Effects of obstacles and deposited coal dust on characteristics of premixed methane–air explosions in a long closed pipe

An experimental system including pressure transducer, electric spark ignition device, data acquisition and control unit was set up to investigate methane–air explosions in a horizontal pipe closed at both ends with or without the presence of obstacles and deposited coal dust. The experimental results show that explosion characteristics depended on the methane content, on the layout of obstacles, and on the deposited coal dust. Pressure fluctuation with a frequency of 150 Hz appeared in its crest when the methane content was close to the stoichiometric ratio (9.5% methane percentage by volume). The pressure rise rate increased locally when a single obstacle was mounted in the pipe, but it had little effect on the pressure peak. Repeated obstacles mounted in the pipe caused the pressure to rise sharply, and the mean maximum explosion overpressure increased with the increase of the obstacle’s number. The amplitude of pressure fluctuation was reduced when deposited coal dust was paved in the bottom of the pipe. However, when repeated obstacles were arranged inside, the maximum overpressures were higher with coal dust deposited than pure gas explosions.     

Effect of concentration and ignition position on vented methane–air explosions

Experiments were conducted in a 1 m³ vessel with a top vent to investigate the effect of methane concentration and ignition position on pressure buildup and flame behavior. Three pressure peaks (p₁, p₂, and P_ext) and two types of pressure oscillations (Helmholtz and acoustic oscillations) were observed. The rupture of vent cover results in p₁ that is insensitive to methane concentration and ignition position. Owing to the interaction between acoustic wave and the flame, p₂ forms in the central and top ignition explosions when the methane–air mixture is near–stoichiometric. When the methane–air mixture is centrally ignited, p₂ first increases and then decreases with an increase in the methane concentration. The external explosion-induced P_ext is observed only in the bottom ignition explosions with an amplitude of several kilopascals. Under the current experimental conditions, flame–acoustic interaction leads to the most serious explosions in central ignition tests. Methane concentration and ignition position have little effect on the frequency of Helmholtz and acoustic oscillations; however, the Helmholtz oscillation lasts longer and first decreases and then increases as the methane concentration increases for top ignition cases. The ignition position significantly affects the Taylor instability of the flame front resulting from the Helmholtz oscillation.     

Effects of cross-wise obstacle position on methane–air deflagration characteristics

A vented chamber, with internal dimensions of 150 mm × 150 mm × 500 mm, is constructed in which the premixed methane–air deflagration flame, propagating away from the ignition source, interacts with obstacles along its path. Three obstacle configurations with different cross-wise positions are investigated. The cross-wise obstacle positions are found to have significant effects on deflagration characteristics, such as flame structure, flame front location, flame speed, and overpressure transients. The rate of flame acceleration, as the flame passes over the last obstacle, is the highest at the configuration with three centrally located obstacles, whereas the lowest is observed at the configuration with three obstacles mounted on one side of the chamber. Compared with the side configuration, the magnitude of overpressure generated increases by approximately 80% and 165% for the central and staggered configurations, respectively. Furthermore, flame propagation speeds and generated overpressures for both the central and staggered configurations are greater, which should to be avoided to reduce the risk associated with turbulent premixed deflagrations in practical processes.     

Explosions of gasoline–air mixture in the tunnels containing branch configuration

In order to investigate the effects of branch tunnels on explosion propagation, experiments were performed in five different configuration tunnels (straight configuration and configurations with 1 branch, 2 branches, 3 branches and 4 branches). Pressure and flame transducers were used to record the history of the pressure development and track the velocity of the flame front. It was shown that the branch tunnels had ability to enhance the maximum overpressure, rate of overpressure rise, and deflagration index (K_G) of the gasoline–air mixture explosion due to the turbulence induced by the branch tunnels. The overpressure rise rate and K_G of the explosion increased as a function of the number of branch tunnels. Experiments also showed that the maximum flame speed increased as the branch number increasing from 0 to 3 due to the enlargement of turbulence induced by the branch tunnels. However, an increase of branch number did not always lead to an enhancement of flame speed because the heat loss was intensified resulting from the increase of flame surface caused by the branch tunnels. When the number of branch tunnels exceeded 3, the maximum flame speed dropped.     

