Validation of closure models for interfacial drag and turbulence of horizontal segregated flows


Validation of closure models for interfacial drag and turbulence of horizontal segregated flows

Höhne, T.

In the last decade, applications of Computational Fluid Dynamic (CFD) methods for nuclear applications received more and more attention, as they proved to be a valuable complementary tool for design and safety. The main interest towards CFD consists in fact in the possibility of obtaining detailed 3D complete flow-field information on relevant physical phenomena at lower cost than experiments. Typically free surfaces manifest as stratified and wavy flows in horizontal flow domain where gas and liquid are separated by gravity. Stratified two-phase flows are relevant in many nuclear applications, e.g. pipelines, main coolant lines, horizontal heat exchangers and storage tanks.

CFD simulations for free surface flows require the modeling of the non-resolved scales. For modeling of interfacial transfers it is necessary to select the adequate interfacial transfer models and to determine the interfacial area. The numerical solution can resolve the statistically averaged motion of the free surface (including waves) which may not be too small relatively to the channel height and to the characteristic length of the spatial discretization. However, the detailed structure of interacting boundary layers of the separated continuous phases and surface ripples cannot be resolved. Instead, its influence on the average flow must be modeled.

The development of a general model 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 modeling of flow related phenomena in nuclear facilities. Here local geometry independent models for mass, momentum, heat transfer, and scalar transport are developed and validated. The new formulation for the drag force at the free surface within the algebraic interfacial area density model (the FSD model inside AIAD) is one result of these activities.

A further step of improvement of modeling the turbulence is the consideration of sub-grid wave turbulence (SWT) that means waves created by Kelvin-Helmholtz instabilities that are smaller than the grid size. So fare in the present code versions they are neglected. However, the influence on the turbulence kinetic energy of the liquid side can be significantly large. A region of marginal breaking is defined according Brocchini and Peregrine (2001). In addition turbulence damping functions should cover all the free surface flow regimes, from weak to strong turbulence.

CFD validation of the new approach was done using experiments of the HZDR HAWAC channel. A discussion of the general requirements of such CFD grade experiments was performed. The CFD calculations were done using the Best Practice Guidelines for two-phase flow modeling. One result of the simulations was that the sub-grid wave turbulence which exists in the area of the free surface follows the slug formations. At the wavy front and back of the slugs the value of the sub-grid wave turbulence is the highest in the channel. The slug frequency analysis was done using fast Fourier transform (FFT). The characteristic slug frequency of the simulation was around 2.0 Hz, which corresponds roughly to the experimental value of approximately 2.4 Hz. The model improves the physics of the existing two fluid approaches and is already applicable for a wide range of industrial two phase flows.

More verification and validation of the approach is still necessary – more CFD grade experimental data are required for the validation.

Keywords: CFD; AIAD; HZDR; two-phase flow

  • Contribution to proceedings
    CFD4NRS-5, Application of CFD/CMFD Codes to Nuclear Reactor Safety and Design and their Experimental Validation, Joint OECD/NEA & IAEA Workshop, 09.-11.09.2014, Zürich, Schweiz
    CD-Rom
  • Lecture (Conference)
    CFD4NRS-5, Application of CFD/CMFD Codes to Nuclear Reactor Safety and Design and their Experimental Validation, Joint OECD/NEA & IAEA Workshop, 09.-11.09.2014, Zürich, Schweiz

Permalink: https://www.hzdr.de/publications/Publ-20823
Publ.-Id: 20823