Research On The Heat Transfer Characteristics Of Phase Change In The Composite Cavity Of Multi-melting Point Materials

The solid-liquid phase transition has applications in energy storage,heat preservation,heat dissipation and other fields,such as solar matching adjustment,power peak shaving and valley filling,building energy saving,and electronic device heat dissipation.Under the background of improving energy utilization and solving energy shortages,phase change materials can store discontinuous and random new energy sources to achieve efficient and reasonable use of energy.Phase change materials have the advantages of high enthalpy,high latent heat,and small size,but their thermal conductivity is low,and the heat absorption/release efficiency is low.Therefore,understanding the solid-liquid phase transition characteristics of phase-change materials and improving the efficiency of phase-change energy storage have important guiding significance for practical engineering applications.In this paper,the common medium and low temperature phase change material paraffin is selected,and the method of combining numerical simulation and experimental research is used to conduct in-depth research on the phase change process and heat transfer characteristics of filled single and multi-melting paraffin composite cavities.First,a solid-liquid phase transition visual observation experimental platform was built,and physical parameters such as the latent heat of paraffin phase transition,melting point,and thermal conductivity were obtained through DSC and Hot Disk tests;Infrared thermal imaging and high-definition video are used to observe the paraffin phase change melting process,analyze the migration change of the phase interface and the temperature field distribution law during the paraffin melting process,and explore the heat transfer mechanism during the paraffin phase change process in the composite cavity.The results show that the phase change interface photographed by high-definition cameras is first approximately parallel to the heating wall,and then gradually bends and tilts to the right under the influence of natural convection,and then falls quickly after touching the right wall;The area of the phase interface increases first and then decreases during the moving process,which affects the speed of the phase transition first and then slow;The temperature gradient in the liquid paraffin zone is higher than that in the solid paraffin zone,and the heat transfer mainly occurs between the liquid zone and the phase change interface;The liquid paraffin area is affected by natural convection driven by buoyancy,and there is thermal stratification along the direction of gravity;The liquid high-temperature paraffin is affected by the volume expansion pressure difference and moves upward and flows into the upper surface of the solid paraffin,which is beneficial to preheat the paraffin away from the heating wall to increase the melting rate.Secondly,based on the experimental data,the corresponding three-dimensional physical model is established,and the experiments and simulations are mutually confirmed,and the phase interface evolution and liquid phase rate changes,heat transfer temperature,flow characteristics,storage The heat,etc.were compared and analyzed,the influence of the melting point temperature difference was discussed,and the dimensionless parameter analysis was carried out.The results show that the numerical simulation and the experimental results are in good agreement in terms of phase field,liquid phase rate change,and temperature field;there is only one solid-liquid phase transition interface in the composite cavity filled with single melting point paraffin,and different layers of phase change materials in the cavity Melting in order,and there are multiple solid-liquid interfaces coexisting in the multi-melting paraffin composite cavity,and different layers of phase change materials can be melted at the same time;In the compound cavity filled with multi-melting paraffin wax,the arrangement of high-melting paraffin close to the high-temperature wall and low-melting paraffin close to the low-temperature wall is beneficial to the phase change behavior;the key to using multi-melting paraffin to change the phase change rate is to realize the synchronization of the phase change material in the phase change process Melting,reasonable use of the order of arrangement of phase change materials,can make full use of the latent heat of phase change in the process of phase change,and control the energy storage rate of phase change materials;Compared with the filling of single melting point paraffin,the thermal diffusion rate is slower when the multiple melting points are arranged from high to low,and the internal temperature difference of the system is significantly reduced,and the internal temperature uniformity is improved;The arrangement of low melting point paraffin close to the high temperature wall makes the flow inside the cavity more violent,while the arrangement of the melting point from high to low reduces the flow speed of the phase change material in the cavity and improves the system stability and the uniformity of the velocity field distribution;Compared with filling with single melting point paraffin,the arrangement of multi-melting paraffin from high to low reduces sensible heat(about 6.4% reduction),increases latent heat(about 16% increase),and increases the total heat storage of the cavity(about 6.2% increase);The melting point temperature difference between the phase change materials is too large or too small,which is not conducive to the phase change behavior.The phase change rate is the fastest when the melting point difference is 6-8K under the calculation conditions in this paper.