Numerical modeling of horizontal annular pipe flow using a droplet entrainment model

Annular flow occurs in many industrial processes, and is characterized by high gas flow at the center of the pipe and liquid film flow around the pipe wall. Due to the high gas velocity, large shear velocities are induced that result in high interfacial shear stress causing continuous entrainment of liquid droplets into the gas core from the liquid film. When the liquid fraction is small in horizontal annular flow, it is possible to cause an extremely important problem that relates to the damage of heat exchanger tubes, because the drainage of liquid due to gravity, as well as the evaporation, leads to the dry-out of thin liquid film near the top of the tube. Therefore, it is important to accurately predict the circumferential distribution of film thickness in horizontal annular flow. Furthermore the thin water film at the cold wall plays a major role for the heat transfer resistance of the condensation process. For better understanding of condensation heat transfer the film formation mechanism and the film distribution need to be known.
One limitation in current simulating horizontal annular flows is the lack of treatment of droplet formation mechanisms. For self-generating annular flows in horizontal pipes, the interfacial momentum exchange and the turbulence parameters have to be modelled correctly. Furthermore the understanding of the mechanism of droplet entrainment in annular flow regimes for heat and mass transfer processes is of great importance in the chemical and nuclear industry.
The development of general computational fluid dynamics (CFD) models closer to physics and including less empiricism is a long-term objective of the activities of the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) research programs. Such models are an essential precondition for the application of CFD codes to the modelling of flow related phenomena in industrial scales. The algebraic interfacial area density model (AIAD), which is one result of these HZDR activities, allows the use of different physical models depending on the local morphology inside a macro-scale multi-fluid framework.
A further step of improvement of modelling interfaces is the consideration of droplet entrainment mechanisms. The proposed entrainment model assumes that due to liquid turbulence the interface gets rough and wavy and forms droplets. The new approach is validated with HZDR annular flow experiments. Important phenomena like the pressure drop, the wave pumping effect, the droplet entrainment rate, the liquid film formation and the transient flow behavior could be calculated, analyzed and compared with the measurement. Verification and Validation is going on – more experimental data are required for the validation.