leak, as can occur during a vehicle collision or pipeline breakdown, the most likely ignition source is a hot surface. Therefore, it is important to be able to predict and thus prevent a situation in which ignition can occur when a flammable mixture is in contact with a hot surface. The use of hydrogen as a fuel requires the ability to ignite it predictably. The problem with refueling spark ignition engines with hydrogen is that the hydrogen-air mixture entering the combustion chamber can be ignited immediately upon contact with a hot surface, such as an intake valve. In diesel engines with direct fuel injection, the pre-ignition problem does not arise. However, hydrogen is difficult to ignite when compressed, an additional device is required such as a glow plug and some ignition helps, usually a glow plug [49]. Therefore, the design of both spark ignition and diesel engines should be based on the analysis of hot surface ignition information. Many experimental works [50] are devoted to the investigation of hydrogen ignition by a hot surface. Most measurements were carried out for gas mixtures at atmospheric pressure. Measurements of the surface temperature required to initiate the ignition of hydrogen (Tign) in air or oxygen at 1 atm, as a rule, were in the range from 640 °C [48] to 930 °C [51]. The value Tign ~ 70 °C was observed for a mixture of 40% hydrogen -60% air above the palladium surface, i.e. with a significant catalytic effect [52, 53, see § 1]. In addition, in some works, there is practically no dependence of Tign on the H2 content [50, 54]. However, in other works, for example, [55], this dependence is observed. We showed earlier that Tign at 40 Torr on a heated palladium foil is ~ 1000 lower than on a heated platinum foil [31]. For thermal ignition, it was found that at pressures up to 180 Torr at 288 °C, the catalytic activity of the palladium surface is higher than that of the Pt surface [31]. We showed in [41] that the dependence of Tign on the hydrogen content for H2 - CH4 - air mixtures at the reactor temperature is determined only by the H2 content in the mixture. Thus the dependence of Tign on [H2] exists. The wide range of measured values also shows that the hot surface temperature required for ignition is not only a property of the gas, but also depends on a number of factors such as mixture composition and pressure, the nature and condition of the surface as determined by the surface history, etc. This section is devoted to the establishment of the regularities of the ignition of mixtures of hydrogen - oxygen and hydrogen - methane - oxygen at low pressures with heated wires of Pd, Pt, nichrome and kanthal (fechral), in order to detect the dependence of the ignition temperature on the fuel concentration and assess the contribution of the catalytic properties of the materials used. Fig. 12. Experimental setup for investigation of initiated ignition. 1 - quartz cylinder 12 cm high and 8 cm in diameter, 2-quartz vacuum lid, 3 - CsI window, 4 - to the pump, 5 - Pt or Pd spiral, 6 - heater, 7 - IR chamber Flir 60, 8 - rotary mirror. The video file was stored in the computer's memory and processed frame-by-frame. The evacuated reactor was filled with a gas mixture from the buffer volume to the required pressure. The wires were quickly heated to initiate ignition of the gas mixture. Before each experiment, the reactor was evacuated to 10-2 Torr. The total pressure in the reactor was monitored with a VIT-2 vacuum gauge, the pressure in the buffer volume was recorded with a model manometer. We used chemically pure gases, 99.99% Pt and 98.5% Pd, commercial nichrome and kanthal. The ignition temperatures of the mixtures under study with heated wires were measured. It was previously shown that replacing methane with nitrogen does not significantly affect the Tign value, in agreement with [41]. Typical results of IR video recording at a total initial pressure of 40 Torr are shown in Fig. 13. The temperature presentation by the Flir 60 camera lags somewhat behind in time due to the inertia of the temperature sensor; therefore, the maximum temperature Texp = 306 °C (shown in the upper left corner of each frame) in the 3rd and 4th frames of Fig. 13 corresponds to the temperature of the wire immediately before ignition. The temperature on the 5th frame (Texp = 380 °C) corresponds to the temperature of the wire heated by the flame. This temperature is underestimated because the ignition is fast, but the temperature during the ignition delay can be measured accurately and reproducibly. The emissivity