A comparative analysis of the characteristics of various piezoelectric transducers for measuring pulse pressures was performed. Their sensitivity and response time were determined, and the effects of the steady-state temperature and pulsed heat flows on the measurement results were investigated. The conditions for the linearity of characteristics under the experimental conditions were established. The conditions were determined for observing the “thermal zero drift,” which characterized the influence of thermal heating on the output data of pressure transducers. The investigations were carried out in a shock tube, at a special calibration test bench, and at a test bench that generated the stepwise pulse of the radiant heat flow.

Infrared visualization was employed to observe the decay of a hydrogen-air flame front propagating in a narrow gap similar to a Hele-Shaw cell. Based on the images, the flame front speeds and sizes for mixtures with hydrogen concentrations of 7 % and 10 % by volume in gaps with widths of 3-5 mm were calculated. The shapes of the individual cells formed were determined, the mechanism of flame front decay due to the critical influence of diffusive-thermal instability was described, and a criterion for flame front decay was proposed. The obtained results may be used in the design of fuel cells and MEMS hydrogen devices to ensure the safety of their operation.

The work studies scenarios of self-ignition of hydrogen-air mixtures after a shock wave reflects from a destructible barrier made of sand or a solid wall. The experiments were carried out using a shock tube. The transverse dimensions of the diagnostic section were 40 x 40 mm. The initial pressure of the hydrogen-air mixture varied from 10 kPa to 50 kPa. The molar excess of hydrogen (equivalence ratio) was 0.3, 0.4 and 0.5. Typical oscillograms of pressure, a photomultiplier tube, and the results of high-speed visualization using the Schlieren method are presented. When the shock wave interacted with the destructible barrier, a thermodynamic unloading of shock-compressed mixture occurred in the direction of the scattering of fragments of the barrier. It was shown that under certain conditions, self-ignition of hydrogen could be prevented by using a destructible barrier. In this case, the prevention was recorded in a narrow range of Mach numbers of the incident shock wave, which are characterized by the occurrence of the so-called "weak" (or "mild") ignition of the hydrogen-air mixture. The conditions for strong ignition behind the reflected shock wave, as well as during the interaction of the reflected shock wave with the flame front behind the incident shock wave, were determined. The results of the presented experimental work are aimed at ensuring explosion safety when working with hydrogen in a confined space.

The conditions for detonation propagation in narrow channels are of great interest for ensuring explosion safety in industrial facilities using hydrogen. The roughness of the channel walls can lead to either flame acceleration or to the detonation decay. In this work the propagation of detonation of hydrogen–air mixtures in smooth and rough narrow channels was experimentally studied. The experimental setup consisted of a 3000 mm long driver section and a 1200 mm long diagnostic section with a 7×7mm cross-section. One or two walls of the diagnostic channel were covered with sandpaper which had a grain size of 500–600μm and 80–100μm. Lean, stoichiometric and rich mixtures were used for hydrogen with air. Based on the flame velocity graphs and shock wave pressure in the diagnostic section, several flame propagation regimes were recorded, depending on the fuel concentration and channel roughness. Non-linear effect of the roughness on the detonation propagation was observed. When two pieces of sandpaper were placed on the opposite sides of the narrow channel, a drop in flame velocity to the sound speed of the combustion products was registered for hydrogen mixtures with air. However, covering one wall with small grain sandpaper could lead to expanding of the detonation limits compared to the smooth channel.

The paper proposes and substantiates the possibility of using an assembly of two thermal conductivity sensors to simultaneously measure the flow rate of the gas mixture and the hydrogen content in it. The concept of measurement was proposed, test assemblies were made, and calibration experiments were carried out in the range of speeds from 0 to 1.84 m/s and hydrogen concentrations from 0 to 100 vol%. The results of the experiments show that the flow rate and hydrogen content of the gas mixture at the location of the sensors can be obtained unambiguously from the signals from the two sensors. Formulas for converting signals into physical quantities were obtained.

