CFD Based Two-Phase Dispersion Model of Flashing Liquefied Jet and Experiment Validation

摘要

The prediction of the hazardous chemical release is important for the risk assessment on the process safety. Though there are various atmospheric dispersion models, it is still hard to analyze the toxic and/or flammable hazard of flashing liquefied jet due to the rapid phase transition from liquid to gas, which usually happens in the condition of the liquefied gas such as ammonia, LPG, LNG releasing from vessels and/or pipes. Based on the CFD method, we propose a two-phase dispersion model to analyze the flashing liquefied jet with the focus on the gas build-up, variation and distribution. The jet of flashing liquid is directly defined as flow inlet boundary according to the liquid discharge rate. Then the Lagrangian particles are introduced to simulate the movement of the liquid phase as discrete airborne spheres propelled through the gas, or as rectangular blocks that collectively form a thin liquid film on solid objects. Simultaneously, the gas phase deriving from the rapid phase transition is calculated based on the mass and energy equilibrium equations between the gas and the liquid. Then the gas dispersion in the atmosphere is determined by the commonly used species transport equations. And the large eddy simulation is worked for the treatment of turbulence influences. The model validation is conducted against the Lathen gas field experiments NO. EEC 86 - EEC 98, in which the liquefied propane is discharged with the rates of 170 -770 g/s, wind speeds of 0.8 - 2.0 m/s, release duration of 105 -330 s and nozzle diameter of 0.1 m in an open area. First of all, the atmospheric environment is simulated through the boundary conditions defined by the on-site temperature, wind speed/direction and atmospheric humidity. Then, the liquefied propane is introduced into the simulation and the model calculates the two-phase transition, liquid movement and gas dispersion automatically. The predicted downwind concentrations of propane are compared with the monitoring data recorded during the experiments. The maximum concentration, duration of gas existence, concentration trends are analyzed at the different monitoring points. Though the average wind speeds are substituted for the real speeds due to the lack of the on-site information. It is shows that the simulated results are quite in accordance with the observed values. The variation in the concentration fluctuations is quit the same. The simulated concentrations are within the acceptable range. The time when gas appears and departures at the monitoring points is satisfied with the experiments. In general, the model proposed is capable of determining the subsequent two-phase dispersion of flashing liquefied jet. And the next study will be concentrated on the model built-up and validation with the two-phase movement influenced by the obstacles which are not concerned in this study and experiments.