set on the device in these experiments was 0.95 (close to a black body). Fig. 13. IR - filming of initiated ignition on a heated Pd wire. 60 frames/s, T0 = 20 °C. A mixture (60% H2 - 40% methane) stoich - oxygen. P0 = 40 Torr. The time in seconds is given from the bottom right of each frame. The red triangle shows the maximum temperature in the rectangle. The blue triangle shows the minimum temperature in the rectangle, the cross indicates the temperature at the point. Emissivity is set to 0.95 (lower left corner of the frame). The recommended emissivity in the range of 8-14 mm for polished palladium wire is ~0.07 (http://www.zaoeuromix.ru/) and 0.07–0.1 (http://www.thermalinfo.ru/) for Pt wire. The value 0.1 for Pt and Pd wires, 0.1 for kanthal and 0.15 for nichrome wire was accepted (http://www.thermalinfo.ru/). The actual ignition temperature on the wire immediately before the explosion at 40 Torr can be estimated from the Stefan-Boltzmann law: 0.95T 4exp ≈0.07T 4 ign. The results are shown in Fig. 14. As can be seen from the Figure, the dependence of Tign on [H2] takes place; Pd shows the highest catalytic activity (Tign values are the lowest) according to [31]. Fig. 14. Experimental dependences of Tign on wires on the H2 content in the mixture, 1 - Pd, 2 - Pt, 3 - nichrome. 4 - kanthal, 5 - crosses - calculations with the values k0cat = 4.1012 exp (-3500/T) cm3/(mol/s) for initiation by the heated Pd surface (lower curve), and k0cat = 2.1015exp (-5000/T) cm3/(mol/s) for Pt (upper curve). The purpose of the numerical calculation was to establish the limiting ignition conditions in temperature at a wall temperature of 300 K, depending on the H2 content in the combustible mixture. The model reflects the experimental fact that at the initial stages of the combustion process, the development of the primary heating source leads to the propagation of the flame front [42] with a velocity U. The surface chain initiation reaction (generation of active combustion centers) was also taken into account, as well as the peculiarities of the mechanism of branched-chain hydrogen oxidation. The reduced kinetic mechanism of hydrogen oxidation in the region of the upper ignition limit can be represented as follows [42]: Н2 + О2 → 2 ОН (0) k0 = 1.92.1014exp(-21890/T) cm3/(mol/s) ОН + Н2 → Н2О + Н (1) k1 = 4.63.1012exp(-2100/T) cm3/(mol/s) Н + О2 → ОН + О (2) k2 = 1.99.1014exp(-8460/T) cm3/(mol/s) О + Н2 → ОН + Н (3) k3 = 1.88.1014exp(-6897/T) cm3/(mol/s) Н + О2 + M → HО2 + M (4) k4 = 6.47.1015(T/298)-0.8 cm6/(mol2/s) Н + HО2 → 2 ОН (5) k5 = 1.69.1014exp(-440/T) cm3/(mol/s) Н2+ HО2 → H2О2 + H (6) k6 = 3.1013exp(-14400/T) cm3/(mol/s) ОН + О → Н + О2 (7) k7 = 9.29.1012 cm3/(mol/s) O + HО2 → ОН + О2 (8) k8 = 3.25.1013 cm3/(mol/s) OH + HО2 → Н2О + О2 (9) k9 = 2.89.1013exp(250/T) cm3/(mol/s) 2 HО2 → Н2О2 + О2 (10) k10 =2.1012 cm3/(mol/s) H2О2 + M → 2 ОН + M (11) k11 = 1.3.1017 exp(-22750/T) cm6/(mol2/s) H + H + M → H2 + M (12) k12 = 2.2.1015 cm6/(mol2/s) A two-dimensional problem was studied. The characteristic scales of the process were chosen as follows: t0= 1/( k10 [H2]0), x0 = (D3/ k10[H2]0)1/2 , U0 = x0/t0 = (D3 k10[H2]0)1/2 (scales of time, length and velocity, respectively, D3 is the diffusion coefficient of H2). We define dimensionless variables and parameters  = t/t0, xx0,  = y/y0, U/U0, Yi = [concentration of the ith component]/[ H2]0 , i= Di/D3 (Di is the diffusion coefficient of the ith component). The velocity and coordinates of the propagating flame front were determined through D3:  = U/(D3 k10[H2]0)1/2,  = x/(D3/ k10[H2]0)1/2 ,  = y/(D3/ k10[H2]0)1/2. Here U, x and y are the corresponding dimensional quantities, k10 is the preexponential factor of reaction (1). Diffusion coefficients (Di/D3, i=0-6) 0, 1, 2, 3 =1, 4, 5, 6 in a hydrogen-oxygen mixture refer to ОН, O, H, H2, O2, HO2, H2O2, respectively. The system of equations for the above reaction mechanism takes the form (m, n = 0 ÷ 6 refer to the reacting particles OH, O, H, H2, O2, HO2, H2O2, respectively): ∂Yi/∂ = i (∂2Yi/∂2 +∂2Yi/∂2) +∑ knYmYn - ∑knYmYn m≠i,n m=i,n ∂T/∂ = 7 (∂2T/∂2 + ∂2T/∂2) + 1/(Cp)∑ QnknYmYn (I) m,n The rate of heat release in the component of the reaction chain [42] is given by the last equation of system (I). Here Cp is the average specific heat at constant pressure, 7 ≈ 3 is the thermal diffusivity for near-stoichiometric mixtures, and 7 ≈ 4 for lean mixtures [42], T is the temperature (K),  is the mixture density g/cm3, taken from [56]. Specific heat Qi and diffusion coefficients were taken from [42]. f is the molar fraction of