Flame acceleration in rough narrow channel was experimentally studied for the mixtures of hydrogen with air and acetylene with air. The experiments were carried out in a 7 by 7 mm smooth channel or a channel with one or two opposite walls covered with sandpaper which had a grain size of 100 mu m or 500 mu m. Using high-speed schlieren and self-luminance visualization several flame acceleration regimes were discovered depending on the channel roughness and composition of the combustible mixture. In all cases, the highest maximum flame velocity was observed in rough channels. Detonation was also obtained only in the rough channels, despite the smaller effective channel size when using sandpaper. It was found that the maximum flame velocity and DDT distance depended non-linearly on the channel blockage ratio (BR). The highest flame velocity and the shortest transition to detonation were recorded at BR of 0.035. At highest BR of 0.16, detonation was not recorded in any of the combustible mixtures used. Using schlieren diagnostics, it was discovered that disturbances of the unburned mixture occur above the rough surface, which lead to an increase in pressure ahead of the flame front. The occurrence of detonation was also detected near the rough surface.

This paper presents experimental investigations of the polyurethane foam influence on the combustion dynamics of hydrogen-air flames propagating in a channel with a sudden change in cross-section (i.e. expansion). The channel is open at both ends. Porous media of various lengths and pore size are considered. The porous inserts are placed downstream of the sudden expansion, inside the diagnostic section of dimensions 20 × 40 mm. A Schlieren visualization technique is used to monitor flame shape and propagation dynamics. Various equivalence ratios ranging from 0.3 to 1.0 are tested. The results show that depending on the equivalence ratio, porous length and pore size, the mixture can either propagate throughout the foam or be quenched. In propagating regime, it is found that the output velocity just behind the foam increases linearly with porous matrix length, indicating that the tortuous flow within the foam plays a significant role in the propagation of the flame. These results could be used both to increase the efficiency of gaseous combustion and to ensure the explosion safety of the gas equipment.

This paper reports on the cellular structure formation on the front of a spherically expanding hydrogen-air flame. The hydrogen-air mixture was considered with hydrogen concentration in experiments from 10 to 50 vol%. This paper aims to analyze cell cascade formation, which occurs due to diffusional-thermal and hydrodynamics instabilities. Using experimentally obtained schlieren images, the flame front radius as the function of an angle was obtained. The cell amplitude dependencies on the normalized time were also analyzed. The values corresponding to cell splitting were obtained by the discrete Fourier transform method. The cell split criterion, which allows taking into account the known instability mechanisms, was formulated.& COPY; 2023 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

The combustion dynamics for hydrogen–air mixtures with a hydrogen content of 15 and 20 vol

An experimental study of ignition and flame front propagation during spark initiation in a hydrogen–air mixture in a semi-open channel with a porous coating is reported. The bottom surface of the channel was covered with a porous layer made of porous polyurethane or steel wool. The measurements were carried out for a stoichiometric mixture (equivalence ratio ER = 1.0) and for a lean mixture (ER = 0.4) of hydrogen with air, where ER is the molar excess of hydrogen. The flame front was recorded with a high-speed camera using the shadow method. Depending on the pore size, the velocity of the flame front and the sizes of disturbances generated on the surface of the flame front were determined. Qualitative features of the deflagration flame front at ER = 0.4, consisting of disturbances resembling small balls of flame, were discovered. The sizes of these disturbances significantly exceed the analytical values for the Darrieus–Landau instability. The effect of coatings made of porous polyurethane or steel wool is compared with the results obtained for an empty smooth channel. Depending on the hydrogen concentration in the hydrogen–air mixture, the velocity of the flame front compared to a smooth channel was three times higher when the channel was covered with steel wool and five times higher when the channel was covered with porous polyurethane.

