Model And Application Research Of Vertical Borehole Heat Exchanger Based On Geological Stratification

The ground source heat pump technology uses shallow geothermal energy for heating and cooling,and has the characteristics of cleanness,high efficiency and stability.In the design of heat pump systems,the soil is often used as the equivalent uniform medium for the design calculation of borehole heat exchangers,ignoring the influence of the geological environment.Based on the actual geological stratification situation and on the basis of the theory of heat transfer in porous media,this paper establishes a numerical model of a borehole heat exchanger considering groundwater seepage,and calculates the water temperature in the U-tube of the borehole heat exchanger Distribution characteristics,the temperature distribution of the soil around the borehole heat exchanger and its heat transfer characteristics,and the effects of the axial layering of geotechnical soil and groundwater seepage on the heat transfer characteristics of the borehole heat exchanger are analyzed.First,based on a comprehensive analysis of the heat exchange mechanism of the borehole heat exchanger,this paper established a three-dimensional numerical model of the homogeneous borehole heat exchanger.Duct Storage System(DST)model verifies the calculation accuracy of the numerical model.Under the condition of constant heat flow,the axial and radial temperature distribution and heat transfer characteristics of the homogeneous borehole heat exchanger model were quantitatively calculated.Secondly,through the established numerical model of the geotechnical axially layered borehole heat exchanger,the thermal response test is carried out,and the homogeneous borehole heat exchanger model is established by using the obtained thermal physical property parameters.Under the condition of 5kW constant heat flow,the calculation results of the two models are used to compare and analyze the influence of the axial layering of rock and soil on the heat transfer characteristics of the borehole heat exchanger.The results show that the average temperature of the circulating fluid in and out of the layer model is 2.2%higher than that of the homogeneous model at 100h;The soil temperature around the borehole varies greatly in different layers along the axis.At 100h,the weighted average value of the total heat flux of the borehole wall is 3.2%higher than the homogeneous model.Finally,by establishing a layered underground borehole heat exchanger model coupled with groundwater seepage,combined with thermal response tests,the thermal physical parameters of the homogeneous model were determined.Under the condition of 5kW constant heat flow,the calculation results of the layered borehole heat exchanger model with and without seepage and the homogeneous model are compared.At 100h,the average temperature of the circulating fluid in and out of the stratified seepage model is 4.3%lower than that of the non-seepage stratified model,and 5.4%higher than the corresponding value of the homogeneous model;the weighted average score of the borehole wall temperature of the non-seepage stratified model at 100h The laminar seepage model is 6.6%higher;when there are multiple layers of groundwater seepage,the U-tube high temperature zone(second layer)seepage has a greater influence.At 100h,the average total heat flux in the seepage layer is 33.7%higher than that in the model without seepage.In this paper,through the calculation of heat transfer characteristics of borehole based on geological layered coupling seepage,the difference between the actual heat transfer characteristics of the borehole heat exchanger and the homogeneous model is compared and analyzed.In order to combine the actual geological prospecting and determine the thermal physical parameters of the stratified stratum,it laid a theoretical foundation for the accurate design of the borehole heat exchanger.