Influence of Cold Walls of Guide Tube on Subcooled Boiling Flow and Heat Transfer Characteristics in Petal-shaped Fuel Assemblies

In order to understand the influence of cold wall on characteristics of subcooled flow boiling in petal-shaped fuel assemblies, a numerical investigation was performed to analyze subcooled flow boiling characteristics in two petal-shaped fuel assembly configurations (with and without guide tubes) by applying the two-fluid model with the RPI wall boiling model. The simulation data of these two geometric models were compared, and the influence of guide tubes on flow and heat transfer characteristics, such as flow field, heat transfer capability, onset location of fully developed nucleate boiling (FDB), and void fraction, all of them were systematically investigated. The results indicate that the transverse flow velocity is primarily concentrated around the fuel rods, forming swirling flows in the assembly with guide tube, no significant transverse flow is observed, cross-flow velocity is zero near the guide tube. Transverse flow similarly concentrates around the fuel rods, but both central and peripheral rods exhibit pronounced swirling patterns in the assembly without guide tube, with peak transverse velocity reaching 0.15 m/s. The guide tubes reduce transverse flow velocity, leading to more uniform lateral velocity distribution around fuel rods. The coolant velocity in corner channels is higher in the assembly with guide tube, exceeding that in corresponding channels of the assembly without guide tube by approximately 30%. In the assembly channel without guide tube, fluid velocity in the central channel increases along the axial direction due to interference from adjacent fuel rods, and the rate of increase gradually diminishes, while velocity with periodic fluctuations decline synchronized to the helical twisting pattern of petal-shaped fuel rods in the central channel of assembly with guide tube. The friction coefficient of the entire fuel assembly channel increases with guide tube, rising by approximately 100% within the simulated conditions compared to assembly without guide tube. In the entrance region of the assembly with guide tubes, heat transfer coefficients are higher. With the flowing, the heat transfer coefficients of both configurations gradually stabilize. The average wall temperature increases gradually along the axial direction in both assemblies (with and without guide tube). At the same axial position, the wall temperature in the non-guide-tube assembly is consistently higher, exceeding that of the assembly with guide-tube by approximately 4-5 K. In the concave regions of the fuel assembly, the wall temperature of fuel rods in the assembly without guide tubes are higher than that in the assembly with guide tubes, with a maximum temperature difference approaching 10 K. No notable difference is observed in the convex regions. The void fraction distribution shows higher in concave zones than in convex zones, with minimal impact from guide tubes. The onset location of FDB is only slightly affected by the guide tube.