CHF experiment and CFD analysis in a 2 × 3 rod bundle with mixing vane

doi:10.1016/j.nucengdes.2009.01.011

Nuclear Engineering and Design

Volume 239, Issue 5, May 2009, Pages 899-912

https://doi.org/10.1016/j.nucengdes.2009.01.011 Get rights and content

Abstract

In this study, the CHF enhancement using various mixing vanes is evaluated and the flow characteristics are investigated through the CHF experiments and CFD analysis.

CHF tests were performed using 2 × 2 and 2 × 3 rod bundles and with R-134a as the working fluid. The test section geometry was identical to that of commercial PWR fuel assembly not including the heated length (1.125 m) and number of fuel rods. From the CHF tests, it was found that the CHF enhancement using mixing vanes under higher mass flux (1400 kg/m² s) and lower pressure (15 bar) conditions is larger than the CHF enhancements under other conditions. Among the mixing vanes used in this study, the swirl vane showed the best performance under relatively low pressure (15 bar) and mass flux (300–1000 kg/m² s) conditions and the hybrid vane performed best near the PWR operating conditions.

The detailed flow characteristics were also investigated by CFD analysis using the same conditions as the CHF tests. To calculate the subcooled boiling flow, the wall partitioning model was applied to the wall boundary and various two-phase parameters were also considered. The reliability of the CFD analysis in the boiling analysis was confirmed by comparing the average void fractions of the analysis and the experiments: the results agreed well. From the CFD analysis, the void fraction flattening as a result of the lateral velocity induced by the mixing vane was observed. By the lateral motion of the liquid, the void fraction in the near wall was decreased and that of the core region was increased resulting in the void fraction flattening. The decrease of the void fraction in the near wall region promoted liquid supply to the wall and consequently the CHF increased. For the quantification of the void flatness, an index was developed and the applicability of the index in the CHF assessment was confirmed.

Introduction

In a PWR nuclear fuel assembly, a spacer grid is used to maintain a gap between the fuel rods. Maintaining the gap is to thermal-hydraulically support uniform heat transfer and to thermal-hydraulically suppress the local occurrence of CHF. If a mixing vane is attached to the spacer grid, the functions of the spacer grid are further reinforced, and the CHF is greatly enhanced. For example, a commercial nuclear fuel assembly with a mixing vane can enhance the CHF by up to 10% and the economic benefits are great due to this enhancement. Despite the demerits of a mixing vane, such as the increase of the pressure drop and FIV and the debris problem, most nuclear fuel development includes the development of a mixing vane because the economical benefits are too great to ignore.

In the existing research, the enhancement of CHF using a mixing vane is one of many considerations in CHF studies on rod bundle geometry. ERPI, CE, and B&W correlations, the well-known correlations for rod bundles, include the effect of a spacer grid, a guide thimble, the non-heating area locations, and so on. In this case, the effect of the mixing vane is taken into consideration in the grid factor (Chang and Baek, 1997).

Fortini and Veloso (2002) and Lee (2000) have attempted to predict the CHF in rod bundles by applying the correction factor to the CHF correlation in a circular tube. They also included the effects of the mixing vanes as a correction factor. The reason that the studies have propriety is because the shapes of the existing mixing vanes are similar to each other and the CHF enhancements were not vastly different. Therefore, a correction factor that does not significantly affect the performance of the prediction should be used. Recently, Chen et al. (2004) and Akhtar et al. (2006) performed a CHF experiment using rod bundles with refrigerants: they showed that using a refrigerant enables proper results to be obtained through fluid-to-fluid modeling.

Some research has investigated the detailed mechanism of the CHF enhancement caused by the mixing vane. Generally, it is well-known that the CHF enhancement through the mixing vane is due to the generation of swirl and cross flows, and the increase of the turbulent intensity. Chung et al. (1996) summarized the effects as shown in Table 1. de Crecy (1994) conducted a radial non-uniform heating CHF experiment to investigate the change of location and the enhancement of the CHF value through the implementation of a mixing vane. According to Crecy's study, the CHF appeared at random radial locations and the CHF value increased in rod bundles with a mixing vane.

Mixing vanes in nuclear fuel assemblies are used to increase the thermal mixing and increase the DNB margin. To date, in most research on mixing vanes, the shape of the mixing vane is not important; rather the research only considers one parameter, such as the mixing vane factor, in the CHF correlation. However, due to the economical benefits gained by incorporating a mixing vane and the technical improvements in the CFD code, much more thermal-hydraulic research on mixing vanes has been undertaken recently.

