Numerical Simulation For LNG Release&Dispersion From Large Scale Tank

With the prompt growth of liquefied natural gas (LNG) trades, more and more LNG projects and storage tanks have been set up, the risk of LNG release from tanks greatly increased. Due to the fact that a great deal of flammable, explosive and low temperature LNG was stored in large-scale tank, the accident of LNG release from the large-scale storage tank will pose a disastrous risk to people, structures and atmospheric environment in proximity to such release. The modeling of LNG release from tank and vapor dispersion in air can provide scientific guidelines for site of LNG receiving terminals, establishment of emergency response plan, delimitation of exclusion zones and evaluation&claims of hazard suffered from accident.In order to choose an appropriate model or modeling tool among a wide range of vapor dispersion models, numerous previous research works were firstly referred to, the methodologies for quantitative assessing the validity of LNG dispersion models were set up. The suitability of Fluent for modeling LNG dispersion was quantitatively evaluated by means of the datasets from the large scale LNG release field trials. The results of evaluation indicated that the Fluent-based numerical dispersion model was more "valid" in predicting dispersion of LNG than the others.In view of great influence of air flow in the vicinity of storage tank on the LNG release and dispersion, wind field simulation of LNG tank become necessary. So the flow characteristics and main Parameters of Atmosphere boundary layer were analyzed, the calculating methods of mean wind profile and turbulence characteristics Parameters were decided and their User-Defined Function (UDF) was written. After this, the calculating domain of numerical simulation was set up according to the theory of Computational Wind Engineering (CWE), and the technology of partition refinement was used in domain meshing and grid quality controlling. The numerical simulation of wind field showed:the flow-field around LNG storage tank is highly complicated, which is filled with series of complicated movements, such as separation, circulation, vortices etc.In next part of this dissertation, the phenomenology of LNG release and dispersion was investigated, the possible flow state of the internal orifice and its determining criterion were studied, and calculating method of discharge rate from LNG storage tank was solved. The Eulerian-Lagrangian multiphase flow models and the stochastic trajectory model were applied to simulate numerically the movement, mass transfer, heat transfer and coupling of LNG droplets phase and vapor phase. On the basis of the mechanism of heat transfer from the substrate, the model of heat transfer form ground which embodied LNG boiling regime was constructed and programmed with UDF. Successful coupling and calculating of the gas phase model, the droplet phase model and ground heat transfer model made the simulation does not only show three-dimensionally the development of the vapor cloud but also model accurately the important dispersion characteristics of LNG dense gas, such as gravity-spreading, lower and significantly wider cloud, bifurcating and sides-accumulating.Low temperature LNG vapor caused the air to be cooled so that atmospheric water vapor condensed to form a white cloud or fog. In order to study the condensation of atmospheric water vapor by the cold temperature and its effect on the behavior of LNG cloud, Volume of Fluid (VOF) model and species transport model were used to solve respective single-phase flow of steam, water and ice. However VOF model can not solve interphase heat transfer and mass transfer, so condensation, ice formation and sublimation models for water were set up in the light of the mechanism of interphase heat and mass transfer, thus interphase heat and mass exchange was solved too. Results of phase change modeling of atmospheric water vapor suggested that:moisture in air had great effect on phase change zones, but the graphs of concentration against time were very similar-the water vapor apparently has essentially no significant effect.In last part of the paper, the numerical simulation was applied to analyze the influence of dike, buildings, release source and wind on LNG cloud dispersion, Results of application indicated:the dike around tank can effectively contain liquid released from storage tank and constrain down-wind dispersion of vapor. Due to obstructing of buildings, dangerous cloud resulted from LNG release tended to accumulate around buildings, and this would pose a risk to people who were working in buildings; release rate had important relation with the hazard consequence, the more rapid of release, the more serious consequence was. All these researches could serve to provide scientific guidelines for site and design of LNG receiving terminals, establishment of preventive measures and emergency response plans.