| With the rapid development of electronic devices and power batteries,heat dissipation has gradually become one of the bottleneck technologies restricting its development.With the advantages of good reliability,simple structure and low cost,the phase change material(PCM)thermal management system(TMS)has broad application prospects for short-term or periodic heating occasions.Form-stable PCM TMSs are proposed in recent years to solve the encapsulation diffculity of conventional PCM.The main limitations of form-stable PCM are installation difficulty in compact space,large thermal contact resistance,high thermal conductivity and high latent heat can not be achived simultaneously and flammability.In order to solve the abovementioned problems,this paper takes the thermal management of battery packs as an example to study the heat transfer characteristics of flexible PCM,and the thermally anisotropic flexible PCM is proposed,and systematic analysis and study are carried out from the aspects of mechanism research,material preparation,thermal control characteristics,and flame retardant improvement.Considering the compact space thermal control requirements,a method for thermal management using flexible form-stable composite phase change materials(CPCMs)is proposed,and taking a small battery pack as an example to investigate the thermal control characteristics of flexible CPCM by experiment and simulation,and the simulation results are in reasonable accordance with experimental ones.Investigation results indicate that the flexible CPCMs embedded in battery pack lower the thermal contact resistance,thereby improving the thermal control performance.The battery temperature drops by 18.0℃ and the temperature rise rate decreases by 38%at 10 C discharge rate due to the better contact heat transfer characteristics of flexible CPCM.Further analysis finds that the simulation result of CPCM with anisotropic thermal conductivity reveals that increasing the radial thermal conductivity helps to improve the thermal control performance,but increasing the axial thermal conductivity does not further improve the thermal control effect.At the same time,it is not only more difficult to prepare CPCM with high thermal conductivity in all directions,but also greatly reduces the latent heat value.Therefore,an excellent thermal control effect can be achieved by increasing the radial thermal conductivity of the CPCM without sacrificing the latent heat value significantly.The above results show that the anisotropic thermal conductivity CPCM plays an important role in enhancing the thermal control effect.However,the current researches on the anisotropic heat conduction mechanism of CPCM is not sufficient,and the existing theoretical analysis models are complicated but not accurate enough.Based on the relationship between the compressibility and the orientation of thermally conductive particles,a simple and accurate anisotropic thermal conductivity prediction model is proposed in this paper,and the predicted value of anisotropic thermal conductivity is in good agreement with the experimental value,the error is within 1.5 W/(m K).The model is simpler and not affected by subjective factors than the scanning electron microscope orientation observation method used in the current study.According to the directional heat conduction and its strengthening mechanism given by the model,thermally anisotropic flexible CPCMs are prepared.The characterization results show that CPCM with an EG content of 15 wt%still has good flexibility and stability.CPCM maintains the latent heat value at 131 J/g,while the radial thermal conductivity can reach 11.2 W/(m K),which is 3 times the axial thermal conductivity.Therefore,this model lays a foundation for solving the contradiction between the thermal control performance enhancement and the latent heat reduction.Based on the above results,this paper studies the thermal control characteristics of anisotropic CPCM with high thermal conductivity in a single direction.A self-built experimental apparatus is used to investigate the heat transfer performance and the temperature control effect on the heat source of the CPCM under different anisotropic thermal conductivity.A thermal control simulation model was established.By combining the anisotropic thermal conductivity prediction model,the simulation results under different PCM parameters are compared in numerical simulation,and the simulation results are in agreement with experimental values.The results show that the temperature at the heat source decreases significantly with the increase of the radial thermal conductivity(TC)of the CPCM and effectively prolongs the usage time of the heat source.As the radial TC of the CPCM increases,the PCM utilization rate is improved.The results combined with the thermal conductivity prediction model indicate that the radial TC is improved as the CPCM density increases.The maximum temperature of the heat source can be reduced by 13.7℃ when the CPCM density is 900 kg/m3 at 1000 s working time.The radial TC has a thermal control performance threshold,when the mass fraction of EG exceeds 20 wt%,that is,the radial thermal conductivity exceeds 10 W/(m K),and the thermal control performance is almost no longer enhanced.The flammability of conventional CPCM may bring huge safety hazards to the thermal control system of battery packs and other equipments.Thus,a high heat storage capacity form-stable composite phase change material(CPCM)with enhanced flame retardancy is proposed,and the thermal and flame retardant properties of the CPCM are measured and compared to other CPCM samples.The flame retardancy of CPCM is improved after flame retardant modification,and V-0 burning rating is achieved while maintaining a high latent heat value of 130 J/g.The peak heat release rate(PHRR)of flame retardant form-stable CPCM drop by 58.8%,and the combustion rate can be greatly slowed down due to the protection of carbon layers formed by modified glass fibers.The glass fibers improve the mechanical properties of the CPCM and prevent it from fracturing during the burning process.The thermal stability of the flame retardant modified CPCM is improved and the degradation process is also retarded,and the material properties remain stable after 1200 thermal cycles. |
PDF Full Text Request