Study On Heat Transfer Characteristics Of Inclined Annular Finned Tube Shell Phase Change Heat Storage Unit

Phase change heat storage technology has a wide range of applications in areas such as air conditioning energy conservation,waste heat recovery and utilization,and power "peak to valley" shifting.However,due to the low thermal conductivity of the materials used for phase change heat storage and the poor heat storage/release performance of phase change heat storage devices,the application of phase change heat storage technology is greatly limited.This article focuses on the thermal storage performance of inclined finned tube shell phase change heat storage units.Numerical and experimental methods were used to study the melting process and the influence of working parameters on the phase change flow and heat transfer characteristics of the inclined heat storage unit.The influence of Natural convection on the heat storage process,the influence of geometric dimensions on the heat storage rate,and the influence of heat transfer fluid inlet temperature and flow velocity on the heat transfer rate were revealed.The main research results obtained are as follows:(1)The research results on the heat storage process of the smooth tube shell phase change heat storage unit show that the tilt angle has an important influence on the melting process dominated by Natural convection heat transfer.Since the hot surface of the area above the heat exchange tube is upward,it is conducive to the formation and development of Natural convection.Therefore,the melting rate in the area above the heat exchange tube is significantly higher than that in the area below.Under the parameter conditions of this article,when the inclination angle is 30° ≤ α ≤ 60°,as the tilt angle decreases,the melting of the area below the bottom of the heat exchange tube gradually accelerates,and the heat storage rate significantly increases.(2)Due to the increase in heat exchange area,the heat storage efficiency of the ring-shaped finned tube shell phase change heat storage unit is significantly better than that of the bare tube tube shell phase change heat storage unit.Under the same operating conditions,the complete melting time of the ribbed heat storage unit was shortened by 20.9 %,while the average heat transfer rate increased by 30.2 %.The melting process of phase change material zone can be divided into four stages: the melting stage of solid phase change material on the outer thin wall surface of the heat exchange tube,the inhibiting heat transfer stage of liquid phase change material in the thin wall,the promoting melting stage of Natural convection and the gradually weakening stage of Natural convection.The change process of melting rate in each stage is: rise,decline,rising,decline.(3)As the number of Ste increases,the latent heat storage of the shell and tube type phase change heat storage unit with annular fins remains unchanged,while the sensible heat storage significantly increases and the total heat storage increases.When the Ste number increases from0.3 to 0.7,the melting time decreases by 75.4 %,and the heat storage and average heat transfer rate increase by 16.1 % and 372.2 %,respectively.Therefore,the increase in Ste number enhances the driving force of the phase change heat transfer process,significantly shortens the melting time,and significantly improves the heat transfer rate.(4)When Re < 500,with the increase of Re number,the convective Heat transfer coefficient in the heat transfer fluid zone increases significantly,the convective heat transfer resistance between the heat transfer fluid and the solid wall decreases,and the total heat transfer resistance of each link of the phase-change heat storage unit decreases accordingly,which can effectively improve the phase change heat storage rate and shorten the heat storage time.Compared with Re = 10,the average wall Nu number increased by 10.9 % when Re = 500.The total heat storage in the phase change heat storage unit remains unchanged,and the melting time is significantly shortened.Compared with the average wall Nu number at Re = 1000,it increased by 4.2 % at Re = 2000.(5)The phase change heat storage material remains unchanged,and as the rib height gradually increases,the melting time decreases.When the rib height is 25 mm,the time required for complete melting is 33.7 % shorter than that of the smooth tube shell phase change heat storage unit.Similarly,the melting time decreases with the increase of the number of fins,as both the number and height of fins increase the heat transfer area and promote the phase change heat transfer process.The arrangement of ribs has a significant impact on phase change heat storage.Compared with the top centralized arrangement of ribs,the time required for complete melting when the ribs are centrally arranged at the bottom decreases by 44.3 %,and the average heat storage per unit time increases by 79.7 %.