Study On Temperature Recovery Characteristics Of Frozen Soil Around Ground Heat Exchanger In Cold Regions

When the ground source heat pump is used for heating in cold regions,there will be a freezing phenomenon in the soil near the ground heat exchanger.Because the freezing and thawing phase change of the soil will bring the exchange of latent heat,it is necessary to evaluate the temperature recovery characteristic of the frozen soil during the shutdown of the ground source heat pump unit,in order to ensure the stable operation of the unit.In this paper,based on the theoretical analysis of phase-change heat transfer,a sandbox experiment platform is built to study the temperature recovery characteristics of frozen soil around the ground heat exchanger.First,the sandbox experimental platform is used to simulate the process of soil freezing and the temperature recovery of the frozen soil,and the influence of internal fluid temperature and soil water content on the temperature recovery of frozen soil is studied by experimental study.The results show that: with the increase of the temperature in the tube and soil water content,the effect of temperature recovery on frozen soil is increased gradually,be expressed in the melting rate of the frozen soil is accelerating,the time needed to melt all the frozen soil is shortened,the recovery temperature and recovery rate of soil at the same location in the same recovery time are increased.When the other conditions are equal,the internal fluid temperature increase from-10? to-2?,the time required for total frozen soil melt reduce from 1.53 h to 0.22 h,and reduce by 85.62%;Soil water content increase from 10% to 25%,the time required for total frozen soil melt decrease from 0.94 h to 0.75 h,and decrease by 20.21%.Then,the method of numerical simulation is introduced,the soil temperature field under different working conditions is simulated,and the calculated values are compared with the measured values in the same working condition,the results show that: the maximum error of simulation value and experimental value is 14.14%,which satisfies the requirements of the engineering calculation precision.It is indicated that the simulation method is applicable to the experimental model in this paper.Then,the effects of soil initial temperature,thermal conductivity and volume heat capacity on the temperature recovery of frozen soil are analyzed by using the validated numerical simulation method.The results show that: the increase of soil initial temperature and thermal conductivity can promote the melting and temperature rise of the frozen soil,be expressed in the melting rate of the frozen soil layer is quickening and the recovery temperature and recovery rate of the soil at the same recovery time at the same position in the frozen soil layer are increased.And the increase of soil volume heat capacity can inhibit the temperature recovery of frozen soil,be expressed in the melting rate of the frozen soil is slowed down and the recovery temperature and recovery rate of the soil at the same recovery time at the same position in the frozen soil layer are decreased.Finally,the effects of forced recovery on the temperature recovery characteristics of frozen soil are studied by using simulation method.The results show that: compared with the natural restoration,the recovery effect of the forced recovery on the frozen soil layer is significantly improved.Forced recovery can speed up the melting rate of the frozen soil and increase the rate of soil temperature rise in the recovery process.When the other conditions are equal,the temperature of the soil is restored by circulating 30 centigrade hot water can make the time needed to melt all frozen soil shorten by 90.99% compared with natural recovery.In addition,the higher the forced recovery temperature,the more significant the effect of the recovery of the frozen soil.The results of this paper can be used as a reference for evaluating the soil temperature recovery performance of ground source heat pump system in cold area under the condition of soil freezing.