Study On The Thermal Characteristics Of Double-Wall Tube Heat Exchangers With Lead Bismuth Eutectic

The Generation Ⅳ nuclear system is developed to be of intrinsic safety and reliability,economical and sustainable.The lead-cooled fast reactor(LFR)is one of the promising schemes,which has broad application prospects in the fields of distributed power supply,island platform,deep space exploration,etc.,due to its easy miniaturization.The Steam Generation Tube Rupture(SGTR)is one of the most important accidents.Double-wall tube heat exchanger is an effective way to prevent and monitor the SGTR.For high-efficient heat transfer performance,researches on double-wall tube design under the premise of safety condition are important in LFR.However,for the low Prandtl characteristic of liquid Lead Bismuth Eutectic(LBE),the Reynolds comparison,which can accurately simulate the heat transfer characteristics of conventional flow in CFD commercial software,is no longer applicable.Therefore,study on LBE-cooled double-wall tube heat exchanger is necessary,i.e.,thermal hydraulics and influencing factors.The Institute of Nuclear Energy Safety Technology has undertaken the research and development of the LBE-cooled reactor in the strategic priority research program of the Chinese Academy of Sciences "Future advanced nuclear fission energy-ADS transmutation system".This research work is carried out in the context of the project.For the double-wall tube heat exchanger scheme of China LEAd-based Research reactor,the thermal hydraulics calculation of the double-wall tube heat exchanger is developed,and the verification and the sensitivity analysis of the structural parameters of the heat exchanger are carried out.The applicability of the different turbulent Prandtl number models in the triangular rod bundle channel is analyzed by the heat transfer correlations verified by the liquid LBE-cooled experiments.And the research of the model applicability between the rod bundle sub-channel and the heat exchanger tube bundle sub-channel is conducted.Finally,the three-dimensional numerical simulation is used to analyze the temperature field and flow field of the double-walled-straight-tube and double-wall bayonet tube heat exchanger,respectively.Sensitivity analysis of operating parameters and the optimization of heat transfer performance are also studied under this program.The main conclusions are listed as follows.(1)The self-developed coding of the double-wall tube heat exchanger thermal hydraulics and sensitivity analysis of the structural parameters are carried out.The results show that:The thermal hydraulic computation of the bayonet double-wall tube heat exchanger in the thermal resistance method can predict the temperature field and heat transfer performance of the fluid on both sides of a single tube bundle.The resistance change of the powder side is more important than that of the heat transfer tube and the convective heat transfer.The increase of the outer diameter of the tube 2 and the reduction of the inner diameter of the tube 3 have the most significant effect on improving the heat exchange performance.The change of the radial structure size of the tube 1 only affects the flow velocity of the water side and the preheating temperature of the water side in the descending pipe.The effect on the overall heat transfer performance is limited.(2)The model of turbulent Prandtl number for liquid LBE in the triangular sub-channel is studied.The results show that:The overall turbulent Prandtl number model is not only related to the Reynolds number Re and Berkeley number Pe,but also to the pitch diameter ratio P/D.The influence of rod diameter on the turbulent Prandtl number model is relatively small.Various turbulent Prandtl number models have the best application range according to pitch-diameter ratio.When P/D is 1.3,the Reynolds model is more suitable to predict;when P/D is in the range of 1.5-1.7,the Jischa model and the Mikityuk relational formula are in good agreement;the Cheng model is suitable for P/D of 1.7 conditions;when P/D is in the range of 1.3-1.7,the Kays model is the best among these models and the Aoki model is the second acceptable.The Kays turbulent Prandtl number model predicted by the sub-channel structure of the pin bundle,can be used to predict the heat transfer performance of the tube bundle in the specific LBE flow rate range of 0.09m/s～0.20m/s for the CLEAR-S heat exchanger.(3)The flow and temperature field analysis of the double-walled-straight-tube heat exchanger and the double-wall bayonet tube heat exchanger has been carried out through the 3D numerical simulation.The results show that:The simulation results are consistent with the experimental data of KYLIN-II and CLEAR-S heat exchangers and the results of the RELAP5 system.The thermal resistance on the powder interlayer side is the main component of the total thermal resistance.The larger thermal resistance on the powder interlayer side weakens the LBE side flow and temperature inhomogeneity on the heat transfer of the tube bundle and significantly reduce the radial temperature difference of the heat exchange tube wall.In the double-wall bayonet tube heat exchanger,under the rated condition,the liquid water that rise in the annular non-effective heat exchange area preheats the water in the downcomer by about 11K,which reduces the overall heat exchange performance of the heat exchanger by about 4%.(4)The sensitivity analysis of the operating parameters and the study of the control strategy of the double-wall bayonet tube heat exchanger has been carried out.Given the system flow rate remains constant,while the inlet and outlet temperature increase linearly with the increase of power,the control strategy can meet the power adjustment requirements of the experimental facility during the regulation of the power.The fluctuation of the inlet temperature of the lead-bismuth side of the primary system and the water side of the secondary system has the greatest impact on the heat exchange performance,followed by the influence of the inlet flow rate,and the influence of the operating pressure of the secondary coolant system is the smallest.The influence ability of the primary coolant system on the heat exchange performance is slightly higher than that of the secondary coolant system.In conclusion,the turbulent Prandtl number model of LBE-cooled turbulent flow and the thermal characteristics of double-wall tube heat exchangers are studied in this paper.The applicability of the turbulent Prandtl number model in the triangular sub-channel is clarified.The thermal hydraulics analysis of double-wall bayonet tube heat exchanger has been done in the thermal resistance method and verified with 1D system coding.The thermal resistance distribution,the influence of structural parameters and operating parameters on the heat transfer performance of double-wall tube heat exchangers are obtained.The research work in this paper can be used to predict the heat transfer characteristics of liquid LBE in sub-channel scale of triangular bundles for various P/D,and provide theoretical reference for design optimization of double-wall tube heat exchangers in LFR.