Explosion behavior of hydrogen–methane/air mixtures

The effects of enriching natural gas with hydrogen on local flame extinction, combustion instabilities and power output have been widely studied for both stationary and mobile systems. On the contrary, the issues of explosion safety for hydrogen–methane mixtures are still under investigation.In this work, experimental tests were performed in a 5 L closed cylindrical vessel for explosions of hydrogen–methane mixtures in stoichiometric air. Different compositions of hydrogen–methane were tested (from pure methane to pure hydrogen) at varying initial pressures (1, 3 and 6 bar).Results have allowed the quantification of the combined effects of both mixture composition (i.e., hydrogen content in the fuel) and initial pressure on maximum pressure, maximum rate of pressure rise and burning velocity. The measured burning velocities were also correlated by means of a Le Chatelier’s Rule-like formula. Good predictions have been obtained (at any initial pressure), except for mixtures with hydrogen molar content in the fuel higher than 50%.     

Numerical simulation of premixed methane–air deflagration in a semi-confined obstructed chamber

In this paper, large eddy simulation coupled with a turbulent flame speed cloure (TFC) subgrid combustion model has been utilized to simulate premixed methane–air deflagration in a semi-confined chamber with three obstacles mounted inside.The computational results are in good agreement with published experimental data, including flame structures, pressure time history and flame speed. The attention is focused on the flame flow field interaction, pressure dynamics, as well as the mechanism of obstacle-induced deflagration. It is found that there is a positive feedback mechanism established between the flame propagation and the flow field. The pressure time history can be divided into four stages and the pseudo-combustion concept is proposed to explain the pressure oscillation phenomenon. The obstacle-induction mechanism includes direct effect and indirect effect, but do not always occur at the same time.     

Experimental investigation of fast flame propagation in stratified hydrogen–air mixtures in semi-confined flat layers

This paper presents results of an experimental investigation on fast flame propagation and the deflagration-to-detonation transition (DDT) and following detonation propagation in a semi-confined flat layer filled with stratified hydrogen–air mixtures. The experiments were performed in a transparent, rectangular channel open from below. The combustion channel has a width of 0.3 m and a length of 2.5 m. The effective layer thickness in the channel was varied by using different linear hydrogen concentration gradients. The method to create quasi-linear hydrogen concentration gradients that differ in the range and slope is also presented. The ignited mixtures were accelerated quickly to sonic flame speed in the first obstructed part of the channel. The interaction of the fast flame propagation with different obstacle set-ups was studied in the second part of the channel. The experimental results show an initiation of DDT by one additional metal grid in the obstructed semi-confined flat layer. Detonation propagation and failed detonation propagation were observed in obstructed and unobstructed parts of the channel.     

Ignition of methane–air mixtures by laser heated small particles

Optical technologies have progressed rapidly in the past 15 years. One application of laser technology in underground coal mines currently under evaluation is the remote measurement of explosive methane gas. Federal regulations require that atmospheric monitoring systems used in gassy underground mines where permissible equipment is required shall be intrinsically safe. Mine Safety and Health Administration criteria for the evaluation and testing of intrinsically safe apparatus and associated apparatus contain no specific guidance for optoelectronic components such as diode lasers. The National Institute for Occupational Safety and Health is conducting a study to help provide a scientific basis for developing appropriate safety guidelines for optical equipment in underground coal mines. Results of experiments involving ignition of methane–air mixtures by collections of small heated particles of Pittsburgh seam coal and black iron oxide are reported. The inert but more strongly absorbing iron oxide targets consistently ignited methane–air mixtures at lower powers than the coal targets. Minimum observed igniting powers for laser energy delivered by 200, 400 and 800 μm core fiber optic cables and directed onto iron oxide targets in methane–air atmospheres were 0.6, 1.1, and 2.2 W, respectively. Comparisons with the results of other researchers are made. A thermal layer theoretical approach to describing the process is included as an appendix.     

