Study On The Mechanism And Flow Field Characteristics Of Methane Explosion During Venting

Gas explosion accidents occur frequently in industrial production and life,which causes severe personal casualties and property losses.To investigate the characteristics and damage effect during explosion venting,a large-scale venting test system with the size of2.0m×1.5m×1.5m was designed in this thesis,the flame propagation images were recorded by using the high-speed camera,the internal and external pressure signals were collected by using the pressure test system,and the external temperature was analyzed by the infrared camera.The effects of ignition position,vent area,solid obstacles,and initial turbulence on the dynamic characteristics of methane-air mixtures were studied during vented explosion.The loading mechanism and flow field characteristics were systematically investigated by comparing experiment,theory,and simulation.The main content includes:The effects of concentration,ignition locations,and vent areas on the explosion parameters of methane were studied during vented explosion,the characteristics of propagation and surface variation of internal flame were first analyzed to reveal the physical mechanism and variation law of internal pressure.The results show that the honeycomb structure of the flame front appears due to thermal diffusion instability,R-T instability,and hydrodynamic instability.The venting process is mainly divided into the opening stage,Helmholtz oscillation stage,and highfrequency oscillation stage.When the methane concentration is close to the upper or lower limit of the explosion,resulting in slow burning,therefore,no high-frequency oscillation flame front appears and only a pressure peak occurs during venting;the concentration is near the optimal equivalence ratio,two pressure peaks appear.The venting opening pressure is affected by the volume expansion rate of combustion products and the dynamic mechanical characteristics of the venting components.With the decrease in the distance between the ignition position and the vent,the amplitude and the duration of Helmholtz oscillation are longer,when the vent area decreases,the Helmholtz oscillation phenomenon gradually weakens and even disappears,and the highfrequency oscillation pressure peak is larger.In the case of front ignition,two pressure peaks in the chamber were recorded;after central ignition,a single pressure peak and the maximum high-frequency oscillation formed;three pressure peaks in the chamber were recorded under rear ignition.The characteristics of external pressure field during explosion venting were experimentally carried out to investigate the formation mechanism and variation characteristics of external pressure,the causes and influencing factors of the second explosion were analyzed,and the relationship between internal and external pressures was established.The results show that after the rupture of film,the rupture shock wave,secondary combustion wave,and highfrequency oscillation pressure peak are formed outside the chamber.The shock wave decays inversely with the distance;the second peak pressure increases linearly with the internal pressure peak;the high-frequency oscillation pressure peak outside the chamber is closely related to that inside the chamber at the later stage of explosion venting.In the case of front ignition,the vented high-temperature burned gas increases the shock wave pressure,when the vent area increases,the difference between the temperature of the internal and external chamber at the instant of venting is small,resulting in the same pressure value.The external flame is the longest at central ignition,which is due to less energy loss.At rear ignition,a large amount of unburned gas is released,leading to a larger external combustion reaction area,and a higher peak pressure generated by combustion wave.The increase in the distance between ignition location and vent,resulting in an increase in the distance between the secondary explosion point and vent.The characteristics of external temperature field during venting were investigated to analyze the influences of the obstacle,ignition position,and initial turbulence on the external temperature,and discuss the relationship between flame and temperature field,and determine the correlation between the external temperature field and the internal pressure.And then the changing laws and influencing factors of the external temperature field were revealed during venting process.The results show that in the presence of obstacles,the coupling between obstacles and flame front is enhanced at front ignition,resulting in the increase in the highfrequency pressure oscillation in the last stage of venting and the high-temperature duration outside the chamber.Under rear ignition and central ignition,the gas released from the vent is hindered by obstacles,which weakens the coupling between the flame front and the sound wave formed during the venting process,causing the weak high-frequency pressure oscillation,and resulting in the decrease in the length and duration of the external high-temperature region.With the increase of the number of obstacles,the internal high-frequency oscillation pressure peak first decreases and then increases and then decreases again.Under strong initial turbulence,D-L instability and thermal diffusion instability significantly increase the combustion rate,leading to the increase in internal pressure peak,the formation of a mushroom cloud,and the long high-temperature field outside the vent.The obstacles and initial turbulence have little effect on the temperature peak.Based on experimental research,the typical configurations of explosion venting were simulated by using the Fluent software,the turbulent combustion model and related parameters for simulating the venting explosion were determined in the large-scale chamber.The flame,velocity,and methane concentration distribution during venting process were simulated to analyze the effects of ignition position and vent area on the generation mechanism of internal pressure peak,the flame structure of external space,and the characteristics of the temperature field.The simulation results ideally reproduce the flame development,and internal pressure changes during venting,the formation mechanism and law of internal pressure are systematically revealed during venting.Based on the analysis of the geometric dimensions of the building space,the test results,and the physical characteristics of the venting process,by differentiating the explosion venting process,and the process in each step was divided into the combustion,venting,and pressure balance processes,then the calculation method of internal pressure was proposed.Comparing with the experimental result and Lee's result,the applicability of Lee model and the calculation method in the large-scale chamber was verified.The results show that the two models are the same for predicting the pressure before the opening of the explosion venting components.The calculation method is better for predicting the oscillating pressure peak,which is suitable for predicting the pressure-time history of explosion venting in the large-scale chamber.