The paper presents experimental investigation results concerning combustion of initially stationary hydrogen-air mixture in a tube, one of the walls of which is lined with steel wool, a fibrous absorbent material. We used a schlieren photograph series imaging the cellular flame to obtain the distance traversed by the flame front as a function of time for the case when lining is absent and for three cases of steel wool lining of varied thickness and porosity. The paper shows that flame propagation rate in the hydrogen-air mixture containing 15 vol % hydrogen in a tube featuring a circular cross-section with a diameter of 54 mm when lining is present is up to 3 times higher than the flame propagation rate in a tube without lining. We calculated the density, heat capacity and thermal conductivity for the steel wool. We also calculated heat emission rate during combustion and the rate at which the steel wool layer absorbs heat from the region occupied by combustion products. We determined the percentage of heat energy absorbed by the steel wool and analysed how this effect affects flame propagation rate. We measured cell dimensions in an unstable flame propagating in a tube with and without lining. For the case of a flame front propagating across a fibrous absorbent material layer, we found that the average cell size observed decreases twofold

Experimental and analytical study of burning hydrogen-air mixtures with 12, 13, and 15 vol % hydrogen concentrations in channels with central and peripheral ignition was performed. Flame propagation speeds were determined by shadow and infrared high-speed imaging in the transverse and longitudinal directions, respectively. It was found that the increase in the flame front speed during the peripheral ignition reaches up to 1.7 times compared to the central ignition depending on mixture content. The pressure growth rate was examined in a closed channel. It was estimated that the time to reach a maximum pressure is 1.1 times less in the case or peripheral ignition than the central one. An analytical model was formed to describe the dynamics of the flame front in both cases. The model of a "reversed finger-flame" generated by a peripheral ignition was presented. The obtained results could be used in designing hydrogen-fueled combustible engines with the reduced knock-effect. (C) 2022 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

The results of an experimental investigation of the effect of acoustic oscillations with a frequency of 250-7000 Hz and an intensity of 105 dB on the dynamics of the acceleration of a hydrogen-air flame in an open channel are presented. Dependences of the position of the flame front and cell sizes were obtained from shadow photographs at different frequencies of acoustic oscillations. The frequency values when the average flame speed increases up to 3 times are found. Acoustic action with a frequency of 250 Hz leads to a slight decrease in the speed of the flame. Keywords: Hydrogen, duct combustion, acoustic impact, cellular flame.

The motivation for this work is to ensure explosion safety when working with flammable gases. One of the passive methods for suppressing combustion of gases, consisting of the use of porous materials, is considered in this work. The propagation of a flame front of hydrogen-air and acetylene-air mixtures when passing through polyurethane foam was experimentally studied. To reduce the velocity of the unburned mixture ahead of the flame front, the initiation was carried out at the open end of the channel. In this event, the foam skeleton has a general effect on the propagation of the flame and not on the flow of the unburned mixture. The flame front velocity was determined with a high-speed camera using the shadow method. It was shown that the flame front does not pass through polyurethane foam containing 45 and 80 pores per inch (ppi). However, the use of foam with 10 and 30 ppi resulted in an acceleration of the flame front, up to 200 m/s for acetylene-air and 550 m/s for hydrogen-air mixtures. It was shown that the relative increase in the velocity of the flame front varied exponentially depending on the length of the foam. The exponent was determined for both mixtures.

