Research On Heat Transfer And Thermal Storage Characteristics Of Phase Change Materials Enhanced By Dual Gradient Metal Foam Structur

The working characteristics and service life of power batteries are closely related to temperature,making it crucial to establish a scientific Battery Thermal Management System(BTMS).Phase Change Material(PCM)latent heatabsorbing and reversible heat-release characteristics during phase change have been widely utilized in battery thermal management.However,the low thermal conductivity of PCMs has been a major challenge,in that this affects their heat storage rate.To address this issue,various materials such as metal fins,metal nanoparticles,and metallic foams have been added to the PCM.Among them,metallic foams have become a research hotspot,due to their high specific surface area,high structural strength,and low production costs.Therefore,this study aims to explore the factors affecting the melting and flow of PCM by using metallic foam as an additive,providing new strategies and ideas for the development of a composite PCM battery thermal management system.This dissertation discusses the heat transfer and thermal storage performance of composite materials by constructing a thermal model for battery cells and a phase change heat transfer model for foam metal composite PCM.Fluent software is used to simulate the temperature field of the lithium battery and the solid-liquid interface change process of the composite PCM.The calculation results show that the temperature of the lithium battery increases with the discharge rate and the ambient temperature.The phase change heat transfer process of the composite PCM is affected by the material properties and structural characteristics of the material itself,and the porosity and pore density are the key factors that determine the structural characteristics of the composite material.Optimizing the foam metal structure can find a balance point between heat transfer and thermal storage performance.Based on this,this article proposes a foam metal structure with a gradient change in porosity and establishes a composite PCM pore scale model.By analyzing the influence of pore size gradient structure on the phase change heat transfer characteristics of composite PCM,this dissertation constructs an ideal physical structure applicable to composite materials.Under the influence of the discharge rate and ambient temperature,the PCM melting process can be divided into four stages.The higher the discharge rate and ambient temperature,the higher the average surface temperature of the battery and the shorter the PCM melting time.The average temperature difference of the PCM surface increases with decreasing height.When the discharge rate is 2C and the ambient temperature is 303 K,the surface average temperature difference of the bottom point is the largest,reaching-5.5K.This paper introduces the effective thermal conductivity equation of the composite PCM and derives the impact of metallic foam materials on the melting performance of the composite PCM.When copper is used as the metallic foam material for the composite PCM,the complete melting time of the composite material is the shortest,which is 32 seconds.The thermal storage density of the composite PCM is mainly affected by the specific heat capacity and density of the material.The thermal storage density of metal aluminum is the highest,reaching 58.7% of the thermal storage density of pure PCM,and has a high thermal storage capacity.Porosity and pore density are important parameters that affect the phasechange heat transfer of metallic foam composite PCM.The larger the porosity,the longer the melting time of the composite PCM;the larger the pore density,the shorter the melting time of the composite PCM.Simply increasing pore density or reducing porosity does not significantly enhance the heat transfer capacity of the composite PCM.The heat transfer process of the composite PCM is affected by both heat conduction and heat convection,and excessively increasing pore density will inhibit the formation of natural convection.The boundary heat flux of the composite PCM increases with increasing porosity.When the porosity is0.8,the maximum boundary heat flux of the composite PCM is 847 J/s,which is0.6% and 24% higher than the composite PCM with porosities of 0.85 and 0.9,respectively.At this time,the thermal storage density is the smallest,only 58.7%of pure PCM.The melting performance of the composite PCM is affected by the pore-size gradient orientation and size.Although the double-pore gradient structure did not significantly improve the effective thermal conductivity of the PCM,it still performed better than any single gradient structure,reaching 25.61W/(m·K).Maintaining uniform battery surface temperature is key to establishing a reliable thermal management system.Gradually optimizing the pore-size gradient size can achieve an ideal composite material structure.Under extreme conditions,the maximum average temperature difference of the optimized structure is 14.7K,with a total thermal storage amount of 75.5% of pure PCM.Based on the research results of this paper,the structural parameters,physical properties,ambient temperature,and battery discharge rate of the metallic foam composite PCM have a significant impact on the melting characteristics and phase-change thermal storage capacity of the PCM.By optimizing the structure of the metallic foam and establishing a composite PCM model with a gradient porosity,the phase-change heat transfer performance of the composite material is explored.The final results show that the metallic foam composite PCM optimized with a gradient porosity can achieve superior heat transfer and temperature uniformity performance and meet the requirements of battery thermal management systems.