Void fractions in a rod bundle geometry at high pressure–part Ⅰ: Experimental study

Highlights

• Extensive data including local, chordal and area-averaged void fractions in a rod bundle was obtained respectively.

• Pressure and mass flux had significant effects on void fraction distribution.

• The non-uniform distribution of void fractions decreased in bubble and annular flow regimes.

• The droplet entrainment occurred in the gas core of annular flow at high mass fluxes.

Abstract

An investigation of void fraction measurement and prediction in a triangular-array rod bundle geometry was carried out under high pressure conditions (P = 5–9 MPa and G = 100–350 kg·m⁻²·s⁻¹). The aims of this work were to expand the existing database and assess and modify the predictive models. The present paper is the Part I of this study, where the experimental work was presented to measure local, chordal and area-average void fractions respectively by a comprehensive measuring system composed by optical probes and gamma-ray densitometry. Non-uniform distribution of void fraction was investigated by local-to-local and local-to-average comparisons of void fractions. The results indicated that non-uniform distribution was not only related to the geometry, but also affected by the transition of flow regimes in rod bundle. The effects of pressure and mass flux on void fractions were also analyzed in detail in consideration of variations of fluid properties and drift-flux parameters. Additionally, both void fraction and interface frequency were compared with the flow regime map in TRACE code. The variation of interface frequency indicated that the droplet entrainment would occur in annular flow at high mass flux. In the Part Ⅱ of this study, an assessment of existing models for void fraction prediction in rod bundles was carried out according to the present data and a newly-developed drift-flux model was proposed.

Introduction

Two-phase flow is one of the most frequent phenomena in a variety of industrial applications such as nuclear power, chemical industry and refrigeration (Hernandez-Alvarado et al., 2018). As one of the fundamental two-phase flow parameters, void fraction plays an important role on flow regime transition and also prediction of some other two-phase parameters such as pressure drop, gas velocity and average density (Zhao et al., 2016). Hence, it is of the great significance to obtain void fraction of two-phase flow accurately in both scientific research and industrial application. In general, the methods for obtaining void fraction contains two aspects (Yun et al., 2008). On the one hand, numerous studies have been carried out on void fraction measurement. Several advanced techniques, including Gamma-ray densitometry (Kumamaru et al., 1994; Kumara et al., 2010; Zhao et al., 2013), conductivity probe (Chakraborty et al., 2009; Paranjape et al., 2012), optical probe (Enrique Juliá et al., 2005; Le Corre et al., 2018), computerized tomography (Jia et al., 2015), differential pressure method (Chen et al., 2012; Clark et al., 2014a) and so on, have been developed and applied to measure void fractions accurately. On the other hand, based on a variety of experimental databases obtained by these techniques under various experimental conditions, models and correlations have been developed for the purpose of void fraction prediction (Bankoff, 1960; Smith, 1969; Thom, 1964; Wallis, 1968). Although a plenty of efforts has been made, accurate measurement and theoretical prediction of void fraction is still a difficult problem that remains unresolved in scientific research and practical industrial monitoring, owing to the complexly random fluctuation of two-phase interface, the limitations of measurement techniques, and the diversity of flow and geometrical condition involved.

Rod (or tube) bundle geometry is a special flow channel and extensively adopted in both reactor core and steam generator of light water reactors (LWR) (Gu et al., 2015). For optimum design and safe operation of nuclear reactor systems, it is essential to investigate the two-phase flow characteristics in rod bundles deeply. Differing from the pipes, rod bundles can lead to a more complex distribution of two phase flow owing to the effect of the rods inside the flow channel, which presents great challenges for accurate prediction of void fraction distribution. In recent decades, great efforts have been made by researchers all around the world to investigate the two-phase flow behavior in LWR fuel rod bundles (Hibiki et al., 2017). As a result, several important void fraction measurement experiments have been carried out aimed at both air-water and steam-water two phase flow in rod bundles, and some well-developed models for void fraction prediction have been also proposed (Bestion, 1990; Griffiths et al., 2014). Compared to the reactor core, the overall performance of two-phase flow in secondary side of steam generator (SG) seems never to have been an issue, because the total heat transferred from primary side to secondary side can be predicted reasonably well. However, the local flow properties such as local distributions of temperature, quality and void fraction are desired to be predicted as accurately as possible, since a series of problems of SG, including heat transfer deterioration-dryout, alternation of wetting and drying, tube vibration, tube corrosion and impurity deposition, are strongly associated with the local conditions (Green and Hetsroni, 1995; Shi et al., 2018). It is also the reason why various three-dimensional thermal-hydraulic computer codes have been developed for the purpose of optimal design of SG. Therefore, more experimental studies concerning the obtainment of local void fraction distributions in rod bundle geometries are definitely need, which is beneficial to deeply understanding of two-phase flow characteristics in rod bundle geometries and provide data foundation for validation and verification of void fraction prediction models and three-dimensional computer codes.

In the present work, an experimental investigation has been carried out for void fraction measurement in a triangular-array rod bundle geometry under high pressure conditions (P = 5–9 MPa and G = 100–350 kg·m⁻²·s⁻¹). The present paper is the Part Ⅰ of this study, where the experimental methodology and measurement results were shown and analyzed. A comprehensive measuring system contained two measuring techniques, optical probe and Gamma-ray densitometry, was established and used in this study. Extensive data including local, chordal and area-averaged void fractions are obtained. Meanwhile, the non-uniform of void fraction distribution and the effect of flow parameters, i.e. pressure and mass flux, on measurement result were analyzed in detail. Additionally, in the Part Ⅱ of this study, a careful assessment of several existing models for rod bundle geometry was carried out against the present void fraction data. A new drift-flux model was developed according to the two-phase flow behavior and compared to a series of data from the present work and the open literatures.