Heat Transfer Properties Of Phase Change Materials With Gradient Porous Framework

In the past few decades,increasing energy demand,the shortage of fossil fuels,and concerns about environmental impact have promoted the development of renewable energy and various energy storage technologies.The medium and low temperature phase change energy storage technology is an important development direction in the field of renewable energy and energy storage technology.At present,researchers have conducted extensive research on the application of medium and low temperature phase change materials.Among them,the addition of solid framework and porous media materials to phase change materials has attracted more and more attention.However,as far as the current research status is concerned,there is a lack of in-depth discussion on the change mechanism of the phase change interface in the solid-liquid phase transition process of the phase change material containing the solid framework,and the lack of in-depth analysis of the local thermal non-equilibrium effect between the solid framework and the phase change material.There is a lack of comprehensive research on the heat transfer characteristics of gradient framework phase change materials in different directions.Based on this,this article conducts research from the following aspects:In this thesis,a solid-liquid phase transition visualization experimental platform was built.Through experimental methods,the solid-liquid interface migration,the thickness of the mushy zone and the temperature field distribution during the melting process of pure paraffin and gradient porous framework phase change materials were observed,analyzed,and composited.The internal temperature change law of the phase change material and the local thermal non-equilibrium effect between the framework and the paraffin were studied.The results show that the phase change interface migration is mainly affected by natural convection and framework heat transfer.The effect of natural convection heat transfer will cause the phase change interface to bend,and the degree of curvature of the phase change interface follows the heat transfer intensity of natural convection.The increase increases.The smaller the porosity of the metal porous framework,the greater the heat transfer strength,and the greater the heat transfer strength,the faster the surrounding paraffin melts,and the faster the phase change interface migration.The main external factors affecting the thickness of the mushy zone are the size of the temperature gradient at the phase change interface and the size of the fluid flow rate.The larger the temperature gradient at the phase transition interface,the thinner the mushy zone,and the smaller the temperature gradient,the thicker the mushy zone.The smaller the fluid velocity at the phase change interface,the thicker the mushy zone,and the greater the velocity,the thinner the mushy zone.Among them,the heat transfer of the metal framework affects the temperature gradient at the phase change interface,and the natural convection heat transfer affects the temperature gradient and flow velocity at the phase change interface.In terms of temperature distribution,the internal temperature distribution of the porous framework phase change material is more uniform than that of pure paraffin wax.Among the porous framework phase change materials containing uniform,X negative gradient,and XZ negative gradient,the XZ negative gradient porous framework phase change material has the most uniform internal temperature distribution in the vertical direction;the temperature change at any point inside the composite phase change material is roughly The tendency to increase rapidly first,then slowly,and then quickly.Experimental studies have found that the local thermal non-equilibrium effect exists in the whole process of the melting of the composite phase change material,of which the most prominent in the beginning of the heating phase and the melting phase of the phase change material.The local thermal non-equilibrium effect has two peaks and one valley in the whole process.Among them,the first peak is caused by the fact that the thermal diffusivity of the metal framework is much higher than that of the paraffin.This delay in the heat diffusion rate produces a local thermal non-equilibrium effect;the second peak is caused by the paraffin.Entering the phase change latent heat stage,the paraffin phase change endothermic temperature does not change,while the skeleton temperature continues to rise;and the valley value is caused by the high temperature circulation formed by natural convection that will flow through the temperature measurement point.At this time,the heat is The high-temperature fluid is transferred to the metal framework.Based on the experimental conditions and experimental environment,the corresponding mathematical and physical models are established,and the content of the experimental research is further enriched by the method of numerical simulation.By numerically simulating seven working conditions of positive/negative gradients in the three directions of X,Z,and XZ,the uniform porous framework formed by the new porous framework units has further improved the heat transfer characteristics of the phase change material with gradient porous framework.Experimental and numerical simulation studies have shown that the size of the framework porosity plays a very important role in the migration process of the phase change interface.The smaller the framework porosity,the smaller the framework porosity,the faster the migration speed of the phase change interface in this region.The thicker the mushy zone of the area.In terms of melting rate,the composite phase change material containing the XZ negative gradient framework melts the fastest,and the composite phase change material containing the X positive gradient framework melts the slowest.In terms of heat storage rate,the order of the heat storage rate of each working condition is similar to the melting rate,working condition 6 is the fastest heat storage,and working condition 1 is the slowest;in terms of temperature uniformity,the internal temperature distribution of working condition 6The uniformity is the best,and the internal temperature distribution of the working condition 1is the worst;among the above 7 kinds of framework distributions,the composite phase change material containing the XZ negative gradient framework is comprehensive in terms of melting rate,heat storage rate,and internal temperature uniformity The best performance.