Contents lists available at ScienceDirect Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes Droplets entrainment ratio in a PWR hot-leg pipe geometry Suleiman Al Issa ⁎ , Rafael Macian-Juan Technische Universität München (TUM), Department of Nuclear Engineering, Boltzmannstrasse 15, 85748 Garching, Germany ARTICLE INFO Keywords: PWR Hot-leg CCFL mechanisms Droplets entrainment ratio Empirical correlation CFD validation data ABSTRACT Droplets entrainment was found to contribute to the occurrence of countercurrent flow limitation (CCFL) at COLLIDER test facility which has a PWR hot-leg pipe geometry with a large diameter value of 190 mm. Previous investigations showed that droplets entrainment starts to become observable at medium water inlet velocities (In Wallis parameters > ∗ J 0.2 w in , 0.5 ). In current article, the occurrence of droplets entrainment at COLLIDER test fa- cility is discussed along with a detailed descriptions of visual observations that corresponds to entrainment mechanisms at the junction between the hot-leg and the steam generator. These mechanisms are compared to typical mechanisms found in literature for annular flows. The significance of COLLIDER large-diameter and pipe geometry against rectangular and narrow channel geometry is explained. Experimental measurements of the droplet entrainment ratio at COLLIDER test facility (using air/water at atmospheric pressure) are introduced and discussed. Measurements cover a range of air superficial velocities between 10.6 and 15.2 m/s, and water su- perficial velocities between 0.05 and 0.12 m/s. A correlation of the critical air velocity at which the droplets entrainment start to occur is given. An empirical correlation of the entrainment ratio was developed for obtained data. The correlation shows a stronger dependency upon the water inlet velocity in comparison to correlations found in literature for annular flows. A comparison between the prediction of available entrainment ratio cor- relations in literature and COLLIDER experimental data is made. The comparison shows the need of a new correlation for the droplets entrainment that takes place at the entrance of a hot-leg pipe geometry. The obtained correlations are useful to predict the onset of entrainment and to quantify its possible contribution to the re- duction of water delivery into the reactor core during the occurrence of an SBLOCA accident. The last result becomes of a special importance knowing that CCFL caused by droplets entrainment occurs at lower air velo- cities than those at which a typical CCFL occurs (caused by large roll-waves at the bend), and that droplets entrainment does not require a transition from a supercritical into a subcritical flow condition in order to take place. 1. Introduction 1.1. Droplets entrainment phenomena and its importance Droplets entrainment is a phenomenon of a high importance for annular and annular-churn two-phase flows in vertical and horizontal pipes (Ryua and Park, 2011; Berna et al., 2014; Lopez de Bertodano et al., 2001; Zhang et al., 2015; Barbosa et al., 2002). Churn flow in particular is a highly turbulent flow condition that is s characterized by the presence of a thick, oscillating, and unstable liquid film, and is still one of the least understood of gas/liquid two-phase flow regimes (Wang et al., 2013). Entrained droplets contribute significantly to heat, mass, and momentum transfer (pressure drop) at the gas liquid interface. The accurate prediction of the entrainment fraction is important for many industrial applications such as: boilers, evaporators, condensers of conventional power generating plants, pressurized water reactors (PWRs) during loss of coolant accidents (LOCA), boiling water reactors (BWRs) during normal operation, geothermal, and oil and gas industry (Ryua and Park, 2011; Okawa and Kataoka, 2005; Patruno et al., 2009). The presence of droplets in the gas phase can produce erosion to the equipment as separators may not be sufficient to remove droplets completely and new droplets can be entrained from the existing liquid film beyond the separator. It also an important phenomenon for the estimation and analysis of dryout and post dryout heat transfer, and the depletion of liquid film which is considered as a good approximation to the onset of critical heat flux condition in light water reactors (Ryua and Park, 2011; Sawant et al., 2009). During LOCA accidental condi- tions in PWRs (AP1000 design), droplets entrainment can transfer li- quid from the upper plenum through the reactor internals and the hot- leg into the containment through the ADS-4 branch line (Sun et al., 2015). Consequently, the prediction of entrainment fraction become of a particular interest for emergency core cooling systems and safety of https://doi.org/10.1016/j.nucengdes.2018.01.010 Received 24 August 2017; Received in revised form 28 December 2017; Accepted 3 January 2018 ⁎ Corresponding author. E-mail addresses: salissa.email@gmail.com, issa@ntech.mw.tum.de (S. Al Issa), macian@ntech.mw.tum.de (R. Macian-Juan). Nuclear Engineering and Design 330 (2018) 1–13 Available online 20 February 2018 0029-5493/ © 2018 Elsevier B.V. All rights reserved. T