Comparative assessment of the explosion characteristics of alcohol–air mixtures

Explosion characteristics of five alcohol–air (ethanol, 1-butanol, 1-pentanol, 2-pentanol and 3-pentanol) mixtures were experimentally conducted in an isochoric chamber over wide ranges of initial temperature and pressure. The effect of temperature and pressure on the different explosion behaviors among these alcohols with various structures were investigated. Results show that the peak explosion pressure is increased with the decrease of temperature and increase of pressure. Maximum rate of pressure rise is insensitive to the temperature variation while it significantly increases with the increase of initial pressure. Among the 1-, 2-, and 3-pentanol–air mixtures, 1-pentanol has the highest values in peak explosion pressure and maximum rate of pressure rise and 2-pentanol gives the lowest values at the initial pressure of 0.1 MPa. These differences tend to be decreased with the increase of initial pressure. Among the three primary alcohol–air (ethanol, 1-butanol and 1-pentanol) mixtures, a similar explosion behavior is presented at the lean mixture side because of the combined effect of adiabtic temperature and flame propagation speed. At the rich mixture side, 1-pentanol gives the highest values in peak explosion pressure and maximum rate of pressure rise and ethanol gives the lowest values. This phenomenon can be interpretated from the combining influence of heat release and heat loss, since the flame speeds of ethanol-, 1-butanol-, 1-pentanol–air mixtures are close at rich mixture side.     

Large eddy simulation of methane–air deflagration in an obstructed chamber using different combustion models

In this paper, simulations of methane–air deflagration inside a semi-confined chamber with three solid obstacles have been carried out with large eddy simulation (LES) technique. Three sub-grid scale (SGS) combustion models, including power-law flame wrinkling model by Charlette et al., turbulent flame speed closure (TFC) model, and eddy dissipation model (EDM), are applied. All numerical results have been compared to literature experimental data. It is found that the power-law flame wrinkling model by Charlette et al. is able to better predict the generated pressure and other flame features, such as flame structure, position, speed and acceleration against measured data. Based on the power-law flame wrinkling model, the flame–vortex interaction during the deflagration progress is also investigated. The results obtained have demonstrated that higher turbulence levels, induced by obstacles, wrinkle the flame and then increase its surface area, the burning rates and the flame speed.     

Experimental study on DDT for hydrogen–methane–air mixtures in tube with obstacles

An experimental study of flame propagation, acceleration and transition to detonation in stoichiometric hydrogen–methane–air mixtures in 6 m long tube filled with obstacles located at different configurations was performed. The initial conditions of the hydrogen–methane–air mixtures were 1 atm and 293 K. Four different cases of obstacle blockage ratio (BR) 0.7, 0.6, 0.5 and 0.4 and three cases of obstacle spacing were used. The wave propagation was monitored by piezoelectric pressure transducers PCB. Pressure transducers were located at different positions along the channel to collect data concerning DDT and detonation development. Tested mixtures were ignited by a weak electric spark at one end of the tube. Detonation cell sizes were measured using smoked foil technique and analyzed with Matlab image processing toolbox. As a result of the experiments the deflagration and detonation regimes and velocities of flame propagation in the obstructed tube were determined.     

The influence of air flow on maximum explosion characteristics of dust–air mixtures

A standard spherical apparatus for measuring explosion characteristics was modified to give increased and controlled turbulence within a dust–air mixture. This was intended to mimic the local effects which may occur during industrial dust explosions, particularly secondary ones which may develop in ducts or mine galleries where the initial explosion causes an increased air velocity and suspension of further quantities of dust.The results show that there may be a doubling of the maximum explosion pressure and of the rate of pressure rise during the explosion under more turbulent conditions. This is significant for modelling of dust explosions and suggests that explosion relief may be inadequate if this factor is not taken into consideration.The modified apparatus therefore gives a laboratory method for assessing the effect of turbulence in dust explosions.     

Effects of initial temperature and pressure on explosion characteristics of propane–diluent–air mixtures

The explosion characteristics of propane–diluent–air mixtures under various temperatures and pressures were investigated using a 20-L apparatus. The explosion limits of propane diluted with nitrogen or carbon dioxide were measured at high temperatures from 25 to 120 °C. The results showed that the upper explosion limit (UEL) increased, and the lower explosion limit (LEL) decreased with the rising temperature. The explosion limits of propane diluted with nitrogen or carbon dioxide were also measured at high pressures from 0.10 to 0.16 MPa. The results showed that the UEL increased, and the LEL almost remainedunchanged along with increased pressure. Under the same initial operating conditions, the concentration of nitrogen required to reach the minimum inerting concentration (MIC) point was higher than the concentration of carbon dioxide. Finally, the study investigated the limiting oxygen concentration (LOC) of propane under various initial temperatures, initial pressures, and inert gases. The LOC of propane decreased approximately linearly with increased temperature or pressure, and the LOC of propane dilution with carbon dioxide was greater than dilution with nitrogen from 25 to 120 °C or from 0.10 to 0.16 MPa, which indicated that the dilution effect of carbon dioxide was better than that of nitrogen.     

