Melting And Solidification Heat Transfer Characteristics Of Paraffin In Rectangular Unit And Structure Optimization

Energy storage technology is widely used in fields such as solar energy utilization,building energy conservation,industrial waste heat recovery,heating and air-conditioning systems and electronic component cooling,due to its advantages in effectively solving the time and space mismatch between energy supply and demand sides.However,at present,there are few studies on improving the total efficiency of heat storage and heat release in rectangular phase change units,and most phase change materials have poor thermal conductivity,which makes it difficult to promote the application of phase change energy storage.Based on this,this paper carries out the following research on the heat transfer characteristics of phase change materials in phase change units and the structural optimization of the units:The numerical model and calculation method for phase change heat transfer with convection outside a horizontal circular tube in a rectangular unit are established by using FLUENT software,and the temperature's variation during the melting process of paraffin in the unit is compared with the experimental results,which verified the reliability of the established enthalpy-porous media modelWith paraffin as phase change material,water as heat transfer fluid,heat storage and heat release characteristics of rectangular element in paraffin wax are analyzed in numerical simulation,respectively.With the change of time,the parameters in the unit such as solid-liquid interface,temperature field,velocity field,flow field and melting/solidification rate shows that during the melting process of paraffin,when the temperature difference between the heating surface and the initial temperature of phase change materials is increased by 20?,the melting rate of paraffin in the unit increases by 24.26%,the proportion of the time that natural convection dominates heat transfer to the total melting time in the unit increases by 36.11%and the intensity of natural convection increases by 41.51%.The effect of natural convection on heat transfer cannot be neglected in the influence of the melting process.During the solidification process of paraffin,when the temperature difference between the exothermic wall surface and the initial temperature of phase change material is increased by 20?,the solidification rate of paraffin in the unit can increase by 29.03%.However,the influence of natural convection on solidification is very weak,and the heat transfer mode during solidification is mainly heat conduction.In order to obtain universal results,dimensionless analyses on the effects of wall temperature and initial temperature of phase change material on the melting and solidification processes are carried out.The results show that improve the temperature difference between the wall temperature and the initial temperature of phase change materials,especially the difference in temperature of solidification process can obviously shorten the total time of heat transfer process in rectangular elements.In order to improve the melting rate of the phase change heat storage unit,the melting process of different rectangular heat storage units are numerically simulated and analyzed,and the influence of the geometric size and the eccentricity ratio of the circular tube on the flow and heat transfer of paraffin in the phase change heat storage unit are studied.The results show that with the increase of the geometric size,that is N,from 1 to 6,the melting rate of paraffin in the rectangular element increases first and then decreases,and the maximum value is obtained at N=1.5.As the eccentricity ratio of the circular tube,that is c,increases from 0 to 0.222,the melting rate of paraffin in the rectangular element first increases and then decreases,reaching the maximum value at c=0.182.On the premise of ensuring a certain heating surface size and heat storage,the influence of different external geometric shapes(square,regular hexagon,regular octagon and circle)on the flow and heat transfer characteristics of paraffin in the melting process of the unit is studied.It is concluded that the total melting time can be shortened most effectively when the section is square,so that the melting time consistency of the upper and lower parts of the tube is the highest,and the melting rate of paraffin in the phase change unit is the highest.Finally,on the basis of the optical tube,longitudinal fins are added to form a finned tube strengthening unit,and the influence of the included angles between fins on the paraffin flow and heat transfer characteristics in the unit is investigated.The results show that with the decrease of the angle between fins from 90°to 45°,the temperature distribution of paraffin in each part of the heat storage unit is more uniform,and the melting time of the rectangular heat storage unit is reduced by 16.24%maximally.