Using the CFD analysis of a single phase flow, Ikeda et al. (2006) predicted the location of the DNB occurrence. They established that the location of the maximum local enthalpy in a single-phase analysis corresponded to the location of the DNB occurrence. However, the analysis could not directly predict the CHF value and could only predict the location of the DNB. Krepper et al. (2007) predicted various two-phase variables in the subcooled flow boiling using a wall boiling model. This study shows the potential of predicting the CHF through the CFD analysis using the wall boiling model in a two-phase flow. The study establishes that the CFD analysis results in proper flow characteristics regarding the CHF prediction. However, the study only shows the analysis results for a single-phase flow: the CFD analysis should also be applied to a two-phase flow to examine the detailed mechanism of the CHF enhancement

The primary objective of this paper is to find the representative mixing factor to mean the performance of the mixing vane on the CHF. To achieve the primary objective, a CFD analysis was performed under identical conditions to those of the CHF test. The two-phase flow parameters under the CHF condition were calculated using the CFD analysis. The CHF test was performed in a 2 × 3 rod bundle with a simple spacer grid and various mixing vanes at KAIST R-134a Loop. The R-134a was a working fluid and, for CFD analysis, CFX 11.0 with a beta version of the wall boiling model was used. By comparing the experimental and numerical results, the representative mixing factor related to the CHF enhancement was obtained. An additional objective of this study was to investigate the CHF characteristics of various mixing vanes and the effect of varying design parameters on the mixing vane. This study also proposes the design direction and provides useful information for designing mixing vanes.

Section snippets

Experimental works

Fig. 1 shows the test loop for the experiment. The loop is closed loop and R-134a was used as the working fluid. The detailed specifications of the loop can be found in the experimental apparatus section of Shin et al.’s study (Shin et al., 2005).

Fig. 2 shows the axial and cross sectional geometry and structure of the 2 × 3 rod bundle test section. The distance between heater rods was 12.65 mm; the heater rod diameter was 9.5 mm; the distance between the heater rod and the insulated wall was 1.575

Two-fluid model

In a two-phase flow, the vapor and liquid phases are treated as continua. The phases may interact with each other through interfacial forces as well as heat and mass transfer across the phase interfaces. The two-fluid model solves two sets of mass, momentum, and energy conservation equations, which are written for each phase as:

(a) Continuity equation $\frac{\partial}{\partial t} (α_{k} ρ_{k}) + \nabla \cdot (α_{k} ρ_{k} U_{k}) = Γ_{k}$

(b) Momentum equation $\frac{\partial}{\partial t} (α_{k} ρ_{k} U_{k}) + \nabla \cdot (α_{k} ρ_{k} U_{k} U_{k}) = \nabla \cdot [α_{k} μ_{k}^{e} (\nabla U_{k} + {(\nabla U_{k})}^{T})] - α_{k} \nabla p_{k} + α_{k} ρ_{k} g + U_{ki} Γ_{k} + M_{k}$

Experimental results

In the CHF experiment with a 2 × 3 rod bundle and a simple spacer grid, the trends of the CHF regarding the major parameters were investigated. Fig. 9 shows the parametric trends of the CHF regarding pressure, mass flux, and inlet subcooling. The CHF value increased as the mass flux and inlet subcooling increased, and the pressure decreased. This trend is similar to that in a circular tube (Bartolomei and Chanturiya, 1967).

Fig. 10 presents a comparison of the experimental data in this study with

Conclusions

The major conclusions from the experimental work and numerical analyses are as follows:

(1)
The CHF enhancement using mixing vanes was enlarged under relatively high mass flux and low pressure conditions. Under a high mass flux condition, the enlarged swirl and cross flows led to the reduction of the wall void fraction and enthalpy near the wall. Under a low pressure condition, the effect of the swirl and cross flows strengthened due to the high density ratio (liquid/vapor).
(2)
Among the mixing vanes

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Citation Excerpt :
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CHF experiment and CFD analysis in a 2 × 3 rod bundle with mixing vane

Abstract

Introduction

Section snippets

Experimental works

Two-fluid model

Experimental results

Conclusions

Int. J. Heat Mass Transfer

Nucl. Eng. Des.

Int. Commun. Heat Mass Transfer

Nucl. Eng. Des.

Nucl. Eng. Des.

Int. J. Heat Mass Transfer

Int. J. Heat Mass Transfer

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