Study On Heat Transfer Enhancement Of Eicosane Phase Change Heat Storage Plate

In order to achieve carbon peaking and carbon neutrality goals,China is vigorously developing green energy-saving buildings.As an important technology in heating system,phase change heat storage technology can realize the peak shifting and valley filling,effectively solve the user’s operating costs,and provide more solutions for the practical application of new energy-saving building technology.Natural convection is an important research factor in rectangular cavity phase change heat storage system.How to make full use of natural convection to enhance local phase change heat transfer in rectangular cavity is of great significance to effectively design the phase change unit of heat storage plate.This paper theoretically and quantitatively analyzes the mechanism of phase change heat transfer in rectangular phase change unit,the simulation research of heat source arrangement and fin composite heat transfer enhancement technology are further deepened.The main contents are as follows:（1）Establish multi-attribute evaluation systems for low temperature phase change materials:AHP and VIKOR method are introduced to comprehensively evaluate the enthalpy,specific heat,thermal conductivity,density,under-cooling and price attributes of eight low temperature phase change heat storage materials.Weather it’s performance oriented or performance-price oriented,the top three materials in evaluation are:Na₂HPO₄·12H₂O,Na₂CO₃·10H₂O and eicosane.Considering the operation stability of the heat storage unit,organic phase change material eicosane is finally selected as the phase change heat storage material because of its better comprehensive performance.（2）Phase change heat transfer mechanism analysis:The energy control equation of phase change heat storage based on enthalpy method is discretized,and the boundary layer theory is introduced to derive the energy balance equation of the cell.By optimizing the above equations,the transient coordination factor Co is obtained,which can quantitatively describe the heat transfer intensity and natural convection degree,increasing the Co value is an effective way to enhance heat transfer.Fluent 18.0 software is used to simulate the discrete heat source location and fin structure design of the phase change unit,and the mathematical model and solution settings are given in the paper.（3）Heat transfer enhancement analysis of heat sources arrangement:The effects of different heating positions on natural convection are studied by numerical simulation.The results show that the arrangement of discrete heat sources can enhance the melting heat transfer of rectangular phase change unit.Combinng the whole melting-solidification process in rectangular cavity phase change unit and comprehensively considering the time required for melting completely,heat storage rate and total heat storage,the complete melting time of heating from the lower part of both sides（Case 5）is 1495 seconds.Under the same heat storage condition of 290KJ,case 5 requires the shortest time,only 1111seconds and is determined as the best heat sources design case.（4）Heat transfer enhancement analysis of adding fins:Heating from the lower part of both sides,the melting and solidification process of the rectangular phase change unit with straight fins,different wave amplitudes and wavy numbers are analyzed by means of numerical simulation,the results show that the addition of fins can enhance the heat transfer of eicosane in the rectangular phase change unit.For the melting condition,the complete melting time of PCM in the phase change unit with straight fins and single wavy fins are shorter,and the total melting time are reduced by 33.4%～34.2%,comprehensive observation of heat storage and heat storage rate show that 2-3 wavy fins phase change unit has the highest heat storage efficiency when storing the same 290KJ heat.For the solidification condition,the phase change units with 5-2 wavy fins,5-3 wavy fins and 2-2wavy fins have shorter solidification time,and the total solidification time is reduced by22.5%-26%,comprehensive analysis of heat release and heat release rate show that 5-3wavy fins have the shortest time when releasing the same 350KJ heat.Considering the melting-solidification heat transfer effect comprehensively,the 2-3 wavy fins design case is adopted as the final phase change unit design case.