| Since the law for heat generation of fuel cell vehicle(FCV)is significantly different from those of vehicles powered by other sources,the thermal management system(TMS)of FCV faces plenty of challenges and difficult problems.Firstly,fuel cell works at a relatively low temperature,and all of its waste heat(41%-62% of gross power),is dissipated by its TMS.Thus,available temperature differents of the TMS is small,and its heat load is 2.5-3 times the amount of that of cooling system of combustion engine.Secondly,lots of electrical components in FCV need to be cooled and strictly controlled temperatres.The TMS is required to cool multiple components and control their temperatures precisely at the same time.Thirdly,heat exchangers from subsystems in the TMS are formed as a cooling module by series-parallel connecting on air flow direction to save space.Due to the thermal coupling among the subsystems,peformances of subsystems will be deteriorated.So,it's neccesary to comprehensively and thoroughly study flow and heat transfer characteristics of multiply working fluids(air,coolant and refrigerant)in heat exchangers,cooling modules and systems in the thermal environment provided by FCV.It is useful in match design and performance optimization of the TMS,so that the TMS could fullfill the demanding cooling requirements and working conditions of FCV.By numerical and experimental approaches,performance studies on heat exchangers,cooling module and TMS have been performed.The main research work has been done as follows.Flow and heat transfer performance of heat exchangers have been investigated.Firstly,the effects of pin-fin arrays with different shapes,diameters,heights and spaces on performance of power control unit(PCU)cold plate were numerically studied.The optimal structure has been selected to meet the requirements of wall temperature,heat transfer and pressure drop.Thus,the cold plate could cool and control temperature of PCU with a little power consumption.Secondly,flow and heat transfer characteristics of mini-channel parallel flow heat exchangers were experimentally investigated.It was found that in the mini-channels(1mm<D<3mm)the flow transition occurred at Rec=1800 with an experimental Re range of [700,4000].Based on the experiment results,a set of correlations had been proposed to describe the flow and heat transfer characteristics in mini-channel and the correlations would be used for high precision performance calculation of minichannel heat exchangers under operating conditions of FCV.Thirdly,a refrigerant-side mal-distributed(MD)model had been developed for multi-pass parallel flow condensers.The MD model has achieved better agreement with experimental data than the uniform distributed(UD)model.The model was applied to analyze effects of aspect ratio,pass arrangement and refrigerant mass flow rate.At last,a model of plate-fin liquid-cooled charge air cooler was built and validated.The model could be applied to the charge air cooler with small flow rate in FCV.All of the heat exchangers researches above were the important bases of the following researches of cooling module and TMS.Flow and heat transfer performance of cooling module in underhood has been studied.Firstly,a distributed parameter model of cooling module has been built.The predicted heat capacities and pressure drops fitted well with the measured data from a wind tunnel test of cooling module.Secondly,the air flow fields of cooling modules with different structures in FCV underhood were simulated with a CFD model.Based on the simulated results,the impacts of vehicle speed,fan rotation speed and module structure on air flow rate and flow quality of cooling module(ratio of reverse flow,non-uniformity of air velocity)have been studied.Thirdly,based on the CFD model and the distributed parameter model,a multi-dimensional model of cooling module had been built to simulate the flow and heat transfer performance of cooling module under the FCV hood.The model has been validated by a module performance test under the hood.Fouthly,the effect of non-uniformity of air velocity on thermal performance of cooling modules was studied and several match and design principles of cooling module were proposed.At last,based on the field synergy principle,method of performance optimization had been found for the given module structure and underhood flow field.Research findings: At low and medium vehicle speed(?60km/h),the air flow non-uniformity of modules deaceased first and then incresed with the rise of fan speed.At high vehicle speed,the non-uniformity kept decreasing with the rise of fan speed.While the non-uniformity of air velocity decreased,the decrease of heat capacity of module caused by mal-distributed air flow in underhood became smaller.The maximun non-uniformity of air velocity was found at vehicle speed of 120km/h,and fan rotation speed of 0,while the decreases of all modules were as high as 7%-9%.Integrated performance of thermal management system has been analyzed and evaluated.Firstly,power consumption model of vehicle and models of main heat source components(such as fuel cell stack,air compressor system,motor system,and so on)in FCV were built,and laws for heat generation of heat sources had been studied.Secondly,thermal management system model has been built by combining cooling system model and air conditioning(AC)model.The TMS model could be used to simulate thermal environment of FCV with various ambient temperatures and driving states.Based on the rule of thermal equilibrium,flow and heat transfer performances of cold and hot fluids in thermal coupled cooling systems and AC system had been predicted.At last,performances of two TMS with different cooling modules for a given FCV had been evaluated by the TMS model.And the "heat balanced vehicle speed" VHB,was used as the criteria for performance evaluation of TMS.The results calculated by the TMS model fitted well with measured data from the thermal management system test on FCV.Research findings: Considering the laws for heat generation of FCV and heat transfer ability of TMS,VHB,which is the highest vehicle speed when cooling systems could cover heat loads,was proposed as criteria for TMS of FCV.The VHB of TMS could practically represent the matching degree of TMS and FCV.And values of VHB of cooling subsystems could be a guideline for matching optimization of subsystems. |
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