Contact Author:
Dr. Wang Kee In
wkin@kaeri.re.kr
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CFD Simulations of a Flow Mixing and Heat Transfer Enhancement in an Advanced LWR Nuclear Fuel Assembly
W. K. In, T. H. Chun, C. H. Shin, D. S. Oh (KAERI)
A computational fluid dynamics(CFD) analysis has been performed to investigate a flow-mixing and heat-transfer enhancement caused by a mixing-vane spacer in a rod bundle. The nuclear fuel assembly used in a light water reactor(LWR) is a rod bundle which is supported by a grid spacer. The commercial LWR fuel assembly is typically a 16x16 or 17x17 array of fuel rods. The coolant flows axially through the subchannel formed between the rods. The fuel spacer affects the coolant flow distribution in the fuel rod bundle, and so a spacers¡¯ geometry has a strong influence on a bundle¡¯s thermal-hydraulic characteristics, such as the critical heat flux and pressure drop. In particular, integral flow deflecting vanes on a grid spacer can improve a departure from a nucleate boiling(DNB) performance by increasing the coolant mixing rate and a rod¡¯s heat transfer ability downstream of the vanes. An understanding of the detailed structure of a flow mixing and heat transfer downstream of a mixing-vane spacer in a nuclear fuel rod bundle is of major interest to the nuclear power industry for a reactors safe and reliable operation.
There have been several studies on a flow mixing and heat-transfer enhancement caused by a mixing-vane spacer in a rod bundle geometry. The previous CFD studies for the rod-bundle geometry are mostly focused on a flow mixing caused by a split-vane grid but there are few CFD studies available on the heat-transfer characteristics in a rod bundle with mixing vanes. It is also noted that the experimental investigations of a heat transfer in rod bundles only heated a small portion of the instrumented rod for the Reynolds numbers of 28000 and 42000. Since the locally heated sensor is only a small portion of a rod, the flow will not be fully thermally developed and its effect on the local heat transfer development is thus not clearly understood as yet. However, the experimental heat transfer data for a fully heated rod bundle with mixing vanes is not available. It is therefore interesting to perform a CFD analysis of a three-dimensional heat-transfer development in a fully heated rod bundle with mixing vane spacers for a normal LWR operating condition, i.e., Re=500000.
This study presents the CFD simulations of a flow mixing and heat transfer in a fully heated 5x5 array of a rod bundle with a split-vane spacer which has been widely used for a LWR fuel assembly. The velocity distribution and fluid mixing pattern are predicted to examine the heat-transfer development downstream of a split-vane spacer. Normalized Nusselt numbers are also predicted and compared with the experimental measurements for a partially heated rod bundle along with their correlations for the low Reynolds numbers. In addition, the present study compares the thermal-hydraulic performance of two different mixing vane spacers, i.e., a split-vane spacer and a hybrid-vane spacer, based on the CFD simulations at the LWR¡¯s operating conditions. The hybrid-vane spacer is a new coolant-mixing device under development to generate a strong swirling flow in a subchannel as well as a crossflow between adjacent subchannels.
A fluid¡¯s mixing pattern and velocity distribution are presented to examine the heat-transfer development downstream of a vaned spacer. The predicted Nusselt numbers at the Reynolds numbers of 28000 and 42000 are compared with the available experimental measurements along with their correlations. The CFD predictions showed a maximum enhancement of the heat transfer a little downstream of the split-vane spacer and appeared to reasonably agree with the Nusselt numbers by using a correlation for a swirling-vane spacer except for the region close to the vaned spacer. The change of the fluid mixing pattern downstream of the split-vane spacer agrees with the streamwise development of the measured lateral flow field. The CFD calculations for the split vane and hybrid vane at the LWR operating conditions predicted hot fuel spots in a streaky structure downstream of the spacer which is believed to be caused by the secondary flow occurring in an opposite direction near the fuel rod. The split vane generates a large crossflow with two small swirls near the vane and a single large swirl further downstream, which diminishes much further downstream. The hybrid vane causes a large central swirl with a small crossflow whose strength continually decreases as it flows downstream. The average Nusselt numbers show a peak near the vaned spacer and a continual decrease further downstream. The split-vane and hybrid-vane spacers are expected to significantly enhance the overall heat transfer of a nuclear fuel assembly.