The effect of orifice plates with different shapes on explosion propagation of premixed methane–air in a semi-confined pipeline

The effect of internal shape of obstacles on the deflagration of premixed methane–air (concentration of 10%) was experimentally investigated in a semi-confined steel pipeline (with a square cross section size of 80 mm × 80 mm and 4 m long). The obstacles used in this study were circular, square, triangular and gear-shaped (4-teeth, 6-teeth and 8-teeth) orifice plates with a blockage ratio of 75%, and the perimeter of the orifice was regarded as a criterion for determining the sharpness of the orifice plate. The overpressure history, flame intensity histories, flame front propagation speed, maximum flame intensity and peak explosion overpressure were analyzed. The explosion in the pipeline can be divided into two stages: initial explosion and secondary explosion. The secondary explosion is caused by recoiled flame. The perimeter is positively related to the intensity of the recoiled flame and the ability of orifice plate to suppress the explosion propagation. In addition, the increase in the perimeter will cause the acceleration of the flame passing through the orifice plate, while after the perimeter of the orifice reaches a certain value, the effect of the increase in perimeter on explosion excitation becomes no obvious. The overpressure (static pressure) downstream of the orifice plate is the result of the combined effect of explosion intensity and turbulence. The increase in perimeter leads to the increase in turbulence downstream of the orifice plate which in turn causes more explosion pressure to be converted into dynamic pressure.     

Structural response for vented methane–air deflagrations: Effects of volumetric blockage ratio

To investigate the effects of cylinders placed parallel to the venting direction on the structural response of the vessel walls to an explosion, 25 batches of vented explosion tests were conducted in a 1 m³ rectangular vessel. Two types of structural response with different amplitudes and frequency distributions were observed and evaluated by comparing the vibration data with both the pressure data and high-speed videos. A low-amplitude structural response of approximately 150–250 m/s², which increased slightly as VBR increased, was triggered by a combination of the initial flame propagation, external explosion, Helmholtz oscillations, and the Taylor instability. A high-amplitude structural response of approximately 9500 m/s² was also observed, which decreased sharply as VBR increased. Additionally, the high amplitude response was never observed when more than two cylinders were present in the vessel. The high amplitude response was triggered due to the coupling between the acoustic wave, the flame, and the resonance of the vessel. The presence of obstacles did not increase the severity of the structural responses under the current experimental conditions. To the contrary, the presence of obstacles in the container attenuated or even inhibited the high-amplitude vibration of the container caused by the explosion.     

Experimental study on advantages of foam–sol in coal dust control

In China, more than 2.65 million coal mine workers are exposed to coal dust. Every year, new pneumoconiosis cases amount to 25,000, among which 6000 cases die of this disease. The figure is twice the death toll in production safety accidents. Occupational diseases seriously endanger life and health of coal mine workers, and restrict the healthy growth of the coal industry.The paper presented a study of foam–sol-based coal dust control. This was an experimental study of characteristics of foam–sol-based coal dust control, which features dust capture, suppression, and isolation. Comparative wettability experiments were carried out to determine contact angles of water, aqueous foam, and foam–sol solution. A new foam–sol generating system with a conical diffuser outlet was proposed.The experiment results clearly showed that the foam–sol features dust capture, suppression, and isolation. The wettability of the aqueous foam solution was better than the foam–sol solution, but the foam–sol technology had the better ability to capture the airborne dust, suppress the static dust and enclose the dust source, due to the excellent surface viscosity, strong cohesiveness and less volatile property. The paper concluded that the foam–sol could greatly improve the dust control efficiency and did not have main deficiencies that the aqueous foam technology had.     

