Numerical Simulation And Heat Transfer Enhancement Of Phase Change Materials Based On Hexadecylamine

With the rapid development of modern industry,environmental and energy issues have become increasingly prominent.Thermal storage technology is one of the effective ways to alleviate these two major issues.Among various heat storage technologies,phase change energy storage has become a research hotspot in the field of heat storage due to its advantages of high heat storage density and constant temperature.The research of heat storage technology includes two parts:the research and development of heat storage material and heat storage unit.The thermal conductivity of thermal storage materials is relatively poor,which makes the phase transformation rate very slow.Therefore,taking effective strengthening measures to improve the heat transfer rate of phase change materials is one of the necessary means.In this paper,hexadecylamine（HDA）was selected as the phase transition material.Fluent software was used to calculate the storage-release rate of the phase transition material by three strengthening technologies,and the changes of temperature,liquid phase volume fraction and flow rate were studied.It mainly includes:the storage and release process of the composite phase change material（CPCMs）formed by three-dimensional graphene（GA）and HDA,the heat transfer process enhanced by the addition of nanoparticle Al₂O₃to HDA,and the heat transfer process optimized by the addition of fin to the shell to improve the heat transfer area.By means of numerical calculation,the storage and release characteristics of phase change materials and the design optimization of storage units for heat absorption and heat release were investigated.The research results are as follows:（1）In this chapter,HDA and HDA/GA CPCMs are used as heat storage media.Firstly,a two-dimensional horizontal tube model is established to verify the grid and time steps,and to determine a reasonable step size and number of grids to ensure the accuracy of the calculation.Fluent software was used to study the evolution of liquid fraction,temperature,velocity distribution,streamline and internal energy with time during the phase transition process.In the process of HDA heat storage,natural convection is the main mode of heat transfer,while HDA/GA CPCMs mainly rely on thermal conduction.The results show that the flow rate of HDA/GA CPCMs is about400 times slower than that of pure HDA,which greatly weakens the natural convection effect but directly improves the thermal conductivity of pure HDA.Compared to HDA,the time for HDA/GA CPCMs to complete solid-liquid phase transition is 79.5%shorter than HDA.The solidification time of HDA/GA CPCMs is1.8%shorter than that of HDA.The above results indicate that the addition of GA accelerates the rate of heat transfer and release.After research,it has been found that HDA/GA CPCMs can be used to recover low-temperature waste heat resources,laying a scientific foundation for further development of low-temperature waste heat recovery methods and providing theoretical guidance for future engineering design.（2）This chapter selects Al₂O₃as an enhanced particle and establishes a three-dimensional shell and tube phase change model.HDA and Al₂O₃/HDA CPCMs are used as thermal storage media,and Fluent software is used to study the evolution of liquid fraction and temperature over time during the phase change process of composite phase change materials with different volume concentrations（1%,2%,3%,4%,5%）of Al₂O₃added to HDA.The results showed that before 600 seconds,Al₂O₃/HDA CPCMs mainly relied on thermal conduction and heat transfer during the heat storage process.At 800 seconds,Al₂O₃/HDA CPCMs with different volume concentrations（1%,2%,3%,4%,5%）of Al₂O₃added exhibit natural convective heat transfer.Causing rapid melting of the top of the interface.Compared to Al₂O₃/HDA CPCMs added with other volume concentrations（1%,2%,3%,4%）of Al₂O₃,the time required for Al₂O₃/HDA CPCMs added with 5%Al₂O₃to completely transform from solid phase to liquid phase is much less,only 987.2 seconds.This is because Al₂O₃has a high thermal conductivity and is in full contact with HDA,thus accelerating the melting process of HDA.Research has shown that the complete melting time of Al₂O₃/HDA CPCMs added with Al₂O₃at volume concentrations of 1%,2%,3%,4%,and 5%is 1098.4 s,1074.2 s,1045.4 s,1014.2 s,and 987.2 s,respectively.The melting rates have increased by 72.8%,73.4%,74.2%,74.9%,and 75.6%compared to pure HDA,respectively.It indicates that with the continuous increase of Al₂O₃volume concentration in Al₂O₃/HDA CPCMs,the melting rate of CPCMs gradually increases.During the solidification process,the complete heat release time of Al₂O₃/HDA CPCMs added with Al₂O₃at volume concentrations of 1%,2%,3%,4%,and 5%was reduced by 73.4%,74.1%,74.8%,75.5%,and 76.3%compared to HDA,respectively.Compared to pure HDA,the addition of Al₂O₃particles reduces the heat release time to varying degrees.During the entire solidification process,the solidification rate of CPCMs increases with the increase of the proportion of Al₂O₃particles.In this study,nanoparticles with a volume concentration of 5%had the strongest enhancement effect.（3）The heat transfer process was optimized with finned device,and the mathematical model of finned reinforced shell-and-tube heat storage unit was established when the phase change material was HDA.The distribution of the flow field in the heat storage unit,the change of liquid volume fraction and the distribution of the temperature field in the heat transfer process are simulated.The influence of fin parameters（number of fins,height of fins and thickness of fins）on the heat storage and release process of HDA was explored by comparing with the heat transfer results of heat storage units without fins.The results show that the heat storage rate of the heat storage unit with fins is 60%higher than that of the unit without fins.The influence of different parameters of the heat transfer unit,namely the number,height and thickness of fins,on the heat transfer process of PCM is analyzed.Finally,determine the optimal number of fins,fin height and fin thickness of HDA fin tube heat exchange structure.The final structural parameters are:the fin with a number of10,a height of h=17 mm and a thickness of d=2 mm is more conducive to heat transfer enhancement.