Study On Heat Transfer Enhancement Technologies Of Evaporator In Organic Rankine Cycle System For Vehicles

The efficient use and recycle of energy have become the most important topic since the government of China call for energy conservation and emissions reduction.Vehicles with internal combustion engine(ICE)are the main consumer of fossil energy.Due to the limit of combustion efficiency,heat transfer loss and the structure of ICE,part of energy in fuel isn’t utilized to convert into mechanical work.The technologies of waste heat recovery of ICE are rapidly developed to reduce the waste of fossil energy when ICE operate,make full use of the heat in exhaust and improve the fuel economy.Organic Rankine cycle(ORC)is suitable for waste heat recovery of ICE exhaust because its simple structure and low operating cost.Evaporator is one of the most important components of ORC system.Enhancing the heat transfer performance and reducing flow resistance of exhaust in evaporator can not only improve the heat to work conversion efficiency of ORC system,but also reduce the influence of evaporator on the ICE operation.The evaporator applicable to this scenario should be specially designed due to the ICE usually runs under variable operating conditions which cause the exhaust parameters are constantly changing.The evaporator in internal combustion engine-organic Rankine cycle(ICE-ORC)combined system is studied from two aspects:the active and passive technology for heat transfer enhancement.Based on passive technology,the star-shaped fin and elliptical tube are used and analyzed in this evaporator to mitigate the influence of ORC system on ICE and reduce the back pressure of exhaust.The model of shell side of evaporator is constructed by COMSOL Multiphysics and its validity is verified by experimental data.The distributions of temperature and velocity of exhaust under ICE typical working conditions can be obtained,and the performance of evaporator can be evaluated by Nusselt number,Euler number,area goodness factor and field synergy module average angle.The results show that the performance of the evaporator can be improved by using star-shaped fin and selecting the parameters of elliptical tube reasonably.The area goodness factor increases by 86.42% when the star-shaped fin is employed,and the Euler number decreases by 1.229% when the elliptical tube with tube rotation angle of0° and ellipticity ratio of 0.4 is used.Working fluid pump is the power source of working fluid flow in ORC system and provides the convenient condition for pulsating flow heat transfer enhancement.Based on active technology,a two-dimensional axisymmetric model of evaporator tube is built.Two-phase flow characteristics using R245 fa as working fluid under continuous flow and pulsating flow are analyzed.The pressure drop of R245 fa pulsating flow,Nusselt number and volume fraction of R245 fa vapor at outlet of tube under different angle frequency,amplitude and oscillation mode can be obtained.The results show that the volume fraction of R245 fa vapor at outlet of tube and heat transfer performance can be improved by high frequency pulsating flow.The maximum increase of Nusselt number and volume fraction are 17.03% and 15.56%,respectively.The sinusoidal pulsating flow with high angle frequency is more suitable for ICE-ORC combined system than other oscillation modes due to it has less influence on flow state and pressure drop of R245 fa while strengthen the heat transfer performance.Furthermore,the optimization models that combine two-dimensional and thirddimensional physical field distribution in COMSOL,optimization algorithm in MATLAB and road condition of ICE are used to optimize the evaporator structure and operating parameters of pulsating flow in ORC system.For the shell side of evaporator,genetic algorithm(GA)is used to optimize the structure of evaporator to obtain the nonuniform structure evaporator.By optimizing of GA,the area goodness factor increases by 173.76%,while the field synergy module average angle and friction factor decreases by 6.17° and 56.67%,respectively.For tube side of evaporator,particle swarm optimization algorithm(PSO)is used to optimize the parameters of R245 fa pulsating flow.Compared with R245 fa continuous flow under the Common Artemis Driving Cycle(CADC)road condition of DE,the average convective heat flux and the volume fraction of R245 fa vapor at outlet of tube with optimized high frequency sinusoidal pulsating flow can be increased by 158.11% and 87.24% respectively,while the flow pressure drop can be decreased by 1.47%.