Yuanzineng kexue jishu (Feb 2023)
Numerical Investigation of Subcooled Flow Boiling Characteristics in Rod Bundle Channel of Petal-shape Fuel Element
Abstract
The petal-shape fuel elements have the advantages of strong heat transfer performance and no need for a spacer grid, allowing the reactor’s power density and economics to be increased even further. The Eulerian two-fluid model in combination with the RPI wall boiling model was used to investigate the boiling properties of the subcooled flow in the rod bundle channel of a 2×2 petal-shape fuel element in the paper. First, the accuracy of the interphase force model and the RPI closure model was evaluated using experimental data from subcooled boiling in a circular tube to accurately replicate two-phase flow heat transfer in the channel of the petal-shape fuel element. For the subcooled boiling phenomenon in the channel of a petal-shape fuel element rod bundle at pressure of 15.5 MPa, the numerical study of the effects of different flow velocities (1.4, 2 and 2.5 m/s) and heat flow flux (450, 650, and 800 kW/m2) on the flow, heat transfer, and void fraction distribution in the channel was carried out. The results demonstrate that the coolant flow velocity in the channel is unevenly distributed, with the maximum flow velocity in the channel’s central region, and the fluid velocity on the windward side of the fuel element being greater than the leeward side. As the inlet flow velocity increases, the flow field on the leeward side (at Line2) becomes more nonuniform. The transverse flow in the channel is primarily found in the inner concave arc of the fuel element, and the intensity of the transverse flow varies with the distance of the adjacent fuel elements. Due to the uneven flow field, the subcooled boiling is more intense and the void fraction is larger in the inner concave arc region, while the void fraction exhibits a huge disparity between the inner concave arc and the outer convex arc of the fuel element. The void percent at the fuel element’s leeward side (15°, 110°, 195°, and 285°) is higher than the void fraction near the outer convex arc’s windward side (70°, 160°, 250°, and 340°). The temperature of the outer convex arc wall surface is significantly lower at the z/H=0.25 position compared to the inner concave arc wall surface, and the temperature of the outer convex arc wall surface is greater than the inner concave arc wall temperature after the z/H=0.25 cross section due to different flow field inhomogeneity and the existence of heat transfer at the inner and outer convex arcs. The phase heat flow density does not show a linear trend of quick drop under the impact of transverse flow, but show a minor rise, and the evaporative heat flow density progressively increases along the axial direction, while the quenching heat flow does not change significantly.
