| Solar energy is one of the most valuable renewable energy sources because of its abundant reserves and easy access.To make solar energy a stable energy,reasonable storage is very important.The phase change heat storage system has high heat storage capacity and heat storage density,and has great potential in the field of solar energy storage.The application of bionic branch fins in phase change storage systems can break through the limitation of heat transfer lag and achieve uniform heat transfer from point to surface,which can significantly enhance the heat storage performance of phase change material(PCM).However,the previous research on branch fins focuses on the melting process of PCM under the two-dimensional model,and there are relatively few studies on the solidifying process,and there are few studies on heat transfer enhancement in three-dimensional space.Therefore,in this paper,the heat storage/release process of branch fin phase change heat storage system in two-dimensional and three-dimensional space is studied by combining theory with numerical simulation.The melting/solidifying characteristics,thermodynamic properties and heat storage/release cycle rate of the branch fin phase change heat storage system are explored and the structure of branch fin is optimized.The main contents are as follows:The transient heat transfer model of the PCM in the heat exchanger is established.The promotion effect of Y-shaped fins on the phase change process is studied and compared with that of straight fins.The effects of Y-shaped fin geometry(length index,width index,fin branch angle)on the phase transition process were studied in a large size range.The results show that the application of Y-shaped fins expands the heat transfer area and heat flow channel,improves the temperature response speed and heat penetration depth of PCM at the outer wall,realizes the uniform absorption/release of heat from point to surface,and accelerates the phase change process of PCM.Compared with straight fins,the melting time of Y-shaped fins is reduced by 51.7%,and the solidifying time is reduced by 57.1%.Reducing the width index can increase the heat transfer area,improve the temperature response speed of the PCM at the outer wall,and significantly reduce the phase change time.Increasing the length index promotes the phase change heat transfer of PCM at the inner tube and weakens the heat transfer at the outer wall.For the melting process,with the increase in length index,the melting time of PCM decreases first and then increases.For the solidifying process,the solidifying time decreases with the increase in the length index,and the temperature curve shows an ’inverted S’ shape.With the increase of the branch angle,the fin divides the whole PCM domain more evenly.The smaller branch angle promotes the heat transfer at the outer wall,and the larger branch angle promotes the heat transfer at the inner tube.When the branch angle is 70°,the phase change time of the PCM is the shortest.Based on the response surface method,the quadratic regression equation between the heat storage/release cycle time and the width index,the length index and the branch angle is obtained.The width index has the greatest influence on the heat storage/release cycle time,the length index is second,and the branch angle has the least influence.Through the steepest ascent algorithm,within the scope of this chapter,when the width index is 1,the length index is 1.13 and the branch angle is 64.49°,the heat storage/release cycle time reaches the minimum.To give full play to the advantages of the branch fins to make the heat evenly distributed,more sub-branches are added to the branch points of the Y-shaped fin,and a new type of umbrella-shaped branch structure is designed.The umbrella-shaped fins add more heat flow channels at the branch points.While expanding the heat transfer area,the heat is evenly released into the PCM region,thereby enhancing the radial heat transfer process.In the melting process,the increase in the number of branches strengthens the heat conduction process and weakens the natural convection.When there are four branches,the PCM melts faster and the temperature field is more uniform.The melting time is reduced by 61.1%compared with the straight fin and 20.7%compared with the Y-shaped fin.During the solidifying process,the increase in the number of branches increases the temperature response speed of the PCM at the outer wall,the cooling speed is accelerated,and the temperature distribution is more uniform.Similarly,when the number of branches is 4,the solidifying speed of PCM is faster,and the solidifying time is reduced by 41.9%compared with the straight fin and 20.1%compared with the Y-shaped fin.On the basis of continuous Y-shaped fin,a discontinuous Y-shaped fin is designed to increase the heat storage/release rate of the PCM.The phase change process of PCM in discontinuous Y-shaped fin heat storage unit is studied from three aspects:fin thickness,fin spacing and rotation angle.The results show that increasing the fin thickness expands the heat transfer area,reduces the heat transfer thermal resistance,and improves the PCM phase change speed.As the fin spacing increases,the fin volume decreases and the PCM volume increases,which significantly weakens the heat conduction process,so the PCM phase transition speed is significantly reduced.The increase of rotation angle changes the arrangement of fins in three-dimensional space,increases the temperature difference of PCM between fins,and strengthens the heat transfer between fins.When the rotation angle is 30°,the fins are evenly arranged in three-dimensional space,and the total phase transition time is the shortest.Based on the response surface method,the quadratic regression equation between the heat storage/release cycle rate and the fin thickness,the fin spacing and the rotation angle are obtained.The fin spacing has the greatest influence on the cycle rate,the fin thickness is the second,and the rotation angle has the least influence.Based on the steepest ascent algorithm,when the fin thickness is 4.24mm,the fin spacing is 1.17mm,and the rotation angle is 31.29°,the heat storage/release cycle rate of the PCM reaches the maximum.Compared with the continuous Y-shaped fin,the melting time of the PCM in the optimized structure is reduced by 8.3%,and the average heat storage rate is increased by 8.9%.The solidifying time decreased by 36.2%,and the average heat release rate increased by 39.5%.The results of this paper have certain guiding significance for the structural design of fins in phase change heat storage system. |
