Investigation On Flow And Heat Transfer Characteristics Of High Efficiency Heat Exchangers Based On Special Medium

With the intensifying of energy shortage and environment pollution in our country,improving the energy utilization efficiency and reducing the heat loss in the energy transfer processe are two important approaches to reducing energy demand and lessening impact of the traditional fossil fuels on the environment.In the fields of advanced nuclear energy and industrial waste heat utilization,various heat exchangers play an important role in improving the energy utilization efficiency.However,with the exhanstion of energy sources,the quality of available heat sources is falling and the temperature difference of heat exchange is also getting smaller,which put higher requirements on the performance of heat exchangers.Therefore,the heat exchangers used in the high-energy-consumption fields are no longer limited to adopt the conventional fluids(such as water and air)as the working fluids.Instead,in order to increase heat transfer efficiency and meet the modern engineering needs,especially to meet the requirements of harsh environmental conditions,the special mediums are selected as the working fluids in these heat exchangers.Therefore,it makes the study of highly efficient heat exchangers based on special mediums particularly important.Based on the above background,numerical simulation and experimental research are carried out to investigate the thermal-hydraulic characteristics of heat exchangers using liquid lead-bismuth eutectic(LBE)and high-temperature flue gas as the working fluids in the thesis.The flow and heat transfer characteristics of the liquid LBE and high-temperature flue gas are analyzed.The numerical simulation method for these two kinds of high efficiency heat exchangers is developed and the heat transfer experimental platforms are designed.The influence of the gravity effect,heat exchanger structure parameters,secondary flow intensity and the experimental operating parameters on the heat exchanger performance are investigated.More importantly,the empirical correlations of pressure drop and heat transfer are proposed.The above research works provide guidance for the optimal design of the heat exchangers based on special mediums.In the numerical simulation of the liquid LBE heat exchanger,the appropriate turbulent-Prandtl-number model and the thermal-physical expressions for the heavy liquid metal with low Prandt number are selected.The forced convection heat transfer characteristics of liquid LBE in the tube with different Reynolds numbers are obtained by the method of large eddy simulation.The influence of the gravity on the mixed convection heat transfer characteristics of liquid LBE in the tube with different inclination angles and length-to-diameter ratios are investigated.And it is found that the dominant factor effecting convective heat transfer of LBE is inclined angles rather than length to diameter ratios.Numerical investigations of liquid LBE-helium heat exchanger with 3D full size model are further performed,and the multiscale model and mesh generation technology is developed.The coupled heat transfer characteristics between the liquid LBE and helium in the main heat exchanger are obtained.And the empirical correlations of pressure drop and heat transfer for the liquid LBE heat exchanger are proposed.By considering the influence of gravity effect on the pressure drop and heat transfer performance,the rational inclined angle of the main heat exchanger is given.For the experimental research of liquid LBE heat exchanger,the LBE and helium gas experimental loop based on accelerator driven sub-critical system(LELA)has been designed and built.The pressure drop and heat transfer characteristics of the liquid LEB main heat exchanger are studied on the experimental platform.By conducting preliminary experiment on nitrogen,the experiments with variable mass-flow-rate and temperature of the liquid and helium and compressor sudden shutdown experiment,the variation rules of the heat transfer performance and the method of heat transfer enhancement for liquid LBE heat exchangers under extreme conditions have been obtained.In the numerical simulation of the primary surface recuperator using high-temperature flue gas as working medium,the half-wall thickness multi-periodic boundary conditions and a 3D fluid-solid coupled heat transfer model for the CW recuperator are proposed.The block division technique between structured and unstructured grids has been developed in the simulation.The numerical results reveal the influence of different boundary conditions and structural parameters on the heat transfer performance of the recuperator.It is verified that the pressure drop characteristics of the working fluids play an important roal in the overall performance of the recuperator.Therefore,a comprehensive account of the pressure loss and heat transfer capacity should be taken for designing the heat exchangers.The influence of the channel amplitude in the flow direction on the secondary flow intensity and thermal hydraulic characteristics of CW recuperator are further investigated.It is found that the secondary flow will be significantly enhanced in the regions where the fluid flow direction is changing,and the secondary flow intensity increase with the amplitude increasing.Although the heat transfer performance of the recuperator will be enhanced with the increasing of the secondary flow intensity,the pressure loss will also increase.The empirical correlations of pressure drop and heat transfer for the CW recuperator are proposed based on lots of numerical results.And the quantitative relationship between the secondary flow intensity and the heat transfer Nusselt number in the recuperator is also obtained.For the experimental research of CW recuperator,the flow and heat transfer experimental platform is designed and the selection of related experimental components is completed in this paper.The process of experimental operation and research scheme are further proposed.Combining CFD numerical simulation with experimental research,the flow and heat transfer characteristics of high efficiency heat exchanger based on special mediums are investigated in this thesis.The influence of boundary conditions and structural parameters on the thermal-hydraulic characteristics of heat exchangers is further studied,and the empirical correlations for predicting heat transfer and pressure drop of heat exchangers are proposed.The investigation of this thesis can provide theoretical foundation for the development of efficient heat exchangers based on special mediums,and make contributions to the improvement of energy efficiency.