Numerical investigation of slug flow in a horizontal pipe using a multi-scale two-phase approach to incorporate gas entrainment effects

Numerical as well as experimental investigations of the highly intermittent slug flow regime of a gas-liquid mixture in horizontal pipes are of particular interest for nuclear reactor safety in post loss-of-coolant accident (LOCA) situations. The strong variation of governing interfacial length scales, as they are characterizing the slug flow regime, pushes common numerical multi-phase approaches to their limits, since they are designed either for interface capturing or for modeling the subgrid behavior of the dispersed mixture. In this work an enhanced hybrid two-phase flow solver is employed to investigate the global and local characteristics of adiabatic, horizontal slug flows in a water-air system. A dynamic switching algorithm for an interface capturing procedure is introduced to examine segregated and dispersed parts in the same flow domain. The interfacial area transport equation (IATE) is used to detect dispersed flow regions as well as to determine variable bubble sizes and their distribution within the slug body. Experimental results of videometry measurements on a horizontal, 10 m long pipe with an inner diameter of 54 mm at atmospheric pressure and room temperature are compared with numerical results of the same geometry in terms of global characteristics such as slug frequency and onset position. Local properties, such as the interfacial area density in the slug body, are also examined. This study demonstrates the capability of a coupled multiscale approach based on the Euler-Euler two-fluid model (TFM) for the simulation of slug flow in horizontal pipes with a high amount of entrainment.