Influence Of Seepage On The Thermal Performance Of Vertical Ground Heat Exchanger With Multiple Layers

The thermal performance of the vertical ground heat exchanger(GHE)is a key factor affecting the energy efficiency of the ground source heat pump(GSHP)system,whose thermal resistance is mainly derived from the rock and soil surrounding the GHE.Due to the complicated geological conditions,vertical ground heat exchangers(GHEs)may traverse different geological layers along the vertical direction,and groundwater flow(seepage)may occur in some geotechnical layers,resulting in different heat exchange capacities of the surrounding layers.Therefore,it is necessary to figure out the influence of different seepage characteristics on the thermal performance of vertical GHEs in multiple-layer substrates,so as to provide relevant research basis for the design of GHE system in the GSHP project.Based on the distribution of confined aquifers obtained from literature investigation,this study established a heat transfer model for the vertical GHE under coupled heat conduction and groundwater advection,that can consider geological stratification and groundwater flow.Based on the similarity principle,the heat transfer model was similarly transformed to obtain the similar control conditions of the model.When the physical parameters of the materials in the similar model are the same as the corresponding prototypes,if the velocity(U-tube heat exchanger flow velocity and groundwater velocity)similarity ratio and geometric similarity ratio are inversely proportional,time similarity ratio is proportional to the square of geometric similarity ratio,and the definite conditions of the two models are similar,then the heat transfer phenomena in the two models are similar.Thus a vertical GHE similar experiment system corresponding to an actual GHE system was constructed,after the dimensions were reduced according to similar control conditions which can simultaneously consider geological stratification and seepage.Through similar experimental data of vertical GHEs under geological stratification and seepage conditions,it was found that the enhanced heat transfer effect of the seepage on the upstream GHE was greater than that of the downstream GHE.Moreover,relative to the direction of seepage,the difference in the spatial position of the two legs of the U-tube affected the heat transfer of the U-tube,thereby affecting the thermal performance of the GHE.The seepage accelerated the recovery of the ground temperature by the transfer of heat after the GHEs stopped operating.The similar numerical model and prototype numerical model corresponding to the similar experiment system were established.Based on the numerical model verified by experimental data,when seepage occured in some geological layers,the establishment and application of the equivalent heat transfer model were discussed.The equivalent model was established by adding the effect of seepage on the thermal performance of the GHE to the thermal conductivity of the layers.,which could better predict the outlet temperature of the GHE and simplify the calculation under certain conditions.The numerical model was used to analyze the influence of rock and soil types,vertical depth and thickness of the seepage layer on the thermal performance of the GHE when there was seepage in some geological layer.Under the condition of one GHE,keeping the inlet temperature constant and running for 4 months continuously,it was found that the influence of the seepage layer on the thermal performance of the GHE was more significant with the increase of time.The seepage layers with different rock and soil types had different heat transfer abilities,resulting in different heat affected radius of the GHE.The degree of influence of the seepage layer at different vertical depths on the thermal performance of the GHE was related to the U-tube flow velocity.When the flow velocity of a single U-tube was 0.6 m/s,the difference in water temperature along the path was small,and the influence of the vertical depth of the seepage layer on the GHE was negligible.As the thickness of the seepage layer increased,the outlet temperature of the GHE decreased,showing an approximately linear relationship.Moreover,with the decrease of the outlet temperature,the temperature of the tube near the outlet of the GHE was lower than the surrounding geological layer,and heat absorption occurred.Since the Pe number only reflects the relative strength relationship between the heat conduction and convection of the seepage layer,when designing the GHE system,it is necessary to re-evaluate the comprehensive heat transfer effect of the seepage layer on the GHE according to the vertical depth and thickness of the actual seepage layer.Through experimentation and numerical calculations,the influence law of seepage on the thermal performance of vertical GHE in multiple-layer substrates was explored in this study,in order to provide a reference for the design of GHE system under relevant seepage geological conditions.