CFD Simulation Studies Of Dispersion Of Combustible Gas In The Atmospheric Environment Under Different Stability Conditions

As a widely used gas fuel,natural gas may be released accidentally in the process of storage,transportation and use.Once the leakage happens,gas concentration gradually increases to the flammable limit,which potentially causes fire and explosion accidents.If these accidents occur in chemical industrial parks,it often results in a series of secondary disasters,such as the dispersion of toxic and harmful substances in the atmosphere.Therefore,understanding the patterns of atmospheric diffusion of hazardous gas can provide necessary guidance for loss prevention,risk assessment and emergency evacuation.In this study,in order to get insight into the patterns of gas dispersion in atmospheric environment,methane was used as the tracer gas,and a CFD model,Fluidyn-Panache,based on Reynolds average(RANS)method,was used to study the CH4 diffusion under different meteorological and topographic conditions.After analyzing the structure,classification and turbulence of the atmospheric boundary layer,characteristics of turbulent motion and temperature distribution near the ground within different types(unstable,neutral and stable)of boundary layer were described based on the Monin-Obukhov similarity theory.Firstly,the effectiveness of simulation results from FluidynPanache was validated by the data of the wind tunnel experiment and foreign MUST field experiment.The results show that Fluidyn-Panache can be used for simulating gas dispersion in flat terrain and urban-like environment under different stability conditions.Secondly,the diffusion process of CH4 on three different terrain(flat,isolated building and regular building array)under different stability conditions were studied in this work.Moreover,in the case of flat terrain,the influences of different wind velocity(0.4,0.9 and 1.5m/s)on CH4 diffusion were also considered.Main conclusions of the study are as follows.1.Gas dispersion is significantly affected by wind velocity under different atmospheric stability conditions.When wind velocity is reducing,the LFL/2 distance and spreading range of the gas cloud will increase,and a greater peak concentration can be maintained at a distance far downwind.A greater wind velocity can accelerate the dilution of the cloud.When atmosphere is in stable conditions and wind speed is low,the atmospheric diffusion ability is usually the worst.2.Building can significantly hinder the gas transmission in the downwind direction,and the mechanical turbulence enhances the mixing between CH4 and fresh air.An interplay flow field will appear in the regular building array with a small aspect ratio.The vortex recirculation caused the retention of CH4 inside the street canyon,which reduces the spread in the downwind direction but aggravates local pollution.The deflection of CH4 plume within the building array is noteworthy when the incident flow is oblique.3.The influences of atmospheric stability on dispersion and flow are similar in different cases.Flow patterns and turbulence around a building are discrepant under different stability conditions.CH4 diffusion is suppressed because atmospheric turbulence is weak under stable conditions.However,when atmosphere condition is unstable,intense turbulent motion contribute to gas dilution.This also results in the difference in the consequence when hazardous gas accidentally releases.