A flashing flow model for the rapid depressurization of CO2 in a pipe accounting for bubble nucleation and growth

Flashing flow is encountered in many industrial systems involving nozzles, valves and decompression of vessels and pipes. In the context of CO₂ capture and storage (CCS), the design of safe and efficient CO₂ transportation systems requires accurate flashing models, e.g., for safety analysis of pipe fractures and to predict the mass flow through relief valves. We propose a homogeneous flashing model (HFM) for flashing flow accounting for the underlying physical phenomena of the phase change: bubble nucleation, coalescence, break-up and growth. Homogeneous nucleation is modeled using classical nucleation theory and heterogeneous nucleation is approximated with constant rates of bubble creation and mass transfer from liquid to vapor. The flashing flow model is fitted for CO₂ pipe depressurization data at various initial conditions. We find that the same, constant model parameters can be applied for the whole set of depressurization cases considered, as opposed to the conventional homogeneous relaxation model which typically is tuned on a case-by-case basis. For depressurization paths where the fluid state passes close to the critical point, we demonstrate that an accurate description of the flashing process along the length of the pipe can only be achieved when both homogeneous and heterogeneous nucleation are accounted for.