Research On Dynamic Characteristics And Theoretical Model Of Premixed Methane-air Explosion Flames In Ducts

Natural gas(the main component is methane with more than 85%volume fraction)plays an irreplaceable role in the industrial and daily usage.However,it should not be ignored that serious explosions occur when natural gas is transported by buried pipelines.The construction of a separate natural gas compartment in the underground utility tunnel can effectively prevent and control such explosions.The scientific vent layouts of natural gas chambers have a significant impact on the prevention and control effectiveness.The evolution laws of characteristic parameters(such as flame structure,flame propagation speed and overpressure)of premixed methane-air explosion in venting ducts are revealed under the coupling effects of key influencing parameters,and then prediction methods of explosion overpressure related to natural gas compartment vents are proposed,which have important theoretical values and practical significance.In terms of current research,under effects of different L/Ds of ducts,vent area and vent layout,evolution laws of flame structure,flame propagation speed and overpressure of premixed methane-air explosions in venting ducts still need to be researched in-depth;the existing vent design standards are not fully applicable to all conditions.Therefore,in this paper,the flame structure,flame propagation speed and overpressure of premixed methane-air explosions in different venting ducts are taken as research objects,evolution characteristics of flame structure,as well as dynamic characteristics of flame propagation speed and overpressure in different flame propagation stages are researched deeply,evolution laws of flame structure,flame propagation speed and overpressure are revealed under effects of different key parameters,effective vent layouts are determined,constructing methods for prediction models of maximum explosion overpressure Pred are proposed,and Bychkov’s theoretical models for characteristic time and position of premixed flame dynamics are improved.The following researches are carried out:(i)The design of visualized experimental platforms for premixed methane-air explosions in ducts.Four types of end-vented straight ducts with L/D of 5,10,15 and 20,and one type of ducts with two side-vented channels are designed and built independently.By scientifically equipped with synchronous control system,real-time automatic gas distribution system and flame sensor,the key problem with low repeatability of experimental results due to the unsynchronized operation of experimental equipment has been overcome.To provide platform supports for experimental researches in Chapters 3,4 and 5.(ii)Research on dynamic characteristics of methane-air explosions finger flame in venting straight ducts.Experiments have been conducted to research dynamic characteristics of premixed methane-air explosion finger flames in end-vented straight ducts with length of 500 mm(L/D=5).Evolution laws of finger flame structure,flame propagation speed and overpressure are investigated deeply,under the effects of different methane volume fractionsφ(CH4),vent area Av and obstacle layout.The results show that only hemispherical and finger flames are generated in the duct with L/D of 5,the most severe explosion is observed whenφ(CH4)is 9.5%,both finger flame deformation degree and maximum explosion overpressure increase with decreasing obstacle diameter and increasing installation height.A new dynamic stage(multi-wave interaction stage)is proposed.Consistent evolutionary relationships among finger flame structure,flame propagation speed and overpressure are revealed during the vent failure stage,multi-wave interaction stage and maximum flame area stage.Data supports for the comparison of experimental results in Chapter 4,and the construction of prediction models of Pred and flame dynamic characteristics in Chapter 6.(iii)Research on dynamic characteristics of methane-air explosion tulip flame and distorted tulip flame in venting straight ducts.Experiments on dynamic characteristics of premixed methane-air explosion tulip flames and distorted tulip flames in end-vented straight ducts with L/D of 10,15 and 20 have been conducted,respectively.Evolution laws of plane flames,tulip flames and distorted tulip flames,flame propagation speed and overpressure are investigated deeply,under the effects of different L/D and Av.The results show that distorted tulip flames are generated in ducts with L/D of 15 and 20 when the interval time tinv and the corresponding interval distance Zinv between the plane flame and the flame skirt touching sidewalls time are 0.5 ms～4 ms and 4.71 mm～56.76 mm,respectively.Both the drop size and drop ratio of Pred increase with increasing Av,but decrease with increasing L/D.Consistent evolutionary relationships among the corresponding flame structure,flame propagations speed and overpressure are revealed during tulip flame stage,distorted tulip flame stage and irregular tulip flame stage,individually.New overpressure peaks Pmwi,Pdis-tulip and Pirr-tulip due to multiwave interactions,distorted tulip flame and irregular tulip flame are identified,respectively.Data supports for the construction of prediction models of Pred and flame dynamic characteristics in Chapter 6.(iv)Research on dynamic characteristics of methane-air explosion wrinkled flame in a duct with side-vented channels.Experiments on dynamic characteristics of premixed methane-air explosion wrinkle flames in a duct with two side-vented channels have been conducted.Evolution laws of wrinkled flames,flame propagation speed and overpressure are investigated deeply,under the effects of different side vent layouts and vent areas.The results show that Helmholtz oscillation stages and oscillating wrinkled flames are generated in the duct when only side vent 1 is open and the side vent area is 100 mm × 100 mm～60 mm × 60 mm.Consistent evolutionary relationships among the oscillating wrinkled flame structure,oscillating flame propagation speed and oscillating overpressure is proposed as a method that can accurately determine the Helmholtz oscillation stage.It is deduced that the interaction between reflected pressure waves,wrinkled flames and side-vented channel 2 is the intrinsic factor during the severe internal turbulence stage.Effective side vent layouts are determined:all side vents and end vent are open,side vents 1 and 2 are open,and side vent 1 and end vent are open.Data supports for the construction of prediction models of Pred in Chapter 6.(v)The prediction models of overpressure and theoretical models of premixed flame dynamic characteristics for methane-air explosions in ducts.Based on the mathematical relationships between Kv/(L/D)、V1/3/D、Av and Pred,a prediction model for Pred of methaneair explosion in end-vented straight ducts is established.Based on the mathematical relationship between Static activation pressure of vent cover Pstat,Vent coefficient Kv and Pred,four Prediction models for Pred of methane-air in ducts two side-vented channels were established under four side vent layouts,respectively.Based on the experimental results,Bychkov’s theoretical models that more accurately predict the characteristic time τwall and characteristic position ζwall when finger flame skirts touch tube sidewalls are improved,respectively,the δvalue(0.8376)in Bychkov’s theoretical model of the plane flame time τplane was modified,the range of β values in the Bychkov’s theoretical model of the plane flame position ζplane is further determined to be 0.4～0.5.To provide methodological and theoretical support for vents design of natural gas compartments.