Study On The Methane-air Deflagration Flames Propagation Characteristcs In An Square Plexiglass Tube

Currently, coal energy plays an important role in the energy structure in our country. In recent years, China’s coal mines gas explosion disasters occurred frequently. We should still study the phenomena of fire damp explosions, though some active gas drainage technologies were successfully used in coal mines. The active measures once failure, the passive prevention and control measures became particularly important. Some gas explosions suppression devices like the flameproof devices or water bags had fixed in the roadway in some large or medium-sized state-owned coal mines, however, the gas explosions occurred, these devices often fail to reach the desired effects, or they didn’t play the role. This fact showed that there are insufficient understanding the mechanism of gas explosion, and the flame propagation mechanism and flame propagation characteristics, etc. for many peoples to some extent. The simulation roadways or small-scale experiment tubes were often used for the investigation into the phenomena of gas explosion. The large-scale simulation laneways were closed to the real coal mine roadways, but, due to its high cost, space requirement, and difficult to test parameters, etc., many scholars preferred to set up the small-scale experiment tubes to study the flame propagation process of gas explosion. The methane-air flame acceleration mechanism and the deflagration wave propagation characteristics under the two conditions are the same and they have the ’size effect’ relations.In order to reveal the methane-air flames propagation mechanism and its acceleration mechanism induced by obstacles, high-speed color video camera, photoelectric sensors, pressure sensors, and fine thermocouples were adopted to test the flame images, flame propagation velocities, deflagration pressures, transient temperatures, and other related flame propagation parameters in an square plexiglass tube with1.5meters long and its cross section is100×100mm2. The deflagration flame propagation characteristics were analyzed with no obstacles, repeated obstacles, solid structure obstacles in tube, respectively. Via the application of various research methods including experimental investigation, theoretical analysis and numerical modeling, the methane-air deflagration flames propagation characteristics and acceleration regulations in tube were studied comprehensively. In the paper, the main research contents and its conclusions are shown in the following aspects:Firstly, the explosion theory, the dynamic coefficient method and the first strength theory were adopted to check the closed tube’s strength loaded by detonation wave under the equivalence ratio of methane-air mixture. The results showed that the tube’s wall thickness fully met with the requirements of explosion flames experiments. Considering the given typical explosion loads, the tube’s wall dynamic response was simulated by ANSYS software. The results revealed that the location at the tube’s axial center mainly showed the pull-shear failure, while the location at the inner wall edge mainly showed the shear failure.The methane-air deflagration flames propagation parameters including flame front locations, flame propagation velocities, and deflagration pressures were investigated by the high-speed video camera, photoelectric sensors, and pressure sensors under the typical methane concentrations with no obstacles, repeat baffles or solid structure obstacles in tube, respectively. The relationship of flame-pressure signal at the same testing point was analyzed and the coupling formula about methane-air flame propagation velocity and its deflagration pressure was deduced and given, also the relationship for the flame front location and time after ignition was consistent with the fitting curve of GaussAmp. The methane-air flames propagation parameters were investigated under the condition of4types of repeat baffles and5forms of solid structure obstacles in tube, and, the results showed that the blockage ratios of repeat baffles had greater contribution to flame acceleration than the numbers and distances of baffles did. And, the plates and triple prisms increased flame speed and overpressure much larger, and cuboids were intermediate, while effects of cylinders were comparatively limited. The irregularities of solid obstacles surface were the main factor contributing to the turbulence degree and flame acceleration. Also, the fixed obstacles in front of the flame had the blocking effect to flame propagation in the initial process of flame propagation, and the flame speed and overpressure was increased obviously when the flame came across the obstacles.The laws of deflagration pressure distribution were investigated experimentally under the condition of different methane concentrations, different repeat baffles, and different sealing materials at open end, respectively. The results showed that the peak-pressure at testing points increased firstly, then decreased, and the pressure pulsation showed at the middle section of the tube, and the pressure decreased sharply near the open end. Also, at the same venting area, the value of deflagration pressure was linked with the thickness and tensile strength of sealing materials, and the bigger thickness or tensile strength was, the greater deflagration pressure was. And the sealing materials constant Kb is an important parameter for representing the materials rupture.The flame photoelectric sensors, the pressure sensors and the fine thermocouples were adopted to obtain the premixed methane-air flame parameters of flame front position and flame propagating velocity along the tube, the transient pressure and temperatures at testing points, respectively. The experiment showed that the transient temperature values of premixed methane-air flame at testing points in coal dust condition obviously revealed the wave of "twin peaks structure", and there was not obvious difference with or without coal dust in tube, but the pressure wave was broadened in coal condition. The values of transient pressure and temperature at testing points became overall decline under the condition of rock dust, and the half-peak width of the temperature was narrowed, the fact showed that the rock dust suppressed the process of flame propagation.The flame propagation parameters were simulated by Fluent software under the condition of the experiment given obstacles of40%blockage ratio fixing in the tube with the concentration of9.5%-methane by volume. The simulation results given the qualitative analysis of the flame acceleration mechanism induced by different forms of solid structure obstacles. The denoising and enhancement treatment for methane-air flame images by Matlab software highlighted the flame features and flame detail parts. At the same time, the flame contours and flame tips were found by the software, then the flame front locations and flame propagation velocities were calculated accurately. And the relationship between the RGB values of flame images with their thermocouples temperatures based on RGB model was preliminarily discussed in the paper.The study has important scientific significance for explaining the propagation mechanism of the methane-air deflagration flames and the mechanism of flame acceleration mechanism induced by obstacles. It has some application value for preventing or controlling the occurrence and development of gas explosion disasters. Also, it has important application significance for optimizing the gas explosion preventing and controlling technologies in mines and the design of the fire resistance performances for the industrial gas pipelines.