Theoretical And Numerical Investigation Of Fin-and-tube Evaporator For A Diesel Engine-organic Rankine Cycle(ORC) Combined System

Based on the energy balance of engine,only a small amount of useful work can be produced from fuel combustion,most of the energy is lost through the cooling system and exhaust.Waste heat recovery from the exhaust of ICE can improve the overall energy efficiency and reduce the fuel consumption.In this study,organic Rankine Cycle(ORC)is used to recover the waste heat of engine.As a key component in ORC system,the evaporator has an important influence on the performance of the system.Therefore,it is of great significance to study the structure and performance of fin-and-tube evaporator.A thermodynamic model of fin-and-tube evaporator of ORC system used for waste heat recovery of a diesel engine is established.Particle swarm optimization(PSO)algorithm is applied for the multi-objective optimization of the fin-and-tube evaporator.Key geometrical parameters such as the inlet radius of the tube side,the inlet radius of the shell side,fin height,fin thickness and fin spacing are selected as decision variables.Total annual cost,volume of tube bundle,and exhaust pressure drop are considered as optimization objectives.The linear weighted method is used to assign a weight coefficient to each objective function.Calculation results obtained by MATLAB R2010 a clearly show a contradictory relationship among the objective functions.For a specific operating condition of the diesel engine,multi-objective optimization of the fin-and-tube evaporator is performed,and the optimal design is obtained.Compared with the results of the original design,the multi-objective optimization results are more satisfactory,the volume of tube bundle is increased by 6.49%,meanwhile the total annual cost is decreased by 71.46% and exhaust pressure drop is reduced by 27.6%.Based on the CFD method,numerical simulation of conjugate heat transfer between exhaust and finned tubes is studied,then the temperature,pressure,and velocity distribution are obtained.The effects of exhaust velocity and fin thickness on the performance of flow and heat transfer are studied.The results show that,as the exhaust velocity increases,the flow velocity of the exhaust gas in the wake region increases,and the area of the wake region can be reduced.With the fin thickness increased,the heat transfer performance of the finned tubes increased,and the pressure drop of the exhaust also increased.Based on the CFD method,numerical simulation of the overall calculation model of fin-and-tube evaporator is studied by using Fluent software.The results are verified by experimental data of diesel engine-ORC combined system test bench.Then the single phase flow and heat transfer characteristics of exhaust and R245 fa are obtained.The results show that the increment of exhaust back pressure is 127.2Pa,maximum exhaust pressure appears on the central region of the first row of tubes.Streamlines on the shell side are relatively concentrated in the central region,while the streamlines in the marginal region are less distributed.The pressure drop is 1.8bar,which affected by the flow resistance.