Experimental Study On The Hydraulic And Heat Transfer Time-space Characteristics Of Buried Pipe Groups

The development and utilization of geothermal energy is an important part of China’s clean,low-carbon and efficient energy system.For the ground-coupled heat pump system,the soil thermal accumulation on the buried pipe group is one of the key problems that need to be solved urgently.In terms of time,with the increase of the operating life,the difference between the building’s heat storage/withdrawal from the soil will continue to increase,and the degree of deviation of the soil temperature field from the initial ground temperature will become more and more obvious;spatially,the soil temperature in the central area will rise above the soil temperature in the boundary area,further worsening the heat transfer conditions of the buried pipes in the central area.At present,most of the research on soil thermal accumulation ignores the effect of the change of the flow rate of the heat transfer medium in the buried pipe on the heat exchange of the buried pipe,so it is important to explore the hydraulic characteristics of different horizontal pipe connection methods,the relationship between the heat exchange of the buried pipe and the soil temperature field,which is crucial to alleviate the soil thermal accumulation in time and space.In this paper,the calculation process of the calculation formula of the normative specific friction resistance is first clarified;secondly,the field test system of the ground-coupled heat pump is built,and the flow,temperature and pressure drop characteristics of the buried pipe heat exchanger under the initial ground temperature and heating working conditions are carried out,and the calculation formula of the buried pipe correction specific friction resistance is obtained;third,the flow distribution and heat exchange spatio-temporal characteristics of the four horizontal pipe connection modes of non-collector pipe heterogeneous,non-collector heterogeneous,non-collector heterogeneous,non-collector and collector homogeneous are explored Finally,the effects of heat exchange equalization and four horizontal pipe connection methods on soil temperature field were analyzed from the perspective of spatio-temporal space.Research indicates:（1）Under any test well,flow,heating power and heating time,the double U-shaped buried pipe heat exchanger composed of one of the buried pipe groups is always less than the normative specific friction resistance,and the percentage of the difference between the two is greater than 20%.If the specification is used to select the friction resistance design,the energy consumption of the pump will be increased and the system efficiency will be reduced.Derive,fit,and verify the calculation formula of modified specific friction resistance based on the test specific friction resistance.The relationship between the modified specific friction resistance and the density,dynamic viscosity,flow rate and inner diameter of the heat transfer medium in the tube is_dP=0.125r^0.75μ^0.25d^-1.²⁵u^{1.7 5}.（2）Different horizontal pipe connection methods will produce different flow rates and heat exchange distribution trends,which will lead to differences in soil thermal accumulation in time and space.When using a different-way connection,the closer to the main trunk,the greater the flow and heat exchange of the buried pipe.After fifteen years of system operation,the soil temperature field presents a temperature distribution difference between the near and far regions in space;when the collector pipe is connected in the same distance,the middle flow of the buried pipe heat exchanger is small,the flow rate on both sides is large,and the difference between the soil temperature in the boundary area and the buried pipe well wall in the central area will be reduced by 50%compared with the uniform distribution of the heat exchange of the buried pipe group,which greatly alleviates the soil thermal accumulation on the space,which is conducive to the uniformity of soil temperature distribution;when the non-collector pipe is connected together,the middle flow of the buried pipe heat exchanger is large and the flow rate on both sides is small.The difference between the soil temperature in the boundary area of the test area and the well wall of the buried pipe in the central area will increase by 125%compared with the average heat exchange of the buried pipe group,which will aggravate the soil thermal accumulation in space.（3）It is recommended to use the header connection method to alleviate the soil thermal accumulation on the space,if the non-collector connection method should be adjusted by adjusting the secondary manifold valve to reduce the heat transfer burden of the soil in the central area,so that the soil temperature distribution is more uniform in space.