Research On Heat Transfer In Porous Medium-deep Underground Heat Exchangers

Compared with the traditional shallow buried pipe heat exchanger,the deep borehole heat exchangers(DBHE)have the advantages of small floor area,high heat consumption per linear meter and high outlet water temperature.At present,there are more and more pipe group projects in many places in China,but the calculation model of DBHE is mainly cylindrical coordinate single hole calculation model.When the operation time is long and the distance between buried pipes is close,there must be heat effect between buried pipes,and the heat effect between each heat exchanger cannot be considered when using the single hole model.In addition,it is difficult to study and optimize the heat transfer characteristics of the multi well deep borehole heat exchangers(MDBHE)in the whole life cycle by using the single hole calculation model.At present,a MDBHE calculation model with high efficiency and accuracy is needed.Based on the above practical problems,this thesis presents a numerical solution model of MDBHE.Because MDBHE is a complex three-dimensional fluid solid coupling unsteady heat conduction problem,the calculation area is huge,and the space step and time step span are very large,so when using commercial CFD software to simulate,it needs to calculate a large number of grid nodes,which means that it needs strong computing power and long calculation time,which is very inconvenient in engineering.Based on the numerical model of single hole heat exchanger,the finite difference method is used in this thesis.The temperature field of underground rock and soil is solved by using the method of weighted average of temperature of intersecting nodes.In each time step,the excess temperature of intersecting nodes in the cylindrical coordinate system is weighted average,which avoids the calculation of a large number of nodes in the threedimensional rectangular coordinate system and greatly speeds up the solution speed.Compared with CFD software,the calculation speed is improved by several orders of magnitude.In this thesis,the numerical model is used to study the heat transfer of double hole and four-hole heat exchanger with the change of pipe spacing,depth and thermal physical properties of rock and soil,and the concepts of heat transfer attenuation rate and heat influence rate of MDBHE are proposed to describe the attenuation range of nominal heat transfer and the heat influence rate between pipes.According to the proposed concept,when the heat effect rate is not less than 5%,the corresponding buried pipe spacing is the recommended spacing.Using the numerical model,it can be concluded that the nominal heat loss rate of the MDBHE approaches to the nominal heat loss rate of the DBHE with the increase of the buried pipe spacing;the heat influence rate of the MDBHE approaches to zero with the increase of the buried pipe spacing;when the buried pipe depth of the MDBHE is deep,it is suggested that the buried pipe spacing should be appropriately increased;the thermal conductivity of rock and soil should be improved,it is suggested to increase the distance between the buried pipes to reduce the heat effect between the buried pipes and effectively improve the nominal heat consumption.