| Thermal management system is critical to fuel cell’s performance,life cycle and safety.For high power fuel cell vehicles,there are some technical difficulties such as high heat dissipation requirement,limited space inside engine compartment,heat dissipation interaction between subsystems.It is necessary to study about heat dissipation performance for fuel cell vehicle driving system which is helpful to increase fuel cell vehicle’s reliability and safety.Since nanofluid has good heat transfer characteristic,this paper used nanofluid to improve heat dissipation performance for high power fuel cell vehicle driving system.Firstly,the stability,the heat dissipation capacity and the p H value of four types nanofluids were compared through theoretical analysis and experimental test.Zn O nanofluid was selected as coolant.Based on heat transfer experiment,the formula of Nusselt number for nanofluid at laminar flow in the tube was obtained and verified.Synthesizing Hamilton & Crosser formula and nanofluid adsorption layer,a theoretical model of nanofluid thermal conductivity was obtained.The model was also verified by experimental test.Based on electrochemical reaction mechanism of proton exchange membrane fuel cell,a single fuel cell model was constructed.The single fuel cell model accuracy was verified by comparing with reference data.The temperature characteristic was also analyzed.The association of 68 k W fuel cell model of output power,heat generation and efficiency were analyzed.The heat generation under low-speed climbing(40km/h,30% gradient)and maximum speed(150km/h)conditions were obtained by using output power and efficiency,respectively.In order to achieve heat dissipation requirement of fuel cell vehicle drive system,single-loop,double-loop and triple-loop structures were constructed.The simulation model of three heat dissipation structures were built with GT-COOL.The triple-loop structure was selected finally.The variation of heat dissipation capacity,radiator size,pump flow rate and pump energy consumption were compared by using conventional coolant and Zn O nanofluid.The temperature field inside the engine compartment was analyzed with computational fluid dynamics(CFD)method.The variation of temperature field under low-speed climbing(40km/h,30% gradient)and maximum speed(150km/h)conditions with conventional coolant and Zn O nanofluids were compared.In addition,the variation of temperature field with different size of radiator and flow rate of pump were compared.The research indicated that Nusselt number formula can calculate Nusselt number of nanofluid on laminar flow inside radiator tube.The thermal conductivity model can calculate nanofluid thermal conductivity accurately.Comparing to conventional coolant,Zn O nanofluid can increase heat dissipation capacity by 8.3% under low-speed climbing(40km/h,30% gradient)condition,and reduce radiator size,pump flow rate and energy consumption by 12%,3.4% and 5.4% respectively.Zn O nanofluid also can keep the surface temperature of fuel cell system below allowable temperature limit.The research can provide theoretical and experimental reference for using nanofluid as coolant in fuel cell vehicle driving system.It also proved that nanofluid has significant value for scientific research and engineering application. |
