Research On Heat Transfer Enhancement Characteristics Of Annular Fins Of Shell And Tube Phase Change Thermal Storage

Phase change heat storage technology has broad application prospects in the field of new energy development and energy saving.However,due to the small thermal conductivity of the phase change material(PCM)and poor heat transfer performance,the performance of the phase change thermal storage is greatly restricted.In this regard,people have proposed a number of enhanced heat transfer measures based on the expansion of the heat conduction route for the PCM side.In fact,the natural convection in the PCM liquid phase region of the phase change thermal storage will enhance heat transfer to a certain extent.Reinforcement measures based solely on the expansion of the heat conduction route will inhibit natural convection,making the final enhanced heat transfer effect insignificant.In order to avoid this phenomenon,it is necessary to study the interference mechanism between the fin-strengthened heat transfer of the tube-fin phase change thermal storage and the natural convection-strengthened heat transfer to improve the heat storage/heat release performance of the tube-fin phase change thermal storage.In this paper,n-octadecane is used as the phase change material,and the vertical arrangement of single-tube tube-fin phase change thermal storages with annular fins is the research object.Through reasonable simplification,a two-dimensional numerical model is established and studied by the enthalpy-porosity method.Through the coupling analysis of fin heat transfer enhancement and natural convective heat transfer,the effect of different fin arrangements on the natural convection in the liquid zone of the phase change regenerator heat storage/exothermic process and the rate of heat storage/exothermic of the entire regenerator are obtained.The main conclusions are:(1)Natural convection has a greater impact on the PCM melting process,and the introduction of fins to enhance heat transfer will suppress the natural convection in the PCM liquid region when the phase change thermal storage stores heat.But for the solidification process,natural convection has little effect.(2)When the phase change regenerator stores heat,the enhanced heat transfer of the fins mainly affects the early stage of the melting process and has little effect on the later stage of melting.And due to the influence of natural convection,the melting proceeds to the late stage,and only the bottom of the unmelted PCM remains,but this part of the PCM completely melts takes a lot of time,especially when the equivalent height of the fin exceeds 0.2,the time required for the remaining 10% of the phase change material to completely melt accounts for more than 20% of the entire melting process time.When the heat accumulator releases heat,only liquid PCM remains at the top and bottom in the later stage of the solidification process,and this part of PCM solidification also requires a lot of time.(3)The height of the fins has a greater influence on the heat storage/heat release performance of the phase change regenerator.For the heat storage process,the greater the height of the fins,the greater the heat transfer area,and the stronger the inhibition of natural convection in the liquid area.Therefore,when the height of the fins continues to increase,the heat transfer enhancement effect obtained per unit of increase shows a downward trend.For the heat release process,the enhanced heat transfer effect of the fins also has this phenomenon,and when the fin heights are 0.8 and 1,respectively,the two liquid phase ratio curves tend to be consistent.(4)For the heat release process,the greater the number of fins,the faster the heat release rate.However,when the fin height is larger,the number of fins continues to increase,and the heat transfer enhancement effect per unit fin number shows a downward trend.When the fin height is small,the trend is not obvious.(5)When the phase change regenerator stores heat,under the same fin structure,the larger the fin spacing,the more natural convection can be fully developed,and the faster the PCM melting rate.And the closer the last fin is to the bottom of the regenerator,the more quickly the remaining solid phase change material melts.When the heat is released,the two liquid phase rate curves are basically the same in the early stage after changing the fin spacing,but the smaller the distance between the fin and the top and bottom of the regenerator,the faster the remaining PCM solidification later.