the initial component. The reaction-diffusion equation for O atoms is shown below as an example: ∂Y1/∂ = 1 (∂2Y1/∂2 +∂2Y1/∂2 ) +k2Y2Y4 – k3Y1Y3 –k7Y0Y1 – k8Y1Y5 The solutions of system (I) satisfy the following boundary conditions for flame propagation from right to left (L is the dimensionless distance between the reactor axis and the reactor wall, symmetry conditions are established on the axis): Yi  0 (i≠34), Т()  300K ,     Y3() fH2, Y4() fO2,   ; Y3()/ 0, Y4()/  0,    (Yi ()/) L= 0; Т(L) = 3000 K When solving system (I), the initial fronts of the initial components Y3 [H2] and Y4 [O2] in the coordinate zero time were determined according to the composition of the mixture. For the numerical solution, a finite-difference approximation of system (I) on a uniform grid of Cartesian coordinates was used. The two-step implicit scheme provided the second order of approximation of system (I) in spatial and temporal variables [57]. The distribution of the initial components in sections parallel to the central plane of the channel was approximated as follows Y3 = ½ -1/(arctg(), Y4 = ½ -1/(arctg(),the initial temperature front was defined as T = Tign exp (() 2/50) [42], where Tign is an a priori (trial) estimate. These initial fronts correspond essentially to the initiation of flame propagation by an external source. In the calculations, we used 500 partition points along the  and 70÷350 points of division in the сoordinate. The Laplace operator was approximated according to the "cross" scheme. The boundary conditions on the wall and the plane of symmetry were also approximated with a second order accuracy reactor, while the partial derivatives with respect to time were approximated by one-sided differences, providing first-order accuracy. which did not change the solution of the original problem with a further decrease in the step. Integration was performed according to an explicit scheme using the “predictor-corrector” procedure. The change in the concentration and temperature distribution was displayed on the display screen, which made it possible to control the calculated parameters during the counting and determine the time of its completion in the dialogue mode. In the course of integration, either the regime of propagation of all the fronts of the Yi concentrations at the same rate was achieved, or the process of chemical transformation was attenuated. It was believed that the traveling wave mode was achieved when the average value of the velocity did not change at a distance of 100 characteristic dimensions of the flame front, which was defined as the distance on the central plane of the channel where the dimensionless concentration of hydrogen atoms decreased by a factor of e (e = 2.71828 ...). If it was necessary to carry out long-term calculations (near the limits of flame propagation), the following procedure was used. Because the progressive wave with the values of the governing parameters used in the calculations occupied no more than 50 grid nodes, then when the wave approached the left boundary of the computational domain, when all changes in concentration and temperature occurred at 100 left grid nodes (there were only zero values on the right), the Yi values from the left half of the net was transferred to the right half and counting continued. The reliability of this calculation method was quantitatively verified in [58]. Obviously, to ensure ignition, a cycle of reactions must occur in which branching occurs (an increase in the number of active centers) [59]. To lower Tign, namely, the flammability limit, the branching rate must increase [59], for example, due to the implementation of an additional branching reaction. Under the conditions of our experiment, this step can be reaction (5), in which a relatively inactive HO2 radical is converted into active ОН, i.e. branching occurs additional to reaction (2). As shown in [59], taking into account reaction (5) makes it possible to explain the expansion of the ignition region in the presence of H atoms generated by an external source. In our case, the source of atoms is the chain initiation reaction (0), because k0 (a, E) increases in the presence of a hot catalyst. This rate is defined below as k0cat (a is the preexponent, E is the activation energy). The higher the catalytic activity of the metal, the greater the value of k0cat. The numerical experiment consisted in the fact that the value of the value k0cat (k0cat >> k0) for all experimental points of each curve in Fig. 14 was sought by means of paired linear regression analysis [60] with