Experimental And Numerical Study On Heat Storage And Release Of Square Tube Shell Phase Change Heat Exchanger

Phase change heat storage is a latent energy storage method that can effectively improve energy utilization efficiency and is commonly used to alleviate the mismatch between energy supply and demand in terms of time,intensity,and location.With the demand for clean heating,peak shaving of power systems,and solar thermal storage,phase change heat storage technology has received widespread attention from researchers.In this paper,the flow coupled phase-change heat transfer process in the phase change heat storage unit with paraffin OP44 E as the phase change medium is studied by numerical and experimental methods based on the research model of the built-in square tube shell type phase change heat storage unit(1)Under the same PCM volume,the melting time of the square tube shell phase change heat storage unit is significantly shorter than that of the circular tube shell phase change heat storage unit,and the complete melting time is shortened by 20.6 %.The research results of the process of phase change heat storage with square tube and shell show that Natural convection heat transfer has an important influence on the process of phase change heat storage.During the melting process,the liquid phase change material flows upward to the top area of the heat storage unit,and the solid-liquid interface moves from top to bottom,and the melting rate above is significantly higher than that below.The phase change heat transfer in the bottom region,mainly based on heat conduction,restricts the overall heat storage rate and complete melting time of the phase change heat storage unit.(2)During the solidification process,the phase-change material near the wall of the heat exchange tube solidifies and forms a thermal conductive layer of the phase-change material.At this time,the solid-liquid interface moves from the inside out and the movement rate slows down significantly with time,indicating that the influence of Natural convection gradually weakens,and the main mode of heat transfer is heat conduction.Compared with the melting process,the solidification process requires a significant increase in time and a significant decrease in heat transfer rate.(3)When the heat exchange fluid inlet flow is less than 40 L/h,with the increase of the heat exchange fluid flow,the convective Heat transfer coefficient of the hot fluid in the heat exchange tube significantly increases,and the convective heat transfer resistance between the heat exchange fluid and the tube wall gradually decreases,thus reducing the total heat transfer resistance between the hot fluid and the phase change material.Therefore,the average heat transfer rate increases with the increase of the heat flow.When the heat flow rate of the heat exchange fluid is greater than 60 L/h,further increasing the heat flow rate will not significantly improve the total heat transfer resistance.At this time,the average heat transfer rate and complete melting time remain basically unchanged.The greater the temperature difference between the inlet temperature and the phase change temperature,the faster the completion of the storage/Exothermic process process,while the PCM initial temperature has little effect on the storage/exothermic process process of the phase change heat storage unit.(4)The research on the vertical and horizontal placement of the phase-change heat storage unit shows that in the early stage of melting,the melting rate is significantly higher than that under the vertical placement because the hot surface of the upper area inside the phase-change heat storage unit under the horizontal placement is upward,which is conducive to the formation and development of natural convection.However,in the later stage of melting,the heat transfer in the bottom area of the phase change unit under horizontal placement is dominated by heat conduction,and the heat transfer condition with the hot side facing downward limits Natural convection.Therefore,the melting rate of the phase change material is significantly lower than that under vertical placement.In terms of complete melting time,the vertical phase transition process in the tube shell has shortened the complete melting time by 40.5 % compared to the horizontal condition.(5)The study on the eccentric arrangement of HTF tubes under horizontal placement conditions shows that for the melting process,the downward movement of HTF tube eccentricity can promote the heat transfer of phase change heat storage materials at the bottom of the phase change heat storage unit and improve the melting rate.As the eccentricity increases,the time for complete melting of the phase change heat storage unit decreases.When the downward eccentricity is 0.4,the time for complete melting inside the tube shell is reduced by 75.4% compared to the HTF tube center arrangement condition.For the solidification process,the downward movement of the HTF tube will reduce the heat transfer effect on the upper region,reduce the solidification rate of the heat storage unit,and the complete solidification time of the phase change heat storage unit is higher than the solidification time under the central arrangement condition.