Study On Heat Transfer Characteristics Of Non-azeotropic Mixed Working Fluid Pulsating Heat Pipe At PEMFC Temperature Range

With the global energy crisis and environmental pollution worsening,proton exchange membrane fuel cell(PEMFC)uses clean energy hydrogen to generate electricity through the chemical reaction of hydrogen oxidation.It has high electric conversion efficiency and no pollution to the environment.Accordingly,it has broad application prospects.However,PEMFC has suffered from high heat generation power and sensitivity to working temperature,which restricts the further improvement of its performance.Therefore,how to optimize the PEMFC thermal management system,keeping it at the normal operating temperature range,and improving its temperature uniformity are extremely important.The existing research shows that the traditional wick heat pipe and micro heat pipe array have been successfully applied to the PEMFC thermal management system due to their high thermal conductivity and high temperature uniformity.As a new passive heat transfer superconductor,the flat-plate closed loop pulsating heat pipe(CLPHP)with a higher heat transfer efficiency.However,for the PEMFC temperature range,it is still not clear whether the flat-plate CLPHP using a binary non-azeotropic mixed working fluid has advantages over the pure working fluid.There is a lack of in-depth analysis in basic research on the heat transfer mechanism of the flat-plate CLPHP,the influence of the state of the internal working fluid on the heat transfer performance with the increasing heat flux,the theoretical analysis of heat transfer performance,and the heat transfer resistance model.Therefore,based on the key problems in the research of heat transfer characteristics of flat-plate CLPHP,the heat transfer characteristics of non-azeotropic mixed working fluid pulsating heat pipe at PEMFC temperature range are carried out in this paper.The main research contents are as follows:Initially,a flat-plate CLPHP with a rectangular flow channel structure is designed.Based on the working temperature range(70–90℃)of PEMFC,combined with the gasliquid phase transformation diagram,the heat transfer performance of flat-plate CLPHP with binary non-azeotropic methanol deionized water of various mass ratios and pure working fluids is contrastive analysed.Meanwhile,on the basis of the thermophysical parameters of the working fluid,a semi-empirical formula for the dimensionless number set and heat flux is established.Moreover,the effects of filling ratio,noncondensable gas content,and horizontal gravity field component on the start-up characteristics,heat transfer performance,and internal relative pressure of flat-plate CLPHP are clarified.The research results show that the binary non-azeotropic working fluid of methanol deionized water with a mass ratio of 5:1 in the flat-plate CLPHP has excellent heat transfer performance,the optimal liquid filling ratio is 40%,the noncondensable gas reduces its heat transfer efficiency,and it is less sensitive to inclination angle.Secondly,the with and without bubble nucleation start-up process of flat-plate CLPHP is studied through visual experiments,and the force balance equation of the bubble formation process is established in combination with bubble dynamics.In addition,the start-up process of CLPHP is defined according to the changing law of heat transfer resistance.It also reveals that the flow pattern of the vapor-liquid working fluid in the evaporation section of the flat-plate CLPHP with the increase of heat flux,and the state of the vapor-liquid working fluid in the internal flow channel and heat transfer performance when the dry-out phenomenon occurs are analysed.The reason for the lower critical heat flux of the edge flow channel for start-up and dry-out compared to the middle flow channel is clarified.It is found that the liquid plug of the flat-plate CLPHP moves erratically and non-unidirectionally from start-up to dry-out.Furthermore,the liquid plug is mainly concentrated in the adiabatic and condensation section,and its pulsation characteristics are similar to mechanical vibration when the temperature of the evaporation section is high.What’s more,combined with the Buckingham π theorem,a dimensionless analysis model is established,and the relationship between the vapor-liquid flow velocity and heat transfer performance of CLPHP is clarified.The force model of the liquid plug movement of the flat-plate CLPHP is constructed,and the influencing factors of the vapor-liquid flow velocity and the effect on the nucleation boiling are investigated.In order to enhance the heat transfer capability of the flat-plate CLPHP,hexadecyl trimethyl ammonium bromide(CTAB),a surfactant,is added to reduce the contact angle of the working fluid,and the optimal concentration of its addition in methanol deionized water binary non-azeotropic mixture working fluid is studied.The results show that the decrease in contact angle reduces the critical heat flux,and increases the vapor-liquid flow velocity and heat transfer performance.When the heating power of the evaporation section is 400 W,the heat transfer efficiency of 20 ppm CTAB is decreased by 29.9%,the internal relative pressure is reduced,the critical heat flux of CLPHP dry-out is increased,and the TUI of the heating plate is reduced by17.3%,which enhances the temperature stability of the heating plate as a heat source of PEMFC.Above all,aiming at the structural characteristics of the flat-plate CLPHP and the thermophysical properties of the working fluid,the theoretical model of the axial heat transfer thermal resistance of the flat-plate CLPHP is established,and according to the heat transfer characteristics of each section,the segmental and integral thermal resistance models are constructed,respectively.Moreover,the calculation accuracy of segmented and integral thermal resistance models is compared and analysed through experimental verification.Simultaneously,based on PEMFC working temperature range of PEMFC,a three-dimensional simulation model of flat-plate CLPHP is designed and established,which reveals the influence of fin density and thickness on heat transfer performance,and determines the effect of increasing the surface heat transfer coefficient of the condensing section on the heat transfer performance of the flat-plate CLPHP.The results show that increasing the thermal conductivity of the flatplate CLPHP shell and reducing the flow resistance of the vapor-liquid working fluid can enhance its heat transfer capability.The segmented thermal resistance model has higher calculation accuracy than the integral model,and the heat transfer ability of the flat-plate CLPHP is enhanced with the increase of the surface heat transfer coefficient on the condensation section.The purpose of this paper is to study the basic key issues such as heat transfer performance of flat-plate CLPHP at PEMFC temperature range.The interior of flatplate CLPHP is filled with a binary non-azeotropic mixture of methanol deionized water.Starting from revealing the heat transfer mechanism and improving the heat transfer performance of flat-plate CLPHP when working at PEMFC temperature range,combined with experiment,theory and simulation,the heat transfer characteristics and performance enhancement of self-stimulated micro rectangular channel flat-plate CLPHP are studied.The purpose is to provide a theoretical basis for the application of flat-plate CLPHP in PEMFC thermal management.