Study On The Evolution Mechanism Of Two-phase Cloud From An Instantaneous Release

It is usual in industry or common life that certain dangerous materials are stored as liquid form in pressurized containers. These materials are often toxic, flammable, or both. Herein, any accidental release can results in potentially disastrous consequences. Although not the most common one, instantaneous flashing release is the most severe mode, which often has explosive characteristics. Although some relevant studies for this issue have been done, some mechanisms of the accidents evolution are not clear and there still are lots of problems. The study for the evolution of two-phase cloud by release can provides scientific basis for the optimal design of the safety system, and it is critical to predict these release by reliable tool.The study on the mechanism of the two-phase cloud evolution from an instantaneous release was presented in this paper. On the basis of super-heat theory, more properties including the phase change latent and saturated vapor pressure were considered in the calculation of the droplets evaporation, with which the model of the heat and mass transfer between droplets and gas-phase. The model for the prediction of cloud expansion was built by the Computational Fluid Dynamics (CFD) approach, and this model was used for the simulation of the small scale instantaneous release by Pettitt in 1990. To validate the optimal performance and scientific properties of model, the averaged velocity of droplets from the calculations of Freon cloud was used to directly compare with experimental data. The model can provides the variation of the microscopic and macroscopic physical change in cloud expansion, and the performance of this model is better than some relevant research before.The simulated results of the Freon release indicate that the expansion of Freon cloud is not violent, and the some important calculated results including cloud size, the distribution of species concentration and temperature, droplets residence and evaporation rate indicated that the first about 0.4 s can be identified as the cloud formation stage, and some phenomena including rapid increasing of cloud size, sharp decreasing of temperature, the distribution of species concentration and temperature tend to inhomogeneous is the stage characteristics of this initial expansion process.The dispersion process of different dangerous materials including liquefied chlorine, liquefied ammonia and liquefied petroleum gas were simulated by successful model in same condition to analysis the characteristics of the expansion process. By the comprehensive analysis of calculated results about droplets and cloud, the simulated results indicate that the law of the cloud evolution is similar with Freon cloud, although the superheat of these three materials is much bigger than Freon's. Because of the violent phase change, the expansion and the decreasing of temperature is more significant than Freon cloud. It is found that the phase change latent is an important factor for the initial cooling.The further analysis of the release consequence was made for three dangerous materials by the reference of damage criterion of each material. According to criterion published by governmental institute, the range of toxic influence of chlorine and ammonia cloud from instantaneous release was predicted. The simulated results show that the chlorine cloud is more dangerous than ammonia's, and the evolution of flammable zone and size of LPG cloud was also presented in this paper.The work presented in this paper develops the evolution mechanism of two-phase cloud from an instantaneous release of liquefied gases. This paper not only rich the mechanism of cloud evolution, but also provides a reliable tool and theoretical basis for the prediction of consequences of vapor explosion accidents and relevant safety system.