Heat Transfer Experiment And Numerical Calculation Of Single U-type Ground Heat Exchanger Under Stratified Seepage Conditions

Ground source heat pump system(GSHPS)has become a mature technology after years of development.The research on GSHPS is getting more and more attention,related research is also carried out from multiple angles.At present,the research on ground source heat pump is not perfect,and there are still insufficient researches on soil stratification,physical properties of backfill materials,and thermal interference evaluation between the branches(TIEB).GSHPS is energy-saving and efficient airconditioning system which use shallow geothermal energy.GSHPS releases heat to soil for cooling in summer and getting heat from soil for heating in winter.The law of heat transfer of single U-type ground heat exchanger(SUGHE)has become the focus of research on GSHPS.There are many factors affecting heat transfer of SUGHE,and many GSHPS were built in seepage zone,so seepage parameters become an important aspect which affect heat transfer of SUGHE.Secondly,backfilling is an important step in the construction of SUGHE,SUGHE is actually exchanged with backfill material,so porosity of backfill material(PBM)is important to heat transfer of SUGHE.Finally,the operating parameters of the GSHPS affect many aspects of SUGHE,and they are also important factors affecting the heat transfer of SUGHE.In this paper,the vertical SUGHE in the geothermal heat pump system was taken as the research object.The influence of seepage parameters,physical properties of backfilling materials and system operating parameters on the heat transfer of the SUGHE was studied.Based on thermal similarity theory,a stratified seepage experiment platform was built,and the numerical calculation model of SUGHE was established based on the stratified seepage experiment platform.Comparing the running data of the experiment platform with the numerical calculation model data,the correctness of the numerical calculation model was verified.The evaluation index of the SUGHE heat transfer was determined,the data fitting method was used to determine the relationship between influential factor and the outlet temperature.The function expression of evaluation index of SUGHE transfer was deduced,and the influence law of influential factors such as seepage parameters,physical properties of backfill material and system operation parameters on the heat transfer of SUGHE was quantitatively analyzed.Based on the numerical calculation model of stratified seepage,the numerical calculation model of annual heat transfer was established,the influence of seepage temperature on the annual heat transfer performance of SUGHE was analyzed.Firstly,based on the thermal similarity theory and Darcy's law,a stratified seepage heat transfer experiment platform was established,and the physical properties of the soil in the experiment platform was measured,the physical parameters of the soil was obtained.According to the physical parameters and dimensions of the experiment platform,a numerical calculation model was established.Comparing the inlet and outlet water temperature and soil temperature in the experimental platform and numerical calculation model,the correctness of the numerical calculation model was verified.The heat exchange per meter(HEPM)was used as evaluation index of heat transfer performance of SUGHE,the thermal imbalance coefficient(TIC)was used as the index of thermal interference evaluation between the branches(TIEB).As the evaluation parameter of influential factors affecting ability,the influence degree was defined.According to the existing literature and norms,the value and the basic value of each influential factor was determined.Based on the numerical calculation model,the influence law of influential factors on SUGHE was analyzed.The data fitting method was adopted to determine the relationship between influential factors and outlet temperature of SUGHE.Combined with the relationship between heat transfer evaluation index and outlet temperature,the functional relationship between heat transfer evaluation index and influential factors was derived.Based on the functional relationship between heat transfer evaluation index and influential factors,the first derivative of heat transfer evaluation index to the influential factors was obtained,and the influence degree of the influential factors on the heat transfer evaluation index was determined.Based on the numerical results,the influences of influential factors on performance of SUGHE,TIEB,and the soil temperature distribution was analyzed.Based on the influence degree expression,the variation law of the influence degree on heat transfer performance of SUGHE and TIEB changed with the influential factors themselves and the running time is analyzed.In the physical properties of backfill materials,the influential factos is PBM.In the seepage parameters,the influencing factors are seepage velocity and seepage temperature.In the operating parameters,the influencing factors are water velocity,inlet temperature,and the operation start-stop ratio.The actual engineering size model was restored by the physical properties of seepage layer of experimental bench model.The influence of seepage temperature on the annual heat transfer performance of SUGHE was studied.It is found that seepage temperature has a positive or negative influence on the annual heat transfer performance of the SUGHE depends on the accumulated load difference in winter and summer.In this paper,the influence degree of each factor changes with the factors themselves and running time was analyzed.The above conclusions have certain reference value for evaluating heat transfer performance of SUGHE and TIEB,providing preliminary prediction and guidance for the corresponding projects.