NATURAL CONVECTION HEAT TRANSFER FROM VERTICAL 7x7 ROD BUNDLES IN LIQUID SODIUM

摘要

Natural convection heat transfer from vertical 7x7 rod bundle in liquid sodium was numerically analyzed to optimize the thermal-hydraulic design for the bundle geometry with equilateral square array, ESA. The unsteady laminar three dimensional basic equations for natural convection heat transfer caused by a step heat flux were numerically solved until the solution reaches a steady-state. The PHOENICS code was used for the calculation considering the temperature dependence of thermo-physical properties concerned. The 7x7 test rods for diameter (D=7.6 mm), heated length (L=200 mm) and L/d (=26.32) were used in this work. The surface heat fluxes for each cylinder were equally given for a modified Rayleigh number, (R_(f,L))_ye and (R_(f,L))_(N_x.xNy,S/D), ranging from 3.08x10~4 to 4.28x10~7 (q=1x10~4~7x10~6 W/m~2) in liquid temperature (T_L=673.15 K). The values of S/D, which are ratios of the diameter of flow channel for bundle geometry to the rod diameter, for vertical 7x7 rod bundle were ranged from 1.8 to 6 on the bundle geometry with equilateral square array. The spatial distribution of average Nusselt numbers for a vertical single cylinder of a rod bundle, (Nu_(av))_(ij), and average Nusselt numbers for a vertical rod bundle, (Nu_(av,B))N_(xxNy,S/D), were clarified. The average value of Nusselt number, (Nu_(av))_(ij) and (Nu_(av,B))N_(xxNy,S/D) for the bundle geometry with various values of S/D were calculated to examine the effect of array size, bundle geometry, S/D, (R_(f,L))_(ij) and (R_(f,L))N_(xxNy,S/D) on heat transfer. The bundle geometry for the higher (Nu_(av,B)_(NxxNy,S/D) value under the condition of S/D=constant was examined. The general correlations for natural convection heat transfer from a vertical N_xxN_y rod bundle with the equilateral square and triangle arrays including the effects of array size, (R_(f,L))_(NxxNy,S/D) and S/D were derived. The correlations for vertical N_xxN_y rod bundles can describe the theoretical values of (Nu_(av,B))_(NxxNy,S/D) for each bundle geometry in the wide analytical range of S/D (=1.8 to 6) and the modified Rayleigh number ((R_(f,L))_(NxxNy,S/D)3.08x10~4 to 4.28xl07) within -9.49 to 10.6 % differences.