| Lead-Bismuth Eutectic (LBE) are currently attracting more and more attention word wildly to be the coolant for fast reactor core due to its low melting temperature, high boiling temperature, outstanding heat transfer performance, chemical stability, and good neutron economy. Owing to the lower prandtl number, the heat transfer to heavy liquid metal is very different from the heat transfer to water or gas. The contribution from thermal conductivity in low prandtl number fluid like lead-bismuth eutectic is much higher than water even through at the conditions with high Reynolds number. Thus one of the most interesting topics in thermal-hydraulic fields is the turbulent heat transfer, especially for complex geometries, e.g. fuel assemblies. Fuel rods of fast reactor cooled by liquid metal are generally arranged in bundles of a triangular configuration and each pin is wrapped with cylindrical wire following a helical pattern around the rod axis. Due to the existence of helical wire, the thermal-hydraulic behaviors of coolant are different from the behaviors in traditional fuel assembly with grid spacers. Thus, to investigate the cooling performance of fuel assembly with wire spacer using liquid metal was a major issue for feasibility of this type reactor.In this thesis, the thermal-hydraulic behaviors of a wire-wrapped rod bundle cooled by LBE were investigated based on KYLIN-II thermal-hydraulic expeirmental loops. And the thermal-hydraulic experiments and numerical analysis of rod bundle with wire spacer were performed respectively, including the flow resistance features, thermal entrance characteristics, temperature distribution and heat transfer. The main contents and results of this thesis are listed as follows:1) From the experimental results of pressure drop, it showed that the friction factor of wire-wrapped bundle has a good agreement with the empirical correlations, e.g. Novendstern (1974) and Rehme (1972). The comparison indicated that LBE has the similar flow resistance characteristics with traditional fluids. And the results showed that the Novendstern model is more suitable for this rod bundle, the relative deviation is around 14%. However, based on the Reynolds similarity, the friction factor results in water were larger than the values in LBE. That is because at the same Reynolds number, the secondary secondary flow induced by helical wires in water was stronger than the value in LBE.2) As for the heat transfer performance, the thermal entrance characteristics, temperature distribution and heat transfer were performed. Firstly, the thermal field did not reach a fully developed and stable condition by measuring the wall temperature of PIN#3 along the flow direction. The phenomenon was attributed to the weak communication among sub-channels in tightly packed bundle. Secondary, the thermal-hydraulic features in inner connect sub-channels were different from inner sub-channels, although the geometry is the same. This indicated that the hexagonal shell has a great influence on the heat transfer of rod bundle geometry especially in the edge and inner connect sub-channels. Thus the average heat transfer coefficient based on inner sub-channel representation the infinit rod bundle was larger than the average heat transfer based on fuel assembly. The experimental results showed that the Borishanskii model was more suitable for this rod bundle, the relative deviation is around 22%. Considing the complex geometry, the results showed a good agreement with the Borishanskii model.3) From the numerical calculation, it could be seen that for low flow rate cases, the results agree better with experimental data, especially in edge sub-channel (SCH #4) coolant temperature. The coolant temperature in SCH#4 calculated by code for high flow rate case is predicting much smaller normalized temperatures than experimental data both at two measuring level. The main reason for the deviations could be mainly attributed to the simplified wire spacer model. Sub-channel analysis calculations of wire spacer bundles have been generally performed employing axially-averaged flow area and wetted perimeter, in which the wire wrap is assumed to be smeared uniformly in all sub-channels. And the pressure drop and heat transfer models were basically aimed at estimating bundle average parameters; they did not consider the differences of flow characteristics in different sub-channels. Thus improving the crossflow and transverse heat conduction model is an important issue for future study in sub-channel analysis. The cladding temperature at outlet of the bundle (without heating) was the same to the coolant temperature for all simulations; thus a temperature drop could be detected. From the results, it could be seen that the sub-channel analysis presented a good agreement with the experimental, although a slightly over prediction could be detected near the outlet of heating region. The results confirmed that the SACOS-PB was a reliable tool for LBE system sub-channel analysis, and much emphasis should be put on the turbulent mixing and wire-wrap crossflow models in the future.4) The three dimensional CFD programs based on eddy viscosity models (RNG^ k-ε standard and SST model) were performed to support the flow and heat transfer analysis. Firstly, the turbulent Prandtl number was investigated and was modified to 1.1. And then the validation tests were performed. It showed that there is no significant differences, and all the turbulent model showed a good agreement with experimental results. But for heat transfer performance, with the Pe<300, the numerical results under-predicted the heat transfer coefficient. This was expected because constant Prt heat transfer modeling exhibits limitations in the mixed and/or natural convection flow regime. This might indicate that the modeling of the turbulent heattransfer is a cause of deviations. An improved heat transfer model is required in future.In summary, the investigation of thermal-hydraulic behaviors of wire-wrapped bundle cooled by LBE were carried out in this paper. Some fundamental experimental data and rules about flow resistance and heat transfer were obtained. Moreover, the CFD code and sub-channel code were analyzed and validated. It provided valuable experience to estabilish the integral study of thermal-hydraulic performance in fuel assembly, and also could support the design of Reactor cooled by LBE. |
PDF Full Text Request