Effect Of Thermophysical Properties Of Layered Soil On Heat Transfer Performance Of Buried Pipe

In order to alleviate China's energy crisis and reduce the proportion of fossil energy in the energy structure,the 13 th Five-Year Plan highlights the importance of renewable energy in the energy structure,and strongly advocates supporting the development of electric-driven heat pumps.The geological exploration,policy support and incentive policies needed for ground source heat pump technology have been also improved.As the heat source of the ground source heat pump system,the thermophysical parameters of the soil play an important role in the design and operation of the system.Accurate measurement of soil thermophysical parameters can make the design of ground source heat pump system more accurate and improve the economy of the ground source heat pump system.The research contents and results are as follows.(1)Distributed optical fiber temperature measuring device is used to measure the thermal physical parameters of underground soil layers.Meanwhile,the average thermal physical parameters of soil layers are measured by conventional methods.Based on these two parameters,the three-dimensional unsteady heat transfer model considering soil layers and the three-dimensional homogeneous and unsteady heat transfer model neglecting soil layers are established respectively,which has been validated by experimental data of thermal response.(2)Based on the established soil layered model and homogeneous model,the heat transfer performance of buried pipes under summer heat storage conditions is studied.Different inlet temperatures(293,303,313,323,333K),different inlet velocities(1.0,1.2,1.4,1.6,1.8,2.0m/s)and different diameter of buried pipes(DN25,32,40,50)were set up to simulate the heat transfer performance of buried pipes under different conditions and different models.The heat transfer performance of buried pipes and the variation law of soil temperature field distribution are analyzed.The heat transfer per unit length,average thermal conductivity of boreholes and thermal radius are used as indexes to evaluate the heat transfer performance.The differences between the simulation results of the two models are compared.The results show that for the heat transfer per unit tube length and the average thermal conductivity of boreholes,the calculated results of the layered model are always larger than those of the homogeneous model.Although the distribution of soil temperature field under the two models is different,the maximum thermal radius of the two models is basically the same.The difference of heat transfer per unit tube length between the two models will increase with the increasing of inlet temperature.When the inlet temperature is 333 K,the difference can reach 6.2%.However,when the inlet temperature changes,the average thermal conductivity of the borehole remains stable.With the increasing of inlet velocity,the heat transfer per unit length and the average thermal conductivity of boreholes of the two models show an increasing trend,while the difference between the two models remains basically unchanged.The difference between the heat transfer per unit length and the average thermal conductivity of boreholes is 2.57W/m and 0.2 W/(m·?),respectively.With the increase of buried pipe diameter,the difference between the two models gradually decreases for the heat transfer per unit length and the average thermal conductivity of boreholes.(3)Based on the established soil layered model and homogeneous model,the heat transfer performance of buried pipes is simulated under winter heating conditions.And the heat transfer per unit length under different inlet velocities and pipe diameters is calculated respectively.Then,the well condition of buried pipes is designed and the initial investment of the well condition is discussed for a building with a heat load of 2250 kW.The results show that when the well depth is 75 m,the flow rate in the pipe is 1.0 m/s and the diameter of the pipe is DN30,the number of drilling holes in the design scheme based on layered and average soil thermophysical parameters is 498 and 528 respectively,with a difference of 6%.The initial investment of drilling is 1.06 and 1.12 million Yuan,respectively.The initial investment of drilling can be reduced by 6% by designing with layered thermophysical parameters.With the diameter of the pipe is kept at DN30,when the flow rate of inlet fluid increases from 1.0m/s to 2.0m/s,both the number of designed wells and initial drilling investment decrease.The number of wells calculated with average thermal physical parameters and initial drilling investment are 3.85% and 3.89% higher than those calculated with layered thermal physical parameters,respectively.The greater the flow rate of imported fluid,the smaller the difference of design results between the two methods.With the flow rate is constant at 1.0m/s,when the diameter of the pipe increases from DN30 to DN40,the initial investment in drilling increases.Because the cost of the pipe increases significantly although the number of drilling wells decreases causing by the heat transfer increased.The number of wells calculated with average thermal physical parameters and initial drilling investment are 5.96% and 1.1% higher than that of layered thermal physical parameters,respectively.The increase of pipe diameter also reduces the difference of design results between the two methods.