Modelling and numerical simulations of heat and mass transfer in multiphase flow during the release of liquid ammonia from a storage tank through the piping system to the open atmosphere

The aim of this study is to develop a contemporary CFD model of the multiphase flow of ammonia from a bulk liquid part of the storage vessel through the release piping to the atmosphere. The model is validated against the experiment with a 12 m3 tank pressurised to 0.5357 MPa(g), i.e. Test No.4 of 560 s duration conducted by INERIS with release from the liquid bulk. The volume-of-fluid (VOF) method is applied for shared velocity and temperature of liquid and vapour phases of ammonia. The heat and mass transfer between liquid and vapour is simulated by modified Lee's evaporation-condensation model, accounting for the heat transfer from/to equipment and environment. A unique feature of the model is the phase change mechanism with varying mass transfer rates based on the transient volume fractions of liquid and vapour in a multiphase flow. This allowed to account for computationally unaffordable simulations of changing in time contact surface area between dispersed liquid and gaseous phases and avoid associated numerical complications. The model effectively captures the complex heat and mass transfer phenomena, including the cooling effect of evaporation in the piping and at the nozzle. The value of the time relaxation parameter and a multiplier accounting for the increase of contact surface between liquid and vapour in the pipe flow are defined by the inverse problem method through the comparison of simulations with the experiment. Simulations accurately reproduced experimental pressure and temperature dynamics in the storage tank, piping system and nozzle, and the total released mass of ammonia of 2352 kg.