Research On Design And Performance Optimization Of Diesel Engine Waste Heat Recovery Bottoming System And Exhaust Heat Exchanger

Nowadays, internal combustion engines (ICEs) have become the most widelyused oil-fired power machinery and they account for more than60%of China’s oilconsumption. However, based on the existing performance indicators, about60%ofthe chemical energy of the fuel is not effectively used and is discharged into theatmosphere in the form of waste heat, causing huge economic losses and seriousenvironmental pollution. In order to make full use of waste heat of ICEs, the wasteheat recovery system performances of diesel engine bottoming organic Rankine cycleare studied in present work.In order to fully understand the characteristics of diesel engine waste heat energyflows, thermal balance test rig of diesel engine is established to obtain waste heatenergy flow parameters under various working conditions. The characteristics ofdiesel engine waste heat energy flows are analyzed from the both aspects of"quantity" and "quality". The exhaust components and the acid dew point temperatureare deduced by the combustion reaction equation, which lay foundation for furtherresearch.The subcritical and transcritical organic Rankine bottoming cycle waste heatrecovery systems with high-temperature exhaust gas as the heat source are established,and the classification and selection principles of organic fluids are discussed. Andthen two practical calculation methods of heat transfer pinch point are proposedrespectively based on the characteristics of subcritical and transcritical cycles. For theconventional refrigerant-based fluids, the results show that: the optimal working fluidfor both the sub-and transcritical cycles is R123, and the thermal efficiencies are18.3%and19.61%respectively. For high-temperature alkane-based working fluid, theresults show that: the optimal working fluid is Cyclohexane and the fuel economy ofdiesel engine is increased by9.3%.Theoretical design and experimental research of exhaust heat recovery organicRankine bottoming cycle system are performed, in which heat conducting oil isadopted as the intermediate medium. Theoretical design results show that the thermalefficiency is13.77%with R123as the optimal working fluid under optimal workconditions. The test bench is established in accordance with the experimental design conditions that are selected based on the optimal work conditions in theoretical design.The preliminary test results show that the improving potentiality of thermal efficiencyof diesel engine can reach3.38%in the best conditions after the organic Rankinebottoming cycle is adopted.In view of various shortcomings including low heat transfer efficiency and hugevolume in shell and tube exhaust heat exchanger, four kinds of designs ofhigh-performance plate fin exhaust heat exchanger are proposed. The comparativeresults show that: the heat transfer coefficients of plate fin exhaust heat exchangersare about50%larger than that of shell and tube exhaust heat exchanger, while thesizes of the former are less than7%of that of the latter. On that basis, in order tofurther improve the performance of plate fin exhaust heat exchanger, the traditionalfin has been optimized by CFD method. And two kinds of high-performance fins areproposed.To take full advantage of the waste heat of the diesel engine, the dual-looporganic Rankine cycle is established, in which the cascade utilization of waste heatcan be realized. The results show that the optimum thermal efficiency of the dual-looporganic Rankine cycle could reach24.67%with Toluene as working fluid, and theimprovement in thermal efficiency of diesel engine run up to19.37%under thiscondition.