Numerical Study On Charging And Discharging Characteristics Of Cylindrical Encapsulated PCM Capsule With Embedded Heat Pipes

Packed bed latent thermal storage systems have received much attention from academic and industrial communities recently due to the high operating efficiency,small space occupation,simple structure and good economics.However,low thermal conductivity of the phase change material(PCM)leads to longer charging/discharging time of the storage system.Thus,based on comparing some main technologies of heat transfer enhancement,heat pipes were employed in this numerical research to strengthen the internal heat transfer process of packed bed system.In this thesis,the 3-D transient model was developed using ANSYS Fluent,and the model was simulated combining Lumped Thermal Resistance Network(LTRN)as the heat pipe model with enthalpy-porosity approach as phase change model.The effects of flow conditions of heat transfer fluid(HTF),heat pipes embedding length,types of HTFs and developing flow on the charging/discharging process of a single EPCM-HP unit were simulated.Then the influence of the configuration of EPCM-HP units array(aligned,staggered)on the charging/discharging process under developing flow condition was also analyzed.In the charging process of single EPCM-HP unit,embedding heat pipe can shorten the melting time of EPCM-HP unit by about 11.26%.The flow condition of HTFs has a great influence on the heat charging process of the EPCM-HP unit.The total melting time of the EPCM-HP unit for laminar flow(3635 s)is ten times more than that for turbulent flow(204.1 s),and the streamline distribution in the turbulent flow is not symmetrical.With the increase of heat pipe embedding length,the total charging time of the EPCM-HP unit decreases,but the enhancement effect is not significant.When the HTF is thermal oil Therminol/VP-1,the charging time(1115 s)of the unit is less than that of air(2021 s).But the enhancement effect on the charging process is more significant when the HTF is air.Compared with fully developed flow,the developing flow can shorten the charging time of the EPCM-HP unit by about 9.58%.In the discharging process of single EPCM-HP unit,embedding heat pipe can shorten the discharging time of EPCM-HP unit by about 13.25%.The total discharging time of the EPCM-HP unit of the turbulent flow(2140 s)is shortened to 1/5 of the laminar flow(11230 s).The enhancement effect of the heat pipe on the discharging process of EPCM-HP unit increases significantly with the increase of embedding length.The embedding length of 25 mm,35 mm and 45 mm can shorten the discharging time of EPCM-HP unit by about 13.25%,27.04%and 42.44%respectively.The total discharging time of the EPCM-HP unit using thermal oil Therminol/VP-1(4630 s)is smaller than that of air(7362 s),but the strengthening effect of the heat pipe is more significant when the HTF is air.The developing flow has less influence on the discharging process of EPCM-HP unit.In addition,the thesis also analyzes the influence of the configuration of EPCM-HP units array(aligned,staggered)on the charging/discharging process under developing flow condition.The results show that the staggered arrangement can significantly increase the disturbance between the HTF and the EPCM-HP units,strengthen the heat transfer process and shorten the charging/discharging time of EPCM-HP units.In the staggered model,alternately change of the liquid fraction,temperature and charged/discharged energy of second and fourth rows units could be observed.The findings of this thesis can provide reference for the design and application of EPCM-HP unit in packed bed system.