CFD studies on the phenomena around counter-current flow limitations of gas/liquid two-phase flow in a model of a PWR hot leg

Abstract

In order to improve the understanding of counter-current two-phase flow and to validate new physical models, CFD simulations of a 1/3rd scale model of the hot leg of a German Konvoi pressurized water reactor (PWR) with rectangular cross section were performed. Selected counter-current flow limitation (CCFL) experiments conducted at Helmholtz-Zentrum Dresden-Rossendorf (HZDR) were calculated with ANSYS CFX using the multi-fluid Euler–Euler modelling approach. The transient calculations were carried out using a gas/liquid inhomogeneous multiphase flow model coupled with a shear stress transport (SST) turbulence model.

In the simulation, the drag law was approached by a newly developed correlation of the drag coefficient (Höhne and Vallée, 2010) in the Algebraic Interfacial Area Density (AIAD) model. The model can distinguish the bubbles, droplets and the free surface using the local liquid phase volume fraction value. A comparison with the high-speed video observations shows a good qualitative agreement. The results indicate also a quantitative agreement between calculations and experimental data for the CCFL characteristics and the water level inside the hot leg channel.

Introduction

Two-phase flow may occur in pressurized water reactors (PWR) following a leakage in the primary cooling circuit. In the event of hypothetical accident scenarios in PWR, emergency strategies have to be mapped out, in order to guarantee the reliable removal of decay heat from the reactor core, also in case of component breakdown. One essential passive heat removal mechanism is the reflux-condenser mode. This mode can appear for instance during a small break loss-of-coolant-accident (LOCA) or because of loss of residual heat removal (RHR) system during mid loop operation at plant outage after the reactor shutdown.

In the hypothetical accident scenario of a loss-of-coolant-accident (LOCA) due to the leakage at any location in the primary circuit, it is expected that the reactor will be depressurized and vaporization will take place, thereby creating steam in the PWR primary side. Should this lead to “reflux condensation”, which may be a favourable event progression, the generated steam will flow to the steam generator through the hot leg. This steam will condense in the steam generator and the condensate will flow back through the hot leg to the reactor, resulting in counter-current steam/water flow. In some scenarios, the success of core cooling depends on the behaviour of this counter-current flow (Deendarlianto et al., submitted for publication).

In the reflux-condenser mode, a part of the condensate will flows back to the reactor core in counter-current to the steam flow. The counter-current flow of steam and condensate is only stable for a certain range of mass flow rates. If the steam mass flow rate increases too much, the condensate is clogged in the hot leg. The condensate is carried over by the steam and partially entrained in the opposite direction to the steam generator. This phenomenon is known as the counter-current flow limitation (CCFL) or flooding, and could affect the cooling of the reactor core. Detailed examples of such LOCA scenarios leading to the reflux condenser mode can be found in Jeong (2002).

A lot of experiments were carried in the past in order to understand the phenomena around CCFL in a model of hot leg PWR. Several experimental correlations were developed to predict the CCFL on them, but they are only valid in specific experimental ranges. Therefore, high resolution experimental data at reactor typical boundary conditions is needed. In order to improve the transient analysis of counter-current two-phase flows, experimental studies were conducted at Helmholtz-Zentrum Dresden-Rossendorf (HZDR). A 1/3rd scale model of the hot leg of a German Konvoi PWR with rectangular cross section was used (Deendarlianto et al., 2008, Vallée et al., 2009a).

The nuclear thermal-hydraulic community is facing today interesting challenges. These include the development and validation of new computational tools that will be used for improved and more detailed analysis as well as new generations of reactors. Current trends are toward multi-dimensional, -scale, -physics approaches for such analyses (Yadigaroglu, 2005). For this purpose, the computational fluid dynamics (CFD) tool is considered to be able to simulate most of two-phase flow configurations encountered in nuclear reactor power plants.

The most widely used analysis to model the CCFL in a PWR hot leg is based on the one dimensional two-fluid models as reported by Ardron and Baneerjee (1986), Lopez de Bertodano (1994) and Wongwises (1996). In this approach, the switching point, where the flow condition changes from sub to critical condition in stratified flow is approached by the experimental correlation in a specified flow direction. Wang and Mayinger (1995) did two-dimensional analyses of counter-current model of UPTF Test TRAM A2 & Test 11 using a two-fluid model. They implemented the interfacial friction factor proposed by Lee and Bankoff (1983) and Ohnuki (1986) into the code FLOW3D. They reported that satisfactory results were obtained, whereas, under the reflux condensation conditions, numerical computation reveals that different flow structures appeared in the region away from the flooding curve and in the region near the flooding curve.

Murase and his co-workers in Tsuruga-Japan are in disagreement with the study of Wang and Mayinger. They claimed that the effects of wall friction can not be correctly evaluated by using two-dimensional analysis. The given boundary conditions at the inlet and outlet of the hot leg in the above work might affect the calculated flow patterns in the hot leg. For this reason they conducted 3-D CFD calculations. Their CFD works can be found in Murase et al. (2009), Minami et al., 2009, Minami et al., 2010b, Kinoshita et al. (2009), and Utanohara et al. (2009).

Murase et al. (2009), Minami et al., 2009, Minami et al., 2010b and Utanohara et al. (2009) conducted 3-D CFD simulations on counter-current flow in a PWR hot-leg air–water two-phase flow in a 1/15th scale model. This calculation model reproduced the size of experimental test facility at Kobe University as reported by Minami et al. (2010a). Their works included the effects of interfacial friction correlation (Utanohara et al., 2009), flow patterns and CCFL (Murase et al., 2009, Minami et al., 2009, Minami et al., 2010b). They used the volume of fluid (VOF) and Euler–Euler two-fluid models on the commercial CFD code FLUENT6.3.26. The required interfacial friction correlations in the Euler–Euler two-fluid model were selected from a combination of available 1-D experimental correlations for the cases of annular and slug flow that gave the best agreement with the experimental data. They concluded that it is better to use the two-fluid model with suitable interface friction correlation than VOF model. The predicted flow patterns, hysteresis behaviours, and CCFL characteristics agree well with their experimental data. Meanwhile those correlations were obtained on the basis of one dimensional analysis, which might affect the calculation results. The use of the 1-D experimental correlation to the 3-D problem might be not accurate when we look into the local physics of the phenomenon.

The development of a general model closer to physics and including less empiricism is a long-term objective of the HZDR research programs. Here local geometry independent models for mass, momentum, heat transfer, and scalar transport are developed and validated. Such models are an essential precondition for the application of CFD codes to the modelling of flow related phenomena in nuclear facilities. One of the developed scientific methods to solve the above problems was the new concept of drag coefficient in the algebraic interfacial area density model (AIAD) (Höhne, 2009).

The aim of this paper is to simulate the phenomena around the CCFL in a PWR hot leg with newly developed of new concept of drag coefficient in the AIAD model to the Euler–Euler problem. It allows the detection of the morphological form of the two phase flow and the corresponding switching via a blending function of each correlation from one object pair to another. The new drag correlation in this model considers the 3D effects of the simulated phenomenon.