Propagation of ethylene–air flames in closed cylindrical vessels with asymmetrical ignition

A study of explosions in several elongated cylindrical vessels with length to diameter L/D = 2.4–20.7 and ignition at vessel's bottom is reported. Ethylene–air mixtures with variable concentration between 3.0 and 10.0 vol% and pressures between 0.30 and 1.80 bara were experimentally investigated at ambient initial temperature. For the whole range of ethylene concentration, several characteristic stages of flame propagation were observed. The height and rate of pressure rise in these stages were found to depend on ethylene concentration, on volume and asymmetry ratio L/D of each vessel. High rates of pressure rise were found in the early stage; in later stages lower rates of pressure rise were observed due to the increase of heat losses. The peak explosion pressures and the maximum rates of pressure rise differ strongly from those measured in centrally ignited explosions, in all examined vessels. In elongated vessels, smooth p(t) records have been obtained for the explosions of lean C₂H₄–air mixtures. In stoichiometric and rich mixtures, pressure oscillations appear even at initial pressures below ambient, resulting in significant overpressures as compared to compact vessels. In the stoichiometric mixture, the frequency of the oscillations was close to the fundamental characteristic frequency of the tube.     

Experimental investigation of hydrogen–air deflagrations and detonations in semi-confined flat layers

This paper presents results of an experimental investigation on the deflagration and deflagration-to-detonation transition (DDT) in an obstructed (blockage ratio BR = 50%), semi-confined flat layer filled with uniform hydrogen–air mixtures. The effect of mixture reactivity depending on flat layer thickness and its width is studied to evaluate the critical conditions for sonic flame propagation and the possibility for detonation onset. The experiments were performed in a transparent, rectangular channel with a length of 2.5 m. The flat layer thickness was varied from 0.06 to 0.24 m and the experiments were performed for different channel widths of 0.3, 0.6 and 0.9 m. The experimental results show flame velocity vs. hydrogen concentration for different thicknesses and widths of the semi-confined flat layer. Three different flame propagation regimes were observed: slow subsonic flame (M << 1), sonic deflagration (M ～ 1) and detonation (M >> 1). It is shown that flame acceleration (FA) to sonic speed is independent of the width of the flat layer. The critical expansion ratio for effective flame acceleration to sonic speed was found to be linearly dependent on the reciprocal layer thickness.     

Leak detection of hydrogen–natural gas mixtures in pipes using the characteristics method of specified time intervals

This paper presents a technique for detection and location of leaks in a single pipe, by means of transient analysis, of hydrogen–natural gas mixtures flows. The method uses transient pressure waves initiated by the sudden closure of a downstream shut-off valve. The presence of a leak in a pipe partially reflects these pressure waves and allows for the location of the leak. Pressure waves are governed by two coupled non-linear, hyperbolic partial differential equations with pressure dependent coefficients. The fluid pressure and velocity are considered as two principal dependent variables. To determine the leak location, the mathematical formulation has been solved by the characteristics method of specified time intervals. The computed results describe the influence of the leak on pressure time-history and the effect of hydrogen mass fraction in the mixture on the leak discharge behaviour. It was found that transient pressure is much important in the case of hydrogen than that in the case of natural gas.     

Effect of ignition position on vented hydrogen–air deflagration in a 1 m3 vessel

This study investigates the effect of the ignition position on vented hydrogen-air deflagration in a 1 m³ vessel and evaluates the performance of the commercial computational fluid dynamics (CFD) code FLACS in simulating the vented explosion of hydrogen-air mixtures. First, the differences in the measured pressure-time histories for various ignition locations are presented, and the mechanisms responsible for the generation of different pressure peaks are explained, along with the flame behavior. Secondly, the CFD software FLACS is assessed against the experimental data. The characteristic phenomena of vented explosion are observed for hydrogen-air mixtures ignited at different ignition positions, such as Helmholtz oscillation for front ignition, the interaction between external explosion and combustion inside the vessel for central ignition, and the wall effect for back-wall ignition. Flame-acoustic interaction are observed in all cases, particularly in those of front ignition and very lean hydrogen-air mixtures. The predicted flame behavior agree well with the experimental data in general while the simulated maximum overpressures are larger than the experimental values by a factor of 1.5–2, which is conservative then would lead to a safe design of explosion panels for instance. Not only the flame development during the deflagration was well-simulated for the different ignition locations, but also the correspondence between the pressure transients and flame behavior was also accurately calculated. The comparison of the predicted results with the experimental data shows the performance of FLACS to model vented mixtures of hydrogen with air ignited in a lab scale vessel. However, the experimental scale is often smaller than that used in practical scenarios, such as hydrogen refueling installations. Thus, future large-scale experiments are necessary to assess the performance of FLACS in practical use.