Study On The Transport Characteristics Of Water Mist And The Mechanism Of Explosion Suppression In Gas Pipeline

Due to the electrostatic spark producing in the pipeline as well as other factors such as possible fire source,low concentration gas is very dangerous in the process of delivery.And once exploding,it will cause great damages.At present,adding water mist to the low concentration gas in transportation is a common practice to assure the safety of it,and this method is also widely applied in engineering field.But most existing studies about explosion suppression mechanism of tube water mist do not consider droplet concentration distribution and the influence of factors such as surface subsidence.Meanwhile,it is still unclear whether the water mist will promote blasting under the conditions of different water mist particle size and obstructions,and so is its promoting mechanism,which is the key to design water mist explosion suppression system.Based on the urgent need of water mist explosion suppression system designing,considering the spatial distribution and deposition characteristics of the water mist,this study sets to do an in-depth research on the internal explosion suppression mechanism of water.It will provide a theoretical basis and technical support to water mist adding to low concentration gas so as to promote the development of water mist explosion suppression technology.The main research contents and conclusions are as follows:(1)First,movement differential equation of the water mist particle is established according to Newton's second law,based on the theory of gas-liquid two phase flow and the force analysis of the water mist particles in the two phase flow field.With the result of the movement differential equation,the main influence factors and influence laws of the settlement characteristics of the water mist in the pipeline are explained.Research has found that with a fixed spray the main factors affecting the characteristics of the distribution and settlement of water mist in the tube are air velocity,spray pressure and water mist particle size.And it is found that with greater water mist particle size and spray pressure,the acceleration is slow under the action of a certain air velocity and the farther the distance required to achieve the state of suspension,the more likely the settlement resulted in the pipe.Moreover,increasing the air velocity can shorten the time of its reaching the state of suspension,thus reducing sedimentation rate.Second,experimental research of water mist distribution characteristics using PIV testing system found that the vortex structure on the cross section and vertical section in the gas liquid two phase flow field has a greater influence on the characteristics of water mist distribution in the pipeline.With a fixed air velocity,smaller particle size water needs more spray pressure for an even distribution,and the larger particle size water mist need smaller spray pressure.(2)Considering its spatial distribution and sedimentation characteristics,the dynamic influencing characteristics of water mist particle size and its concentration,pipe size and obstructions on the flame propagation speed and the explosion pressure in the explosion was studied with the water mist explosion suppression experimental system.And the influence law of water mist under different conditions on the gas explosion in the pipe was comprehensively explored.With the distribution and concentration of the three types of water mist being the same,study found that when water mist of 45 ~ 160 microns diameter was added to the pipe,the water mist promoted the gas explosion;its explosion promoting ability decreased with the decrease of gas concentration and length to diameter ratio of the pipeline.With water mist concentration under the same conditions,the ultrasonic water mist,(about 5 microns particle size),showed good effect on explosion suppression,and its suppression effect would get better with the increase of water mist concentration.When the water mist concentration was 1176.5 g/m3 in the pipe,the gas could not be lighted.But under the condition of the obstacle,45 ~ 160 microns water mist would suppress the blasting,and the closer the location of the obstacles from the ignition source,the better its explosion suppression effect.(3)The numerical calculation of explosion flame temperature,rate of liquid evaporation rate,the change rule of gas-phase reaction,droplets-flame coupling effect and the process of droplet breakage was carried out respectively using large eddy turbulence model,EDC combustion model and VOF model.Results showed that ultrasonic water mist in the gas pipeline could completely evaporate,showing very good explosion suppression effect;But for water mist of 45 ~ 160 microns diameter,it could not completely evaporate in the pipe and explosion airflow velocity was not enough to make the droplets break;its explosion flame front would fold and transform repeatedly under the action of water mist droplets;the flame surface area increased rapidly and turbulent intensity of the gas phase would also become significantly larger,which all helped increase the reaction rate,resulting in the increase of the gas explosion strength.If the water mist droplets broken rate was met in the tube,water mist droplets would be crushed under the action of explosion airflow,the small droplets after crushing could have very good explosion suppression effect.The conclusion of this study will provide a new basis for the comprehensive analysis of water mist explosion suppression mechanism;it will also supplement and advance the existing gas explosion suppression theories.On the other hand,it can provide theoretical basis and technical support for the technology of adding water mist to the gas pipeline.