EGR Waste Heat Recovery System Optimization Design And Dynamic Simulation Of Marine Diesel Engine

Two-stroke marine diesel engines are widely used in the ships all over the world because of their high efficiency and reliability.Exhaust Gas Recirculation(EGR)system is adopted by the main engine manufactory for the purpose of decreasing the NOx emission in marine diesel engines.However,the fuel economy of the diesel engine is poor due to the poor burning efficiency in the cylinder and additional power consume of the EGR auxiliary device.For the purpose of increasing the fuel economy of the diesel engine,the recovery of the waste heat of the recirculation gas are studied.In this paper,the EGR system of the diesel engine,which model is 6S80ME-C9.5,is treated as the study object,and the EGR system is combined with a Waste Heat Recovery(WHR)system to recover the waste heat of the EGR gas.The new EGR and WHR combined system can simultaneously reduce the NOx emission and the EEDI of the diesel engine.The simulation method is adopted in the paper to analysis the waste heat recovery of the EGR gas.The mathematical models of the heat exchanger of the WHR system,which include the steady,dynamic heat exchanger model,are first developed in the paper,then the mathematical model of the pump and expander,at last these models are assembled together according to the relationships of the components to establish the WHR system steady and dynamic simulation models.The NSGA-II algorithm and the WHR system steady simulation model are adopted to obtain the optimization parameters of the WHR system and the dynamic simulation model are adopted to evaluate the dynamic characteristics of the WHR system.The main works and conclusions of the paper are:(1)On the basis of the merits and drawbacks of the steady simulation models in the literature,the Moving Boundary(MB)and Finite Volume(FV)Coupling algorithm for the heat exchanger with fluid phase change is put forward by the author independently.The computational accuracy and speed are well accomplished by the new heat exchanger steady simulation model.The dynamic simulation model based on the Finite Volume method are also developed in the paper.(2)The out of order solving model of the components of the WHR system is raised by the author because of the special property of the WHR system model already exist.The components in the WHR system are solved by the attribute of the components,that is,the components which are able to solve are solved first,and the other conponents which are not able to solve are solved later.The WHR system simulation model established with the out of order solving model can solve any type of WHR system without or small modify the simulation program.(3)The case studies of the heat exchanger using the MB-FV coupling algorithm are carried out in the paper,including the sub-critical and super-critical heat exchanger.The computational accuracy and speed of the MB-FV coupling algorithm are obtained by comparision the simulation results of the MB-FV coupling algorithm with the FV model and MB model of the heat exchanger.The case studies of the dynamic heat exchanger model are also given in the paper.(4)The parameters of the EGR gas are obtained by the backsteeping of the results of the CEAS program and the backsteeping steps are given in the paper.The simple thermaldynamic analysis results of the EGR gas are acquired and the necessity to recover the waste heat of the EGR gas are given in the paper.(5)Two structure of the EGR and WHR combined system,which are series and parallel system,are proposed in the paper to reduce the NOx emission and EEDI of the diesel engine simultaneously according to the structure of the EGR system of the diesel engine 6S80MEC9.5.The merits and drawbacks of the series and parallel system are discussed in the paper.Results show that the series system not only can recover the waste heat of the EGR gas but also can recover the waste heat of the pressured air when EGR system is shutting down.Moreover,the volume and the pressure lose of the series system are also very good compare with the parallel system.(6)Five structure of the WHR system of the EGR gas of 6S80ME-C9.5,which are RORC cycle,S-CO2 brayton cycle,dual-loop RORC cycle,S-CO2 and RORC combined cycle and SCO2 and ORC combined cycle,are proposed in the paper.In order to evaluate the performance of the five combined cycle,the maximum power and minimum heat transfer area of the combined cycle are calculated using NSGA-II algorithm.Results show that S-CO2 and RORC combined cycle with the advantage of low system investment,low space volume and high power is more suitable to recover the waste heat of the EGR gas.On the basis of the results of the multi-objective optimization,the heat exchanger geometry parameters of the S-CO2 and RORC combined cycle are obtained through the MB-FV coupling algorithm.(7)The dynamic model of the ORC system is established in the pape.Moreover,the dynamic characteristics of the low temperature RORC cycle of the S-CO2 and RORC combined cycle are obtained in the paper.Results show that the thermal lag exists in the evaporator and condenser,and the response time of temperature is greater than pressure.The mass flow rate of the expander lags behind the pump in the dynamic process of the ORC system.