| The vehicle industry is an important pillar industry of the national economy,which has greatly improved people’s living standards and travel convenience.According to the latest statistics of the Ministry of Public Security,the number of vehicles in China is 281 million in 2020,ranking first in the world,but the problems of environmental protection and energy waste have become more serious.The automobile thermoelectric generator(ATEG)can convert the waste heat energy of high-temperature exhaust gas into electricity,which is in line with the development theme of energy conservation and emission reduction in the vehicle industry.However,the complexity and variability of the vehicle operation conditions lead to the fluctuation of the exhaust temperature and mass flow rate under transient conditions.At present,most of the related studies at home and abroad focus on the steadystate performance of the ATEG,and the transient research is centered on thermoelectric refrigeration.At the same time,the working process of ATEG touch on the flow-thermalelectric complicated multi-physical field coupling.Hence,a transient CFD(computational fluid dynamics)-thermoelectric coupling model of the ATEG is established in this work,supported by the National Natural Science Foundation Project of China “Study on multi-field coupling mechanism and energy dynamic programming of vehicle thermoelectric generator system under variable working conditions”(No.51977100).The model is solved by the finite element simulation software and used to predict the dynamic response characteristics of the ATEG under variable working conditions.The main research contents are as follows:Firstly,the thermal-electric energy conversion phenomenon inside the thermoelectric semiconductors is introduced.On the basis of the basic theoretical formula of thermodynamics,the steady-state and transient-state thermoelectric coupling numerial model for a single thermoelectric module(TEM)are constructed respectively,and several different transient heat source excitations are set for the model solution.The transient changes of temperature,output power and conversion efficiency are analyzed by means of COMSOL finite element software to explore the time delay rule of dynamic response characteristics.In the next place,ATEG is summarized,and its structure,operating principle and categories are described.Due to the long time of finite element solution,the physical structure of ATEG and TEM are predigested,and the parameters of equivalent structure are derived by the formula.Considering the flow and heat transfer of the internal air in a heat exchanger and cooling water,the steady-state and transient-state CFD governing equations are proposed,including the formula of mass conservation,momentum conservation and energy conservation.In order to accurately study the actual working state of ATEG,highway fuel economy test cycle is selected as the simulation working condition,and the transient temperature of the hot and cold sides of TEM is analyzed as the boundary condition of the subsequent chapters.Then,the analysis flow of transient response characteristics of ATEG is described in detail,and the coupling relationship between transient CFD-thermoelectric model is explained.Taking the whole thermoelectric system as the research object,based on the above theoretical basis and simulation results,taking the transient average temperature at both sides of the thermoelectric module as the boundary input,the dynamic response characteristics of ATEG under HWFET condition are studied.The output performance of ATEG under steady and transient conditions is compared to illustrate the predictability of the two models.In addition,the temperature uniformity of heat exchanger hot side of ATEG is evaluated.Finally,the transient test bench of ATEG and thermoelectric module are built to verify the transient thermoelectric model and the transient CFD-thermoelectric coupling model respectively.The test results are considered to be in accord with those obtained by solving the numerical model fundamentally,and the causes of time delay and error are analyzed reasonably,thus the validity of transient numerical model are verified.The theoretical analysis and experimental study show that the average output power of TEM is improved by 3.53% and 4.99% respectively under the triangle and sine wave transient heat source excitation compared with the results of steady-state heat source input,but it has a negative impact on the thermoelectric conversion efficiency.And the time delay is positively correlated with the change rate of temperature.The average output power and average thermoelectric conversion efficiency of ATEG are 26.808 W and 2.966%respectively when the vehicle drives under HWFET cycle conditions for 300 s.The prediction of transient characteristics of ATEG by steady-state model may overestimate the output power and underestimate the conversion efficiency.The transient-state CFD thermoelectric coupling model can effectively evaluate the transient response characteristics of ATEG under under variable operating conditions. |
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