Simulation Of Heat Transfer Performance Of Buried Pipe Group In Layered Soil

With the increasing contradiction between energy and environment,GSHP(Ground Source Heat Pump),a green and energy-efficient cold and heat source system,is an efficient way to use shallow geothermal energy.The heat transfer performance of buried pipe heat exchanger and surrounding rock and soil medium is always one of the main research objects of GSHP technology.In present practical application,deep ground is regarded as even medium while disregard its possible stratification when design the buried pipe heat exchanger.Different rock and soil physical properties of each layer will lead to the difference of actual heat exchange performance for the system with a large number of buried pipes.Therefore,it is of great significance to establish the heat transfer model of stratified tube group to evaluate the performance of tube group heat transfer accurately.The analytical model of heat transfer of buried pipe heat exchanger has the characteristics of fast calculation speed and convenient application.It has always been a powerful tool for scholars to analyze the heat transfer performance of buried pipes and design and calculate the parameters of buried pipes.With the development of the application of GSHP system,the existing analytical model of heat transfer of buried pipes can't directly solve and analyze the corresponding heat transfer problems.Therefore,the analytic model of stratified tube group heat transfer under the condition of considering multiple finite line heat source models is constructed after coupling based on the existing finite line heat source model outside the borehole,combined with the quasi three-dimensional heat transfer model of single U-tube in the borehole.With the help of MATLAB,it can realize the solution and calculation of the model.At the same time,the accuracy and reliability of the analytical solution model are verified by using the software Fuent to build the corresponding numerical solution model of stratified tube group heat transfer.The heat transfer efficiency of buried pipe group and the temperature response of rock and soil in the area of pipe group are calculated by using analytical model and numerical model respectively.The "regional thermal efficiency" is introduced to evaluate the overall heat transfer of the pipe group,and the quantitative analysis of the overall heat transfer of the pipe group is carried out from the perspectives of layered geotechnical thermal properties,the temperature at the inlet and outlet of the buried pipe and the heat transfer layer by layer.The heat transfer performance of the pipe group heat exchanger in layered rock and soil and the thermal interference between the boreholes are further studied by changing the parameters of thermal physical properties of rock and soil,the number of pipes buried in the ground,the layered structure and the seepage velocity of groundwater.The results show that the stratification of rock and soil brings about different physical parameters and stratification structure of different layers,and the different degree of thermal interference between different boreholes in different rock and soil layers is one of the main reasons for the difference of heat transfer efficiency of pipe groups in layered rock and soil.Layered rock and soil will affect the regional thermal efficiency,but the homogeneous model can't correctly reflect this effect,while the seepage speed has a significant impact on the thermal efficiency.The seepage of groundwater makes the thermal interference coefficient of the pipe group quickly tend to a fixed value,and the heat transfer of the seepage layer should be analyzed separately.At last,it will be more practical to calculate and analyze the heat transfer of the buried pipe group heat exchanger by using the layered tube group heat transfer model,especially for the large-scale GSHP system with a large number of buried pipes.The layered analysis will provide a more accurate basis for the design of the buried pipes.