| The louvered fin and flat tube heat exchanger(LFHE)has been widely used in automotive cooling systems for its compactness and lightweight,and its cold side heat transfer performance has an important impact on automotive energy conversion efficiency and fuel economy.Currently,studies on the LFHE mainly focus on improving its cold side heat transfer performance by reasonably designing and optimizing the geometric parameters of louvered fins(LFs),while studies on heat transfer enhancement by vortex generators(VGs)are mostly focused on tube fin heat exchanger(TFHE).The above studies mainly have the following problems:Firstly,there is a lack of arranging VGs on cooling tubes and exploring their impacts on the cold side heat transfer performance of the LFHE;Secondly,after applying VGs to the LFHE’s cold side,there is a lack of studies on the optimization of the corresponding VGs’ geometric parameters;Finally,experimental studies on enhancing convective heat transfer of the LFHE’s cold side with VGs is not yet sufficient.For the above reasons,this paper conducts an in-depth study of the LFHE using VGs to enhance cold side convective heat transfer through numerical simulation,multi-objective optimization,and experiments based on similarity theory.The main contents are as follows:(1)A simplified basic physical model for the cold side of the LFHE and numerical simulation method was established and verified by wind tunnel tests of heat exchangers.The results show that the numerical values of pressure drop and convective heat transfer coefficient of the LFHE are in good agreement with the experimental ones,with errors of±5.2%and±7.02%under different operating conditions,respectively.Therefore,the simplified model and numerical simulation method established have good accuracy and reliability,and which can be used as the basis for subsequent studies.(2)The physical model of louvered fin-common flow up vortex generator(LF-CFUVG)is proposed,and its flow and heat transfer characteristics are explored based on numerical methods.The results show that the separated vortices formed at the openings of the LFs and the "development-destruction" effect of these openings on the thermal boundary layer are the main reasons for the increase of flow resistance and the enhancement of convective heat transfer,respectively.However,the decrease of the minimum free flow area and the increase of pressure drag caused by the CFUVG,as well as the suppression of the thermal boundary layer near the welding position of the LFs and cooling pipe by the longitudinal vortex induced by CFUVGs will further increase the flow resistance and convective heat transfer intensity.It is also found that the pressure drop of LF-CFUVG increases with the CFUVGs’ geometric parameters,but the heat transfer coefficient and the comprehensive performance decrease when the angle of attack reaches 50°.In the correlations of pressure drop and convective heat transfer coefficient obtained by multiple linear regression method,their errors are within 18%and±6%respectively,the flow and heat transfer performance of LF-CFUVG can be evaluated and predicted efficiently by these correlations.(3)The physical model of louvered fin-common flow down vortex generator(LF-CFDVG)is proposed,and its flow and heat transfer characteristics are studied.The results show that the position with the highest flow resistance and the most intense convective heat transfer in LF-CFDVG is still at the openings of the LFs.The longitudinal vortices induced by the CFDVGs enhance the convective heat transfer intensity of the cooling pipe wall between the CFDVGs,and the CFDVGs increase flow resistance.The pressure drop of the LF-CFDVG increases with the CFDVGs’geometric parameters,but the convective heat transfer coefficient and comprehensive performance tend to decrease when the angle of attack reaches 50°,and the correlations for pressure drop and convective heat transfer coefficient are with errors of±7%and±6%,respectively.After comparing the heat dissipation of the LF-CFUVG and LF-CFDVG,it is found that the average difference between them is no more than 3%.Therefore,the effectiveness of the CFUVG and CFDVG in enhancing the convective heat transfer of the LFHE’s cold side is the same.(4)Based on multi-objective optimization methods,an optimization study was conducted using the geometric parameters of the VGs as optimization variables,and the pressure drop minimization and convective heat transfer coefficient maximization of LF-CFUVG and LF-CFDVG as objective functions.The results show that the regression models for pressure drop and convective heat transfer coefficients of the LF-CFUVG and LF-CFDVG have good goodness of fit,with R2 above 98%.Through response surface analysis,it was found that a smaller value for the geometric parameters of the VGs can reduce the pressure drop,while when the angle of attack is smaller and the other geometric parameters are larger,it is beneficial to enhance convective heat transfer.After optimization,it was found that the average growth rates of comprehensive performance of the LF-CFUVG and LF-CFDVG under different operating conditions were 20.76%and 22.47%,respectively.(5)Based on the similarity theory,the scaled-up physical models of the LF-CFUVG and LF-CFDVG corresponded to the optimal results were designed and fabricated,and a direct-flow wind tunnel test platform for measuring the flow and heat transfer performance of heat exchanger was built.The results show that the pressure drop and convective heat transfer coefficient of the enlarged models increase with air velocity,and the theoretical value of the proportional relationship between the pressure drop and convective heat transfer coefficient of the prototype and the enlarged physical model tends to be consistent with the actual value,which indicates the correctness of the optimal schemes of the LF-CFUVG and LF-CFDVG. |
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