Heat Transfer Research And Thermal Characteristics Comparison Of Novel Double-channel Heat Exchanger With Phase Change Material

The latent heat thermal energy storage technology based on phase change materials has the advantages of high heat storage density,small temperature fluctuations and easy control of the process.It can effectively solve the problem of inconsistent supply and demand of thermal energy in time or space.It has broad application prospects in building energy saving,solar thermal utilization,and waste heat recovery and many other fields.As the core of the application of latent heat thermal energy storage technology,the thermal energy storage heat exchanger is composed of phase change material and heat exchanger to realize the encapsulation of the phase change material and the heat transfer between the heating/cooling fluid and the phase change material.In most applications,the heating and cooling fluids are usually different types of media.For example,the heating fluid is high temperature flue gas and molten salt or waste hot water,refrigerant,etc,while the cooling fluid is water,air,etc.Therefore,the thermal energy storage heat exchanger with multi-channel structure has more practical application value.In order to increase the rate of heat storage and release,the heat transfer performance of thermal energy storage heat exchanger must be strengthened.The research group of the author of the thesis independently developed novel shell-and-tube thermal energy storage heat exchanger,pillow-plate thermal energy storage heat exchanger and spiral-wound thermal energy storage heat exchanger with double-channel structure,and experimentally studied the heat transfer performance of the above three new types of thermal energy storage heat exchangers.The method of numerical analysis was used to comprehensively compare three thermal energy storage heat exchangers’thermal performance under the same heat release conditions,revealing the applicable scope of different types of thermal energy storage heat exchanger.The specific research content is as follows:（1）This paper designs and develops a double-channel shell-and-tube thermal energy storage heat exchanger.Different from the traditional shell-and-tube heat exchanger,the tube side of the new-type shell-and-tube thermal energy storage heat exchanger adopts a double-pipe structure.In order to enhance the heat transfer of heating and cooling process,a high thermal conductivity paraffin/expanded graphite composite phase change material was prepared and filled in the casing annulus.The tube side and the shell side are respectively fluid channels to realize double-channel heat exchange.In order to facilitate experimental research,both the tube side and the shell side used water as the heat transfer fluid.The heating and cooling performance of the tube side and the shell side of the thermal energy storage heat exchanger at different flow rates were explored through experiments.The hot water temperature was constant at 55℃and the cooling water temperature was 30℃,and the flow rate was 300 L/h,500 L/h,800 L/h,1000L/h and 1200 L/h.The results show that the heat storage and heat release of the shell side are as high as 13.5 MJ and 10.1 MJ,and the average working power is about 4 kW to 14 kW.The total heat storage and heat release of the tube side are 10.1 MJ and 9.9 MJ,and the working power is 2.5 kW to 10.0 kW.The heat transfer numerical model of the shell-and-tube thermal energy storage heat exchanger was constructed,and the influence of the different density of paraffin/expanded graphite composite phase change materials,the diameter of the inner tube,the expansion surface of the fins and the spacing of the baffles on the improvement of the thermal performance of the thermal energy storage heat exchanger was explored.The optimized parameters obtained are:the density of the composite phase change material is 600 kg/m³,the thermal conductivity is 5.16 W/（m·K）,the diameter of the inner tube is 19 mm,the height of the rectangular fin is 6 mm,and the thickness is 2 mm,the interval of baffles is 100 mm.（2）A new type of double-channel pillow-plate thermal energy storage heat exchanger was designed and developed.The pillow-shaped plate is formed by overlapping and sealing two stainless steel plates.Two flow channels are distributed in the plate to carry heating and cooling fluids respectively.Sodium acetate trihydrate was used as the phase change material to fill the gap between the plates.The thermal performance of the pillow-plate thermal energy storage heat exchanger was explored through experiments.The heating water temperature is 75℃,the cooling water temperature is 25℃,and the flow rate is 100 L/h,200 L/h,300 L/h,400 L/h and500 L/h.The experimental results show that the total heat release energy of the pillow-plate thermal energy storage heat exchanger is 4.3 MJ to 6.3 MJ,and the average working power is maintained at 2 kW to 5 kW.The total heat transfer coefficient and the heat transfer area-UA value were used to evaluate the heat transfer performance of the heat storage heat exchanger.For the flow rate from 100 L/h to 500 L/h,the UA value changes from 25 W/K to 70 W/K,and the UA value of the heating process is slightly higher than that of the exothermic process.（3）We further designed and developed a double-channel multi-layer spiral-wound thermal energy storage heat exchanger with a simple preparation process,in which the heat exchange tube is spirally wound,and sodium acetate trihydrate is used as phase change material filled in the gap of the tube.The heat storage and heat release performance of the spiral-wound heat storage heat exchanger at different flow rates were explored through experiments,and the flow rates were 100 L/h,200 L/h,300 L/h,400 L/h and 500 L/h.The experimental results show that when the flow rate is 500 L/h,the average heat release power of the thermal energy storage heat exchanger is 1.73 kW to 10.0 kW,and the total heat release energy is 15.23 MJ.The UA value shows a linear increase trend with the increase of the flow rate.When the flow rate increases from 100 L/h to 500 L/h,the UA value increases from 72 W/K to 83 W/K.（4）Using the method of numerical simulation,under the same type and filling amount of phase change material condition,the heat release performance of the above three double-channel thermal energy storage heat exchangers with different structures was comprehensively compared.The simulation analysis results show that the shell-and-tube thermal energy storage heat exchanger has the highest outlet temperature and the highest heat release power,which is suitable for applications with high outlet temperature requirements,but is not suitable for small volume occasions.The spiral-wound thermal energy storage heat exchanger has good thermal performance as a whole.The heat transfer rate per unit area and unit temperature difference is180 W/m^2.K,and the heat release power per unit volume is 115 kW/m³,which proves that this type of thermal energy storage heat exchanger has comprehensive good thermal performance,but the working pressure drop of this type of thermal energy storage heat exchanger is relatively high,which is not suitable for applications where the pressure drop is strictly required.Compared with the other two types of thermal energy storage heat exchangers,the pillow-plate thermal energy storage heat exchanger has the most stable water outlet temperature and is suitable for occasions that require a stable outlet temperature.