Study On Heat Transfer Enhancement Of Composite Inorganic Salt Phase Change Cold Storage

The peak-to-valley power consumption ratio in my country is far below the average level of developed countries,and the contradiction between power supply and demand is prominent.The large amount of power supply at the peak increases the construction cost of power facilities,and the remaining power at the valley threatens the security of the power grid.Cold storage technology has become an effective way to alleviate the contradiction between building energy supply and demand because of its role in promoting power load peak shaving and valley filling.The energy storage density of inorganic salt phase change cold storage is higher than that of water cold storage,and the phase change temperature is widely adjustable to higher than that of ice cold storage.Because of its unique advantages over traditional cold storage methods,it has become an option for air conditioning cold storage.In order to improve the shortcomings of inorganic salt itself,such as supercooling and phase separation,and improve its thermal conductivity,a composite inorganic salt phase change material was prepared.Then a high-efficiency heat transfer tube-fin type was designed using it as a cold storage medium.The cold storage tank strengthened the overall heat transfer performance of the cold storage tank,and operation experiments were carried out under different temperature conditions.The overall heat transfer performance of the cold storage tank was strengthened.Operation experiments were carried out under different temperature conditions.A phase change material was prepared with Na₂SO₄·10H₂O and Na₂HPO₄·12H₂O as the composite substrate,supplemented with additives such as cooling agent,nucleating agent,thickener,and thermal conductivity agent.The composite substrate of Na₂SO₄·10H₂O:Na₂HPO₄·12H₂O=4:6 is less stratified than pure Na₂SO₄·10H₂O,and the supercooling and phase transition temperature are lower than a single pure substance.Adding 4g of NH₄Cl to20g of the above composite substrate reduces the phase transition temperature to about 6°C.Adding 0.4g of borax basically eliminates supercooling.Adding 0.4g of CMC obtains a stable gel system.Adding 0.2g of nano-TiO₂ significantly improves the thermal conductivity.The proportion of phase change material is 32%Na₂SO₄·10H₂O+48%Na₂HPO₄·12H₂O+16%NH₄Cl+1.6%Borax+1.6%CMC+0.8%nano-TiO₂.The phase transition temperature is6.3℃.The latent heat is 139J/g.The liquid thermal conductivity is 0.7989 W/（m·K）.After 50cycles,it has good thermal stability and is suitable for building air conditioning cold storage.Numerical simulations of the heat transfer characteristics of tube-fin cold storage tanks with different structures have been carried out.The structural optimization results show that the greater the tube spacing,the longer the cold storage completion time t₀.Three types of fins with different structural parameters are compared.By analyzing the temperature curves,cooling storage completion time t₀,fin efficiency F,average fin enhanced heat transfer efficiency F₀ and other parameters,it is concluded that the enhanced heat transfer efficiency of vertical fins is higher than that of fractal fins and horizontal fin.Shortening the length of the fins can maintain a high enhanced heat transfer efficiency under less metal consumption.Increasing the temperature difference between the cold/hot medium and the phase change material can significantly improve the heat transfer efficiency.However,the increase in temperature difference will adversely affect the refrigeration host and the user side.The tube-fin cold storage tank is finally arranged in a row of tube bundles with a spacing of l=35mm.Vertical fins with structural parameters of N₂=4 and l₂’/l=0.6 is added to the outside of the tube.Build a cold storage system test platform.Five temperature measurement points are arranged along the radial and edge of the heat exchange tube structural unit to analyze the heat transfer performance of the cold storage tank.During the cold storage/release process,the temperature curves of the five measuring points are basically the same.As the distance from the measuring point to the wall of the heat exchange tube increases,thermal hysteresis occurs in turn,and the temperature curve becomes slower.The temperature changes at points A and B and the temperature changes at points C and D are almost synchronized.It shows that most areas of the cold storage tank cavity have a fast heat transfer rate and a relatively uniform temperature.The corner point E is the area where the heat exchange is finally completed.The higher temperature difference between the cold/hot medium and the phase change material,the faster the heat exchange rate of the phase change material.In the actual heat transfer process,the heat transfer speed is slightly slower than the simulation results due to factors such as the temperature of the cold/hot medium at the measuring points,the inlet and contact thermal resistance.