| The automobile thermoelectric generator(TEG)system can convert the low-grade waste heat contained in engine exhaust gas into high-grade electric energy,so that the fuel economy can be improved.Accordingly,the TEG system has attracted great attention in the field of automobile waste heat recovery.In previous studies,a great number of TEG system prototypes have been developed.Combined with enhanced heat transfer methods and structural optimization,the output performance of the TEG system has been greatly improved,exhibiting great application potential.However,in the existing studies concerning the structural optimization of the TEG system,the traditional analytical models or computational fluid dynamics(CFD)models are widely used to predict the output performance of the TEG system under different structural parameters.The error of the model itself may lead to the unreliability of optimization results.In addition,the traditional steady-state model can not meet the requirement of predicting the performance of the TEG system under transient conditions,and the study concerning the transient modeling of the thermoelectric generator system is insufficient.To address this issue,this work first analyzes the multiphysical field coupling mechanism of the TEG system and then conducts an in-depth study on the energy maximum utilization of the TEG system and its output response characteristics under transient driving cycles through theoretical modeling,simulation analysis,structural optimization,and experimental validation.The main contributions of this work are shown in the following five aspects:(1)The measurement method concerning thermoelectric parameters and the thermalelectric coupling model of TEMs.Aiming at the problem that the thermoelectric parameters are not provided by manufacturers or the interface contact effect is not considered in the thermoelectric parameters,the experimental measurement method of equivalent thermoelectric parameters or that of contact thermal and electric resistances are proposed respectively,which provides a basis for accurately analyzing the output performance of TEMs.Besides,the coupling mechanism between the thermal and electric fields of the TEM is revealed,a thermal-electric coupling numerical model is established,and the accuracy of the model is verified by bench tests.On this basis,the influence of different parameters on the TEM performance is analyzed via simulations,and the optimal parameter requirements of the TEM in different application scenarios are clarified,which provides theoretical guidance for researchers to select and design the optimal TEM parameters in practical applications.(2)The fluid-thermal-electric multiphysical field coupling model of the TEG system and its numerical solution methods.Aiming at the problem of multiphysical field coupling of the fluid,thermal,and electric fields involved in the working process of the TEG system,the CFD model and thermal-electric coupling model are respectively adapted to characterize the coupling process between the fluid and thermal fields and between the thermal and electric fields.Accordingly,a fluid-thermal-electric multiphysical field coupling model is proposed,which is solved by ANSYS,COMSOL separate solver,and COMSOL coupling solver,respectively.Besides,the analytical model of the TEG system is improved and compared with the fluid-thermalelectric multiphysical field coupling model.The essential reasons for the difference in simulation results predicted by different models are analyzed in detail,the general conclusions of the multiphysical field modeling for the TEG system are obtained,and a test bench is designed to verify the correctness of the theoretical analysis.(3)A multi-objective optimization method of the TEG system based on the fluidthermal-electric multiphysical field coupling model.Considering the additional power losses when the TEG system is applied to the automobile exhaust waste heat recovery,including backpressure loss,gravity loss,and pumping power loss,the net power and net efficiency models of the TEG system are established.Taking net power and net efficiency as evaluation criteria,a multi-objective optimization method of the TEG system is proposed,and the heat exchanger parameters are comprehensively optimized.Based on the concept of phased optimization,this method first optimizes the length,width,and height of the heat exchanger channel,and then optimizes the thickness and spacing of internal fins according to the optimal channel parameters.Finally,the influence law of heat exchanger parameters on the performance of the TEG system is revealed,and the TEG system prototype is fabricated according to the optimization results.(4)The transient fluid-thermal-electric multiphysical field coupling model of the TEG system.In order to fill the gap of transient modeling for the TEG system,this work introduces a transient term into the governing equations of the steady-state fluid-thermal-electric multiphysical field coupling model and proposes a transient fluid-thermal-electric multiphysical field coupling model,which provides a powerful tool for the performance analysis of the TEG system under transient heat source excitations.Besides,considering the huge consumption of calculation resources,a hybrid transient CFD-TE numerical model and a hybrid transient CFD-analytical model are also proposed,to shorten the calculation time.The differences in simulation results predicted by different transient models are compared in detail,and some general conclusions are obtained.Finally,the accuracy of the transient model is verified by transient experiments.(5)Transient heat source-based performance optimization strategy of the TEG system.Considering that the thermoelectric performance of the TEG system is higher than the TEM itself under the transient driving cycle,this work regards the complex transient driving cycle as the combination of the basic transient waveforms of exhaust heat source and vehicle speed respectively,to reveal the essential reasons why the performance of the TEG system under the transient driving cycle is improved.A meaningful finding that the transient heat source can improve the output performance of the TEG system,especially for the conversion efficiency,is obtained,which provides a new optimization strategy for improving the output performance of the TEG system.The research results of this work show that: i)In the fluid-thermal-electric multiphysical field coupling model,the model using the COMSOL coupling solver enables higher accuracy than other models;While ignoring the influence of Peltier heat,Joule heat,and Thomson heat on the coupling process between the fluid and thermal fields,the output performance and the parasitic internal resistance of the TEG system will be underestimated.ii)The fin parameter optimization can greatly improve the output performance of the TEG system;Compared with the structure without fin optimizations,the output power,net power,conversion efficiency,and net efficiency are increased by 26.85%,42.22%,9.52%,and 22.95%,respectively.iii)Although the output performance predicted by the hybrid transient CFD-TE numerical model and CFD-analytical model has the same transient response characteristics as the prediction results of the transient fluid-thermalelectric multiphysical field coupling model,these two models will cause the overestimation of output power and conversion efficiency.iv)Under the driving cycles of HWFET,NEDC,and WLTP,compared with the prediction results of the transient fluid-thermal-electric multiphysical field coupling model,the output power obtained from the corresponding steady-state analysis is overestimated by 2.28%,7.00%,and 7.05%,respectively,while the conversion efficiency is underestimated by 9.22%,8.89%,and 14.44%,respectively. |
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