Effect Of Subsurface Seepage On Heat Transfer Efficiency And Soil Temperature Field Of Buried Pipes

With the development of double carbon target in China,shallow geothermal energy as a clean,efficient and pollution-free energy has been paid more and more attention.The extraction and utilization of shallow geothermal energy is usually realized by ground heat exchanger,whose heat transfer effect is directly related to the economy and reliability of the whole ground source heat pump system.The heat transfer effect of buried pipe is influenced by many factors.At present,there are many researches on the influence of different factors on the heat transfer of buried pipe,at the same time,the influence of groundwater seepage on the ground heat exchanger is controversial.In order to study the influence of multi-factors on heat transfer of buried pipe and soil temperature field around borehole,a coupled model of heat and permeability was established by using COMSOL multi-physical field simulation software,and multi-factor experiments were carried out based on Taguchi method,then,the effects of groundwater seepage and drill hole spacing on heat transfer of buried pipe and soil temperature field around drill hole were studied.The specific research contents and conclusions are as follows:(1)COMSOL is used to establish three-dimensional coupled model of single tube and two-dimensional model of Tube Group,and Taguchi method is used to establish orthogonal table,taking the temperature difference between the inlet and outlet of the buried pipe,heat transfer rate per meter,and the average temperature difference of the soil around the borehole as evaluation indexes,the effects of groundwater seepage velocity,soil thermal conductivity and borehole spacing on heat transfer and soil temperature field around the borehole are studied.The results show that the most important factor affecting heat transfer rate per meter is soil thermal conductivity,followed by groundwater seepage velocity.For the soil temperature field around the borehole,the borehole spacing and seepage velocity are important factors.(2)In order to further study the influence of seepage on heat transfer of buried pipe,the influence of seepage velocity,seepage direction and seepage layer thickness on heat transfer of buried pipe was analyzed.The results show that the larger the seepage velocity and the thickness of the seepage layer,When the Ushaped pipe is arranged perpendicular to the seepage direction,heat transfer rate per meter of the borehole is the largest.(3)Finally,the effects of drill hole spacing on soil temperature field were studied,and a three-dimensional pipe group model was established to study the effects of drill hole spacing,intermittent operation and load ratio on soil temperature field.The results show that increasing drill hole spacing can reduce soil heat accumulation and improve heat transfer efficiency of buried pipe.When the seepage velocity is high,the soil area can be saved by decreasing the space perpendicular to the seepage direction,and the soil temperature rise can be kept small.Intermittent operation can reduce soil heat accumulation and temperature rise.When the seepage velocity is large,the temperature difference between different load ratios is small.