Numerical Study On The Flow And Heat Transfer Characteristics Between Liquid Lead-bismuth Eutectic And Helium

The utilization of nuclear energy could greatly relieve the energy crisis. However, one of the most important problems with nuclear energy is the management of radioactive wastes arising from the spent nuclear fuels, which would bring a long-term potential hazard to geological environment. In order to solve this problem, accelerator driven sub-critical system (ADS) was proposed, which could transmute the long-lived nuclides to the stable and short-lived nuclides and generate the electrical power simultaneously. Chinese Academy of Sciences (CAS) launched the "Strategic Priority Research Program" of ADS at2011, which would strive for the self-development of the series key technology from test facility to demonstration facility. Liquid lead-bismuth eutectic (LBE) has some favourable properties such as good chemical inertness, strong buoyancy and high atomic number. Hence LBE was extensively used as the coolant of the sub-critical core and the material of the spallation target providing the external neutron source. Helium is a good working fluid for the secondary loop of ADS because of its good heat conducting property and chemical inertness. Therefore it is very meaningful to investigate on the flow and heat transfer characteristics between liquid LBE and helium for ADS. Based on this point, this dissertation investigated on the turbulent-Prandtl-number (Prt) of liquid LBE and the flow and heat transfer characteristics between liquid LBE and helium.Firstly, computational fluid dynamics (CFD) analysis was employed to evaluate the applicability of various Prt models for LBE in the circular tube under the boundary conditions of constant-heat-flux and constant-wall-temperature. The research results indicate that Reynolds analogy, which is applicable to conventional fluids, is invalid to simulate the flow and heat transfer of LBE because of its low molecule Prandtl number. And Reynolds analogy always over-predicts Nusselt number for fully developed pipe flow of LBE. Unlike the conventional fluids, Prt has a significant difference in turbulent LBE flow between constant-heat-flux and constant-wall-temperature boundary conditions. The reliable Prt models are recommended for CFD applications to LBE flows under the boundary conditions of constant-heat-flux and constant-wall-temperature, respectively.Further, the flow and heat transfer characteristics between LBE and helium in a LBE-helium heat exchanger was investigated.3D numerical simulation of fluid-solid coupled heat transfer with variable property in the LBE-helium heat exchanger was conducted. The pressure drop of LBE in the tube-side obtained from the simulation was in good agreement with Colebrook correlation. The overall heat transfer coefficient of the heat exchanger obtained from the simulation was plausibly consistent with calculation result of heat transfer correlations. The effectiveness of the heat exchanger calculated from the simulation also fit well with the effectiveness-NTU relation. The research results indicate that helium in the shell-side dominates the heat transfer performance of the heat exchanger. The increment of LBE mass-flow-rate cannot significantly improve the overall heat transfer coefficient and heat exchange quantity unless helium mass-flow-rate in the shell-side increases simultaneously.In order to easily and quickly obtain the outlet temperatures of helium and LBE, a concept of modified effectiveness of heat exchanger was introduced and correlated as the function of tube-side to shell-side heat capacity rate ratio. The modified effectiveness of LBE-helium heat exchanger decreases with increasing tube-side to shell-side heat capacity rate ratio. That is, the outlet temperature of helium is closer to the inlet temperature of LBE with decreasing the helium mass-flow-rate if LBE mass-flow-rate remains unchanged. On the contrary, the outlet temperature of helium decreases with increasing the helium mass-flow-rate. Therefore there should be a tradeoff between good heat transfer performance and high outlet helium temperature when optimizing the design of LBE-helium heat exchanger.Finally, the dissertation introduced the design of LBE-helium experimental loop of ADS and its construction progress.