Numerical Simulation Of Heat Transfer Performance Of Buried Pipe Heat Exchanger Based On Geotechnical Layering

Based on the geotechnical layered structure and relying on reasonable assumptions and simplified conditions,a three-dimensional unsteady model of a vertical single U-shaped buried tube heat transfer system is established.The effects of different inlet speeds and inlet temperatures on the heat exchanger outlet temperature and single well heat transfer were studied by using ANSYS software,which used stratified thermal properties,average thermal properties and weighted thermophysical parameters.The difference between the outlet temperature and the single well heat exchange under the physical parameters was analyzed.The results show that compared with the average thermal property parameters,the relative error between the calculated results of the weighted thermophysical parameters and the stratified thermophysical parameters is small.When the geothermal parameters are simplified,the weighted average method is recommended.Based on the structural characteristics of layered geotechnical soil,the buried tube heat exchanger is divided into corresponding sections to study its heat transfer characteristics.The results show that when the system is operated for 96 h,Compared with the downcomer,the energy efficiency coefficient of the riser is reduced by 40%,42.9%,49.1%,and 14.4% from top to bottom,and the heat exchange capacity per unit length of the downcomer is 44.29 W/m,53.5 W/m,47.84 W/m,22.78 W/m,respectively.Therefore,the greater the thermal conductivity of the soil around the heat exchanger,the smaller the rate of decline of the energy efficiency coefficient of the riser,and the heat exchange capacity per unit length of the riser of each section is smaller than that of the downcomer,and the unit tube of the rock layer with the highest thermal conductivity,and the unit pipe length of the rock layer with the largest thermal conductivity is the largest.The study on the spatial distribution of geothermal temperature around the buried heat exchanger of the layered heat transfer model shows that Due to the different thermophysical parameters of the layered rock,the axial temperature distribution of each layer is uniformly distributed in a circular shape,and the axial direction presents a non-uniform distribution law.Therefore,in the actual design,the difference of the maximum heat affected radius of different geotechnical layers caused by the stratification factors should be fully considered.The maximum heat affected radius of each layer should be selected as the reference value of the ground tube heat exchanger spacing,and the group tube spacing should be reduced.Thermal short circuit phenomenon,strengthen the heat transfer performance of the buried tube heat exchanger.The effect of thermal conductivity and volumetric heat capacity of the backfill material on the outlet temperature and heat transfer performance of the heat exchanger was analyzed by layered backfilling.The results show that compared with the reference thermal conductivity,the increase in the heat transfer rate of the three tubes with different thermal conductivity conditions is: 3.5%,8.4% and 12.9%,and the increase of the imbalance coefficient is 7.01%,17.47%,and 25.57%.Compared with the reference volume heat capacity,the increase in heat exchange capacity per unit length of the three increased volumetric heat capacity conditions is: 1.11%,0.41%,and 0.07%,and the imbalance coefficient is reduced by 0.1%,0.2.% and 0.3%.It can be seen that the heat transfer coefficient increases,the heat exchange amount per unit length increases,but the imbalance coefficient also increases.Although the volumetric heat capacity increases,the heat exchange amount per unit length can be increased,but the increase is small.Therefore,the thermal conductivity of the backfill material is proportional to the volumetric heat capacity,and the former has a dominant effect on the heat transfer performance of the buried heat exchanger.In the study of strengthening the heat transfer of the buried heat exchanger,the backfill material should be considered.The thermal conductivity,which can reduce the number of wells while reducing the footprint of the drilling area,and reduce the initial investment,is of great significance for the further promotion of soil source heat pump applications.