Theory And Design Research On Heat Transfer Performance Of The U-pipe Vertical Ground Heat Exchanger Based On Coupled Heat And Seepage Model

The heat transfer performance of ground heat exchanger(GHE)is always a research key in the field of ground-coupled heat pump(GCHP)related technologies.At present,the heat transfer models of GHE used in practical project designs are mainly based on single heat conduction theories,which can’t consider the influence of groundwater seepage on the heat transfer performance of GHE.It may cause the designed length of GHE to be oversized.As a result,the initial and operational costs of the systems are improved.Therefore,this paper analyzes the heat transfer process of GHE considering groundwater seepage,multi-pipes,dynamical load and combines some heat transfer models of GHE,such as one-dimension model,two-dimension model and quasi three-dimension model,infinite moving line-source model and infinite line-source model.This paper developes the application software on GHE design calculation in the Visual Studio environment by hybrid programming with Visual Basic.NET language and Matlab language.This software can implement some functions of GHE design calculation.For example,it can calculate the thermal resistance inside and outside borehole,the length and number of the borehole.The results indicate that,for the thermal resistance inside the borehole,the result of quasi three-dimension model is the least and the results of onedimension model and two-dimension model are close.As for the thermal resistance outside the borehole,the result of infinite moving line-source model is less than the result of infinite linesource model,which shows that the groundwater seepage can enhance heat transfer in a way.On account of the fact that the existing GHE analytical model considering the influence of groundwater seepage can’t solve the coupled heat transfer problem from the in-tube fluid to the soil with groundwater advection comprehensively.Therefore,in this paper,a coupled heat transfer analytical model is established to take the influence of groundwater advection on GHE with single and double U-pipe buried inside the borehole into consideration,with the temperature of the borehole wall coupling the finite moving line-source model outside the borehole with the quasi three-dimension model inside the borehole,which applies to the single borehole and the solution of the analytical model can be obtained by iterative optimizing calculation.In addition,the heat transfer analytical model of multi-pipes and variable heat flux line-source analytical model are established based on the single borehole model in terms of superposition principle and step load.On the basis of the GHE design calculation application software,this software is further developed to calculate the soil temperature around the GHE by using these established model and this software are validated by the measured data.The effects of soil temperature properties,backfill material,groundwater seepage,seepage velocity on the heat transfer performance of multi-pipes in summer are studied by taking the heat transfer per meter borehole and outlet temperature of the buried pipes as evaluation indexes.The soil temperature around multi-pipes is dynamically simulated by this developed software.The results indicate that the groundwater seepage can effectively promote the heat transfer around the multi-pipes and the deviation of 25% will occur if the groundwater seepage is not considered in the process of GHE design.The conclutions being got by analyzing the whole year hourly soil temperature at the different radius and different depths show that,the thermal disturbance around the multi-pipes have obvious influence on the soil temperature field of the multi-pipes central location.Moreover,the soil temperature of the whole year hourly dynamic load at the 80 meters depth and outside the radius of 3m from the multi-pipes are close to the initial temperature after being stable,and basically remain unaffected.In addition,the injected heat is greater than the extracted heat,indicating that there is some imbalance in the heat transfer capacity between summer and winter,which results in a certain rise in the soil temperature.