Study On Heat Exchange Characteristics Of Ground Tube Heat Exchanger With Phase Change Material Backfill

In recent years,the energy crisis caused by the irrational economic structure has increased.Under the background of energy saving and emission reduction,the ground source heat pump system has been widely used in the construction industry in China,and scholars at home and abroad have exchanged heat exchangers for underground heat exchangers.A large number of studies have been carried out on the properties,of which studies have shown that the heat transfer properties of the backfill material are of crucial importance.In recent years,some scholars have proposed to use phase change materials as backfill materials in underground heat exchange systems.Studies have shown that the use of phase change materials has the advantages of increasing the heat storage capacity of individual wells and reducing the radius of heat impact.In this paper,pure phase change materials and ternary phase change materials are used as backfills,respectively,to investigate their effects on the heat transfer characteristics of ground heat exchangers.The results of the study have theoretical significance for the practical application of phase change material backfill.In this paper,the three-dimensional unsteady heat transfer model of the buried pipe heat exchanger is established by using FLUENT.The backfilling of pure phase change material and fixed phase change material is simulated using the helium-porosity model and equivalent specific thermal model,respectively.First of all,for the backfilling of pure phase change material,the heat transfer characteristics of both ordinary backfill and backfill of pure phase change materials are compared and analyzed.The results show that under the calculation conditions of this paper,after adding PCM,the heat transfer can be effectively improved and the buried pipe can be relieved.Heat builds up around you.Subsequently,the heat transfer performance of the buried pipe heat exchanger under intermittent operating conditions in summer was studied.The results show that under the operating conditions of this paper(the range of phase transition temperature is 18-26°C),the heat exchange capacity is at the initial stage of intermittent operation.As the phase change temperature increases,the PCM with a lower phase transition temperature can significantly improve the heat exchange capacity of the buried pipe.However,as the time progresses,the heat transfer rate of the PCM with a higher phase transition temperature to the heat exchanger improves.The effect is better than the PCM with lower phase transition temperature in the same period.In addition,during the operation,PCMs with different phase transition temperatures exhibit different melting and solidification characteristics.When the PCM continues to store and release heat,the fluctuation of the soil temperature during operation can be reduced.For other influencing factors: latent heat of phase change,initial soil temperature,flow rate,etc.,the effect of heat transfer performance on the heat transfer performance was analyzed by the single factor method.For the backfilling of the phase change material,the heat transfer performance of the buried pipe heat exchanger was simulated using the equivalent specific heat model based on the sensible heat capacity method.The results of the numerical simulation were compared with the data in the reference and verified.The correctness of the equivalent specific heat model used is correct.On this basis,the phase change materials with volume fractions of 20%,40%,60%,and 80% were evenly mixed with the backfill media,and the effect of backfill ratio on the heat transfer performance was explored.The results showed that when the backfill media are thermally conductive When the coefficient and specific heat capacity are smaller than the phase change material,the higher the backfill ratio,the greater the effect on the heat transfer amount.In addition,for the different types of buildings,this paper selects four typical types of buildings: shopping malls,hotels,offices,and residences.According to their load characteristics,they take different types of intermittent operation ratios,and compare and analyze the effects of intermittent operation on heat transfer.The longer the time between the system shutdown and the operation,the more favorable the recovery of the surrounding soil and the long-term efficient operation of the ground-tube heat exchanger.Finally,in this paper,the backfilling of the phase change material with 100% volume fraction is used to analyze the heat transfer characteristics of the ground heat exchanger in the complete operating cycle,and the heat transfer in the refrigeration and heating conditions is emphatically introduced.Performance,provide reference for the application of phase change material backfill ground heat exchanger.In summary,the phase change backfilling can increase the heat transfer capacity of the buried heat exchanger to a certain extent,but it needs sufficient time for intermittent recovery,so that the temperature field around the buried pipe can quickly return to the initial state.The research results of this paper can provide theoretical guidance for the application of phase change backfill engineering.