parameters and E, so that the calculated curve was the closest to the experimental one. The experimental points on each curve were the average of the experimental temperatures above which combustion occurred and below which there was no combustion. The calculated profiles of the chemical temperature of the components and the flame velocity U for one of the experimental points are shown in Fig. 15. In Fig. 15, the shade of green determines the spatial distribution of Yi concentrations. While the boundaries between the shades are isoconcentration (for the temperature distribution by isotherms), a darker color corresponds to a higher concentration (temperature) value within the range of variation between isoconcentration lines (isotherms). The interval between adjacent isoconcentration (isothermal) lines corresponds to a change in concentration (temperature) by 30%. In each of the given two-dimensional distributions, the top of the “frame” is the channel wall, and the bottom of the “frame” is the central plane. The calculation results are shown in Fig. 14 (crosses). As can be seen from Fig. 14, taking into account the elementary reaction H + HO2 → 2OH (5) allows one to explain the experimentally discovered dependence of Tign on the H2 concentration. It also allows one to obtain satisfactory agreement with experimental data at k0cat (a, E) values for catalytic materials: k0cat = 4.1012exp (-3500/T) cm3/(mol/s) for palladium and k0cat = 2.1015exp (-5000 / T) cm3/(mol/s) for platinum. Fig. 17. Calculated profiles of chemical components and temperature at a given point in time (when it is clear whether the reaction has started or not) above (a) and below (b) Tign. The top of each frame is the reactor wall; bottom - the axis of the reactor. The front of the flame moves from the center to both sides. P = 40 Torr, wall temperature - 300 K. a) Initial T = 680K, ignition occurs; b) Initial T = 670 K, no ignition. U - flame speed, arbitrary units. It should be noted that the activation energy of the gross process for a hot palladium surface (~ 7 kcal/mol) is higher than for a cold palladium surface (2.4 kcal/mol), while for a hot platinum surface (~ 10 kcal/mol). On the contrary, it is lower than for the cold platinum surface (19 ± 4 kcal/mol), which may be due to both the use of the reduced hydrogen combustion mechanism and errors in temperature measurement. Establishing the nature of this discrepancy requires further research. Conclusions for Chapter 7 It was experimentally established that the temperature of the ignition limit above the palladium surface at P = 1.75 atm, measured by approaching from the bottom up in temperature, for mixtures of 30% methane + 70% hydrogen + air ( = 0.9, T = 317 °C) and 30% propane + 70 % H2 + air ( = 1, T = 106 °C) noticeably decreases after subsequent ignitions to T = 270 °C for H2 - CH4 - air and to T = 32 °C for a mixture H2 - C3H8 - air. The flammability limit returns to the initial value after the reactor is treated with oxygen or air, i.e. there is a hysteresis phenomenon. The ignition limit of mixtures 30% (C2, C4, C5, C6) + 70% H2 + air ( = 0.6, 1.1, 1.2, 1.2, respectively) above the surface of metallic palladium is 25 ÷ 35 °C at P = 1.75. There is no hysteresis effect. It was found that a lean mixture of 30% C2H6 + 70% H2 + air ( = 0.6) has the lowest ignition limit temperature: 24 °C at 1 atm. The estimate of the effective activation energy for the ignition of mixtures over Pd is ~ 2.4 ± 1 kcal/mol, which is typical for the surface process. It is shown that the use of Pd makes it possible to ignite combustible 30% hydrocarbon + 70% H2 at 1–2 atm at the initial room temperature without using external energy sources. It was observed that the separation of the CH and Na emission bands in time during the combustion of a mixture of 30% propane + 70% H2 + air ( = 1), found in this work, is due to the occurrence of hydrodynamic instability of the flame when it touches the end of the cylindrical reactor. It was found that the ignition temperatures of hydrogen - oxygen and hydrogen - methane - oxygen mixtures under the pressure of heated wires of palladium, platinum, nichrome and kanthal (fechral) at a total pressure of 40 Torr increase with a decrease in the hydrogen content in the mixture. Only heated palladium wire has a noticeable catalytic effect. A qualitative numerical calculation made it possible to reveal the role of the additional branching reaction H + HO2 → 2OH in the process of ignition initiation by a heated wire.

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