Contents lists available at ScienceDirect Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng A hybrid multiphase flow model for the prediction of both low and high void fraction nucleate boiling regimes Muritala Alade Amidu a,c , Yacine Addad a,c, ⁎ , M.K. Riahi b,c a Department of Nuclear Engineering, Khalifa University of Science and Technology (KU), Po Box 127788, United Arab Emirates b Department of Mathematics, Khalifa University of Science and Technology (KU), Po Box 127788, United Arab Emirates c Emirates Nuclear Technology Center (ENTC), Khalifa University of Science and Technology (KU), Po Box 127788, United Arab Emirates HIGHLIGHTS • A hybrid VOF-Eulerian multifluid model is presented. • A turbulent rough wall function is proposed to enhance the prediction of the velocity profile. • The hybrid model reasonably predict the features of low void fraction flow boiling. • The hybrid model could captures large scale interface in high void fraction flow boiling. • Need to extend the wall boiling model to accommodate heat transfer beneath vapor slugs. ARTICLE INFO Keywords: Wall function, hydrodynamic roughness, subcooled flow boiling Hybrid model Dispersed flow Slug flow ABSTRACT The improvement of a hybrid (a combination of Volume-of-Fluid (VOF) and Eulerian model) multiphase flow solver for the numerical prediction of high- and low-void fraction flow boiling regimes is presented in this article. These flow regimes could simultaneously occur in core-catcher and in-vessel retention-external reactor cooling systems during a severe accident in nuclear power plant. To enhance the prediction of the low void fraction boiling regime, a turbulent rough wall function model is implemented in the hybrid model to reproduce the impact of coarseness induced by the existence of growing bubbles along the heating wall on the liquid velocity profile. With this wall function, a more accurate prediction of the radial velocity profile is achieved within the uncertainty of the velocity measurements. Moreover, an improved prediction of the radial void fraction is achieved using the model proposed by Lopez de Bertodano for turbulence dispersion force without compromising the prediction of the radial gas velocity profile and radial liquid temperature profile. Although the hybrid model shows potential in capturing the interface and dynamic behavior of large-scale bubbles (vapor slug) for high void fraction regime, the predicted wall superheat is higher than the measured values. This highlighted the need for the extension of the present wall boiling model to cover flow boiling involving sliding vapor slugs on the heated wall. 1. Introduction In the last decade, computation fluid dynamics (CFD) code has been applied for the safety analyses of nuclear power systems. This is espe- cially useful for the safety analyses of severe accident scenarios where accurate predictions are needed to evaluate the performance of severe accident management systems. Examples of such severe accident management systems are core-catcher and In-Vessel Retention-External Reactor Cooling (IVR-ERVC). The boiling heat transfer in such severe accident management systems is characterized by the presence of both a low and high void fraction as the heating surface is facing the downward direction. The complex nature of the interface in a high void fraction regime, somehow, discouraged the research activity in this area. However, most of the reported advancements in multiphase flow modeling centered on the prediction of bubbly or dispersed flows and these advancements in bubbly flow are not adequate to meet the needs of the current severe accident management systems involving different boiling regimes (dispersed, slug, and annular flow regimes) occurring all together. Therefore, a physical modeling concept for regimes in- volving high void fraction needs to be examined to fully capture the https://doi.org/10.1016/j.applthermaleng.2020.115625 Received 11 September 2019; Received in revised form 28 May 2020; Accepted 17 June 2020 ⁎ Corresponding author at: Department of Nuclear Engineering, Khalifa University of Science and Technology (KU), Po Box 127788, United Arab Emirates. E-mail address: yacine.addad@ku.ac.ae (Y. Addad). Applied Thermal Engineering 178 (2020) 115625 Available online 23 June 2020 1359-4311/ © 2020 Elsevier Ltd. All rights reserved. T