Study On Hazardous Liquefied Gas Release And Dispersion In Urban Environments

The scientific knowledge of hazardous liquefied gases release and dispersion in urban environments is the prerequisite for accurate assessment and precise rescue of such accidents.The flows resulting from liquefied gases release are in multi-phase,and the process can be divided into three stages,namely liquefied gas discharge,two-phase cloud dispersion,and gas turbulent dispersion.In this paper,the characteristics of leakage source and the dispersion and evolution mechanism of two-phase cloud were studied with liquefied carbon dioxide(CO₂).Furthermore,the characteristics of CO₂turbulent dispersion in urban environments and the influence factors were investigated.Based on the results,the dangerous area of liquefied choline(Cl₂)leakage accident in urban areas were determined.To investigate the characteristics of leakage source and the dispersion and evolution mechanism of two-phase cloud,the experiment apparatus for two-phase cloud release and dispersion process test was established.In the experiment,the storage temperature of liquefied CO₂and the cloud temperature at the source were measured.The parameters at the source,such as vapour mass fraction,injection velocity and mean droplet diameter,were calculated based on thermodynamic method.Discarding the pseudo-source concept and considering the relationship between droplets phase transition and cloud concentration distribution,the numerical model for the dispersion of CO₂two-phase cloud was established.The cloud temperature measured in downwind area in the experiment was used to verify the results of the numerical model,and the maximum relative error between them was 5.8%.The movement and phase transition of droplets can promote the dispersion of cloud.The maximum dispersion distance along wind direction and crosswise direction obtained by two-phase model were larger than those by single-phase model,with the relative differences of 57.8%and 53.6%respectively.The results can reflect the necessity of establishing two-phase model.The numerical model for the dispersion of CO₂two-phase cloud with water phase transition was established,and the influences of water phase transition on characteristics of two-cloud dispersion process were studied.The results show that the maximum dispersion distance was increased when taking the influence of water phase transition into account.To study the CO₂ dispersion characteristics in urban environments and the influence factors,the wind tunnel for dispersion in urban environments test was designed and constructed.The profile of average wind velocity and turbulent structure in the urban boundary layer were provided,with the power-law exponent of 0.308.Fog experiments were conducted to seek a qualitative analysis for the flow structure in street canyons and the influencing mechanism of the key parameters.In CO₂dispersion experiments,the sensors were hung at the typical points of the canyon to acquire the CO₂concentration data.The experimental data of wind tunnel enriched the essential data in this research area and have important guiding significance for building the gas dispersion model in urban area.Based on the atmosphere boundary theory,the calculation model for CO₂dispersion in urban environments was established with SST k-?model,which was ascendant for gas dispersion in urban terrain.The three-dimensional flow field in the canyon were simulated,the results show that the flow structure contained various types of patterns,including vertical circulation flow,horizontal vortex and the channeling flow.Based on these results,the CO₂dispersion characteristics and influence factors were investigated.The results show that the entire releasing and dispersing process can be divided into two stages.At the beginning of the release,CO₂was jetted vertically upwards from the source at an initial speed,and the motion of cloud was determined by jet speed and wind.The next stage began with the collapse of cloud.The gravity and wind together influenced the concentration distribution of CO₂.CO₂behaved as dense gas,and only 0.5%of volume rate of flow was removed through the top opening.Under the vertical vortex,the CO₂accumulated along with the leeward side.Increasing the windward-side building height and aspect ratio,the CO₂volume fraction were reduced from 22.9%to14.5%and 15.9%respectively.The numerical calculation model for two-phase cloud release and dispersion in urban environments was established,through which the integration of three stages of liquefied gases release and dispersion was implemented.The droplets phase transition and cloud dispersion of Cl₂and ammonia(NH₃)were studied.The results show that the evaporation time of Cl₂droplets is shorter than that of NH₃droplets due to the former one has lower latent heat of vaporization and the smaller diameters than later one.Meanwhile,cloud concentration and droplet number density at the windward side is higher than that at the leeward side.Based on the risk levels of 0?14ppm?14?35ppm?35?900ppm and?900ppm,the dangerous area for liquefied Cl₂leakage accidents in street canyon was determined.In addition,the influencing mechanism of the key parameters was studied.The results show that the leakage rate and wind velocity had an important impact on the dangerous area.The effect of ambient temperature on leakage source was major,but the effect on dangerous area was minor.The dangerous area in the early stage decreased with the decreasing street length.The results can further enrich the database in this research field,and bring insight into two-phase cloud dispersion characteristics and the law of gas dispersion process in urban environments.The conclusion has very important guiding significance for the consequence prediction and emergency response to such accidents.