One of the modern energy safety concerns is ensuring explosion safety when working with flammable gases. For this purpose, various methods are used, such as active chemical inhibitors and passive combustion suppression. The use of various porous materials is prevalent among passive methods for combustion suppression. This study focused on the experimental investigation of hydrogen-air and acetylene air flame propagation through polyurethane foam and through the empty channel. The use of high-speed infrared camera enabled visualization of flame propagation inside the porous material. Two types of diagnostic channels were used with the dimensions of the diagnostic section 16 x 20 mm and 40 x 20 mm. The length of the foam was 120 mm. In hydrogen-air mixtures with 10 pores per inch (PPI) polyurethane foam the flame velocity was higher than in the empty section; with 30 PPI polyurethane foam the flame was quenched in the mixture with equivalence ratio ER = 0.3, but for mixtures with ER = 0.4 and ER = 0.5 the average flame velocity was higher than in the empty channel. In the acetylene-air mixtures flame velocity was usually lower in polyurethane foam and only reached the flame velocity in the empty channel with ER = 0.8 and 10 PPI foam. Critical Peclet numbers that allowed the flame to propagate through porous materials were calculated based on the obtained experimental data. The critical Peclet numbers were 23-24 for the hydrogen-air mixture and 37-38 for the acetylene-air mixture. The channel was open near the ignition point, which allowed the obtained results to be less dependent on the channel length. Information obtained in this work can be used to improve the means of ensuring explosion safety, including in the areas of nuclear power and transport using hydrogen.(c) 2022 Institution of Chemical Engineers. Published by Elsevier Ltd. All rights reserved.

The risks and destruction assessment of gas explosions is based on the potential burning rates and pressure impulses. Such issues should be considered in designing any infrastructure for using or producing hydrogen or having a gas outburst possibility due to the hydrogen high flammability and explosive hazards. An experimental and theoretical study of the propagation dynamics uncertainty in premixed laminar unstable hydrogen-air flames is presented and its statistical analysis was performed. The main objective of this work was to quantify the potential velocities of the hydrogen-air flame front propagation. Experiments were repeated up to 30 times in the mixtures with a hydrogen content of 10-60 vol.%. Based on the hydrogen content in the mixture data scattering was estimated. Some confirming results were obtained using the numerical integration of the Sivashinsky equation of flame propagation. The obtained results could update the Rules, Guidelines, Codes and Standards of Hydrogen Safety.

The study focuses on the interaction between a gas flow and liquid drops and describes the proposed experimental setup in detail. First, we experimentally studied the vibrations of a drop with a diameter of 1.4 mm in the airflow with a speed of 3.1--10 m/s and by a high-speed Phantom camera with a frame rate of 2000 fps, we took a series of photographs of the drop in the airflow at regular intervals. Then, we calculated the spectra of free vibrations of the drop in a gas flow, as well as the vibrations affected by sound vibrations of the air with a frequency of 0--1000 Hz and a sound pressure level of 0--121 dB, and found the gas flow and acoustic radiation parameter regions, at which the drop vibrations amplitude is higher or lower. Next, by the Kelvin --- Helmholtz instability theory, we theoretically analyzed the airflow-acoustic radiation mutual influence on the liquid drop and considered the drop possible values of the Weber number under experimental conditions. Findings of the research show that the drop does not fragmentize in the airflow. Finally, we found the drop vibration frequency values in the airflow, at which the amplitude of oscillations is constant

Small scale experiments examining the propagation of a hemispherical flame in a hydrogen–air mixture were performed under conditions of constant pressure. The shape of the propagating flame front in a 15% hydrogen–air mixture was reconstructed with the use of near-infrared imaging. The wrinkled flame surface was restored from experimentally obtained two-dimensional IR photographs using data on the radiation of vibrational valence transitions of water in the combustion products and the features of the propagation of the curved flame front. The values of the wrinkling factor were obtained using reconstructed 3D surfaces and 2D boundary dependencies.

The force arising on a wedge during the interaction of a sliding discharge with a supersonic flow is investigated. The sliding discharge was created in a supersonic atmospheric-vacuum wind tunnel with Mach number M = 2, a Reynolds number of about 10(6), and static pressure in the flow p = 0.15 bar. It was found that the effect of the discharge is not limited to the rise of a shock wave, but also significantly changes the character of the flow around the wedge due to pulsed heat release. With an increase in the discharge energy, its effect on the wedge also increases. The force created by a sliding discharge in a supersonic flow is several times greater than in static air, regardless of the pressure: 0.15 or 1 bar.