Performance Evaluation And Enhancement Design Research On Exhaust Heat Exchanger For Engine Waste Heat Recovery

As the most widely used power component in mobile devices,internal combustion engine（ICE）’s energy saving and emission reduction are of great significance to the sustainable development of economy and environment.Because the exhaust of ICEs has the characteristics of high temperature and high energy,the waste heat recovery（WHR）of exhaust based thermal cycle is recognized as one of the potential and efficient technologies.Exhaust heat exchanger,as a key part in the recovery system,its heat transfer performance directly affects the overall system efficiency,and the exhaust back pressure rise and additional load will cause power loss of ICE.Therefore,in view of the requirements of high efficiency,low pressure drop and light weight of exhaust heat exchanger in WHR system of ICEs,this paper carried out a study on the performance evaluation and enhancement design of exhaust heat exchanger based on metal foam.In view of the problem that the current evaluation index of exhaust heat exchanger cannot guide the actual performance due to insufficient factors,this paper established the comprehensive evaluation index of exhaust gas heat exchanger applicable to the WHR of mobile ICEs and gave its mathematical model.Based on the index,an optimization design method for the whole process of exhaust heat exchanger covering three levels of structural design,performance evaluation and parameter optimization was constructed,which provided a method basis for the subsequent whole process analysis of the exhaust heat exchanger.According to the first-level of the method,the structure design of exhaust heat exchanger based on metal foam baffles and fins was studied.Firstly,the single-tube model with metal foam fins was established to complete the exploration of key parameters variable rules.Then,the internal structure parameters of the heat exchanger were determined based on the optimization analysis results of single-tube parameters,and the structure design of six kinds of heat exchangers were completed according to different layout methods of metal foam baffles and metal foam fins.According to the second-level of the method,the performance evaluation research for the six shell and tube heat exchangers designed in the first level is carried out.CFD simulation of six heat exchangers was completed to obtain basic performance including flow,pressure drop,heat transfer and weight,then comprehensive performance evaluation of different heat exchangers was carried out.The results show that the shell and tube heat exchanger with metal foam baffles and annular fins is the best structure.Compared with the shell and tube heat exchanger with solid baffles and no fins,its comprehensive performance promotes 3.33 kw.According to the third-level of the method,the parameter optimization research based on the best structure of shell and tube heat exchanger determined by the second level was carried out.The influence of key parameters on the comprehensive performance was explored,and the optimization method combining CFD,ANN and GA algorithm was used to complete the global optimization process within the given range of independent variables.The results show that when the number of metal foam fins is 94 m^-1,the inner tube pitch is 58.60 mm,and the porosity is 0.702,the comprehensive performance of the best structure shell and tube heat exchanger reaches the maximum value of 4.28 k W.Compared with the existing conventional serpentine tube exhaust heat exchanger in the laboratory,the new-type of shell and tube exhaust heat exchanger obtained through the above whole-process optimization design method can increase the heat transfer performance by 9.0%,lower the pressure drop performance by 76.0%,lessen the weight by 78.1%,reduce the volume by 85.7%,decrease the power loss by 74.6%,and improve the comprehensive performance by 100.9%.