Two-scale Two-Flux Model for Simultaneous Density Wave and Slug Flow Instabilities

A CLAUSSE; A LÓPEZ DE BERTODANO; M LÓPEZ DE BERTODANO

Washington

Encuentro; 20th International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH-20); 2023

American Nuclear Society

The Two-Flux Model (TFxM) presentedherein is a new variational approach to the 1D incompressible Two-Fluid Model(TFM) averaging based on the drift-flux variables. The insight of this two-scale formulationusing the new Two-Fluid Short Wave Theory (TF-SWT), similar to single phaseShallow Water Theory (SWT), and the Drift-FluxModel (DFM) equationsis that the local and global scales unfold naturally, and the linear stabilityanalyses [9] become separate. Furthermore,the DFM and SWT theories are better understood than the TFM. The objective is to performseparate linear stability analyses for local and system waves, and combined nonlinearsimulations, enabled by the new two-scale formulation. A linear stabilityanalysis shows how the instabilities decouple mathematically into theirrespective local and system scales. First, for the local interfacial waves, aslug flow inertial coupling makes the TF-SWT well-posed parabolic and anexperimental drag correlation turns the model unstable because of a kinematicinstability. Second, aseparate DFM linear stability analysis for a boiling channel results in thewell-known DWI stability boundary map. Finally, nonlinearsimulations of the DWI with and without slugging illustrate the new two-scalecapability in practice. The new well-posed and objectiveformulation, based on two separate and rigorous physical models, demonstratesthe inherent TFM versatility. It also demonstrates that the KHI must beaddressed within the perspective of the TF-SWT to avoid regularization of anill-posed TFM. Finally, it is shown that the unconditionally well-posed andobjective DFM is the best regularization for system analysis and simulations, if local flow details are notdesired.