Flash boiling and pressure recovery phenomenon during venting from liquid ammonia tank ullage

Abstract

Liquid ammonia stored at elevated pressures undergoes phase change during tank venting. The paper describes a CFD model able to reproduce the INERIS experiment performed using 12 m3 volume tank when 300 kg of ammonia were vented during 460 s from the ullage through the pipe to the atmosphere. The model is based on the volume-of-fluid method combined with Lee’s model for mass transfer between phases. Extensive calibration trials were conducted to examine the impact of surrounding temperature conditions. It allowed to establish the range of time relaxation parameter values of Lee evaporation-condensation model specific for liquid ammonia tank venting. Simulations reproduced accurately the experimentally measured final pressure in the tank and the amount of ammonia released during venting. Liquid ammonia boiling is triggered by reduction of pressure and takes place throughout the liquid causing the pressure recovery phenomenon. Numerical simulations demonstrated that pressure recovery phenomenon is driven by the characteristic time delay associated with the rise of vapour bubbles generated by flash boiling and the subsequent release of evaporated ammonia into the tank ullage space. The developed CFD model can be used as a contemporary tool for safety engineering and development of tank management strategies for liquid ammonia storage.