Diffusion Mechanism Of Heavy Gas Sulfur Hexafluoride Leakage In Underground Space

Heavy gases are gases with relative molecular mass greater than air,such as chlorine,sulfur hexafluoride（SF₆）,etc.They are widely used in chemical industry,material science,environmental protection technology and energy field,among which,sulfur hexafluoride has an irreplaceable role in underground hydropower plants.However,in the past decades,several accidents caused by heavy gas leaks such as poisoning,fire and explosion have caused serious damage.Heavy gas leakage accidents in underground spaces have their special characteristics due to their enclosed environment,therefore,it is necessary to study the mechanism of heavy gas leakage dispersion in underground spaces in depth in order to develop better preventive and countermeasures.In response to the lack of current research on the influence of leak port characteristics,obstacles in space and other factors on leak dispersion,this paper solves the control equations discretely by establishing a numerical computational model of indoor sulfur hexafluoride leak dispersion and using the finite volume method.The influence of leakage direction,height,shape of leakage outlet and indoor obstacles on indoor sulfur hexafluoride leakage dispersion is analyzed for single source leakage;the influence of factors such as the number and spacing of leakage outlets on indoor sulfur hexafluoride leakage dispersion process is discussed for multiple source leakage.The following conclusions were obtained:（1）The jet velocity decays fastest in the upward direction of the leak,decaying to1/10 of the initial velocity at x/d=42（dimensionless distance）;within ten minutes of the leak,the most dangerous when the direction of the leak is downward,and the concentration of sulfur hexafluoride at the breathing surface is 21.8%higher than the other directions,and the direction will be horizontal so that the concentration of sulfur hexafluoride is higher on the downwind side of the room.（2）The relative position of the leak and the breathing surface will have a great impact on the concentration of the breathing surface.Within ten minutes of sulfur hexafluoride leakage,the concentration of sulfur hexafluoride at the breathing surface is the lowest when the height of the leakage port is 1 m,and the concentration at the bottom of the room is the highest;when the height of the leakage port is in the range of 3 m to 9 m,the concentration of sulfur hexafluoride at the breathing surface and the bottom of the room decreases sequentially with the increase in height.（3）The concentration field formed by the diffusion of leaks of different shapes is different.In the case of equal leakage velocity and leakage area,compared with the circular shape,the slit-shaped leaks will lead to higher concentrations in the room,and the highest concentration on the detection surface will rise by 7%.（4）Obstacles will change the trajectory of heavy gas movement,resulting in its accumulation on the surface of the obstacle,between the obstacle and the wall,when the height of the obstacle exceeds the critical value,the velocity field and concentration field caused by the height of the obstacle changes very little.（5）The effect of different spacing on the velocity field near the jet port is more significant,and the effect on the concentration of sulfur hexafluoride at the breathing surface is smaller;multi-hole leaks are more dangerous than single-hole leaks,and the average concentration of sulfur hexafluoride at the breathing surface and bottom of the room is proportional to the number of leaks,which increases with the number to make the concentration of sulfur hexafluoride rise by 8.4～14.3%.The work of this paper can provide some reference basis for the prevention of sulfur hexafluoride gas leakage accidents in underground space and targeted ventilation methods.