Numerical Simulation Study On The Enhancement Of Heat Transfer Characteristics Of Phase Change Material

Global social-economic development is closely related to sufficient energy supply,and thermal energy storage technology have been identified as key technologies to ensure that energy is relatively balanced in time and space.Phase change energy storage technology is using the phase change materials to absorb or release the latent heat of phase change for energy storage and release during the phase change process.It has been widely used due to its suitable physical properties and relatively stable chemical properties.In this paper,numerical simulation methods are used to study the widely used rectangular square cavity heat storage units and tube-in-shell storage units in industry.By adding nanomaterials with high thermal conductivity and optimizing the corresponding structure to enhance its heat transfer characteristics,The main research contents are as follows:First,for rectangular square cavity heat storage units,using methyl laurate as a phase change material,based on the EMT model,the strengthen effect of graphene nanosheets with different volume fractions on their heat storage process was studied.The study shows that the addition of graphene nanosheets did not change the melting trend of phase change materials.When the content of graphene nanosheets was 1 Vol%,2 Vol%and 3 Vol%,the complete melting time was shortened by 38.89%,55.22%and 62.22%,the heat storage did not reduced significantly.Increasing the wall temperature can increase the heat exchange power significantly,meanwhile,the intensity of natural convection also increases,and the melting speed is accelerated.For the square-cavity heat storage unit,the natural convection intensity will weaken in the late melting process,and heat will accumulate at the top of the square cavity,which will reduce the heat storage efficiency.Secondly,in order to solve the problem of heat accumulation at the top of the square cavity unit,the layered model was raised.The position and the number of the separate plate and the filling method of the nanoparticle-enhanced phase change material will affect the heat storage result.When the separate plate is in the middle of the square cavity,the melting speed is the fastest.On this basis,the top and bottom of the square cavity are filled with 1 Vol%and 3 Vol%nanoparticle-enhanced phase change materials respectively has the best melting performance,compared to the model of 2 Vol%,the melting time shortened by 21.8%,the overall average temperature of the cavity is slowed down,and the heat storage efficiency has improved,and the natural convection lasted longer.Further increasing the number of the separate plate,the melting effect was best when the volume fraction of the composite phase change material in the cavity from top to bottom increased sequentially,but the increase in the melting rate gradually decreased.Finally,a tube-in-shell storage unite was constructed to study the enhanced heat transfer process of phase change materials in the heat accumulator when the inner tube was heated.It was found that in oxide nanomaterials,Al2O3 promotes the melting process of fatty acid esters better than CuO and ZnO.At the same time,as the volume fraction of nanoparticles increases,the effect of viscosity on natural convection is greater,and the melting rate is lower.Aiming at the melting characteristics of the tube-in-shell storage unite,combined with the effect of nanoparticles on fatty acid esters,an optimized setting of the separate plate deflect downward based on enhanced heat transfer was performed.The result shows that the melting speed is the fastest when the deflection of the separate plate is 10° downword,considering the economic factor comprehensively,the best angle is 200 downword.The liquid phase ratios of the tube-in-shell storage unit at different times when the separate plate deflect 20°downward is obtained by fitting.