Study On Optimization Of Vertical Ground Heat Exchanger Based On Geotechnical Stratification And Groundwater Seepage

According to the geological conditions in the Loess Plateau region,a three-dimensional unsteady heat transfer model of vertical ground heat exchanger considering geotechnical stratification and groundwater seepage is established by Fluent.Through numerical simulation,the heat transfer performance and geotechnical temperature field under the action of groundwater are analyzed and studied.The method to judge the best buried depth of ground heat exchanger is put forward,which provides a basis for the design of ground heat exchanger in the loess plateau area.The layered rock-soil body surrounding the ground heat exchanger were simulated for a period of 10 years.By analyzing the change of the temperature of the rock-soil body with time,the reasonable suggestions for the temperature balance of soil under the operation of the ground heat exchanger were provided.The results show that the heat transfer of ground heat exchanger in the aquifer is intense,which leads to ineffective heat transfer in the latter half of ground heat exchanger.Excessive depth of the ground heat exchanger not only affects the heat transfer efficiency,but also increases the investment.In the Loess Plateau region,the best buried depth of ground heat exchanger is related to the thickness of aquifer.When the thickness of the aquifer is large enough,there is a"typical aquifer",which is defined as the area in the inner part of the aquifer where the heat transfer efficiency of the ground heat exchanger is high,that is,the point from the groundwater table to the point where the exchanger has ineffective or inefficient heat transfer.When the thickness of aquifer is less than that of"typical aquifer",the optimal depth of ground heat exchanger is located at the bottom of aquifer.When the thickness of the aquifer is greater than that of the"typical aquifer",the optimal depth of ground heat exchanger is the bottom of the"typical aquifer".The thickness of the"typical aquifer"is related to the seepage velocity of groundwater.With the increase of the seepage velocity,the thickness of the"typical aquifer"decreases gradually and reaches the maximum at 1×10^-6m/s.When the seepage velocity is less than 1×10^-6m/s,the influence of groundwater on the heat transfer of ground heat exchangers can be ignored.The height of the groundwater table has no effect on the thickness of the typical aquifer,but it will increase the position where the ground heat exchanger has invalid heat transfer,leading to the decrease of the optimal buried depth of the ground heat exchanger.The increase of thermal conductivity of backfill material will increase the heat transfer of ground heat exchanger,but it has no effect on the optimal depth of ground heat exchanger.The branch distribution of double U-shaped ground heat exchanger has little influence on the heat transfer and the optimal depth.The heat transfer of three supply and one return ground heat exchanger is greater than both two-supply-one-return and four-supply-one-return ground heat exchanger.These three types of ground heat exchanger do not appear the best buried depth in the simulation range,which proves that several-supply-one-return ground heat exchanger can give full play to the role of groundwater and reduce the thermal interference between branch pipes.The Peclet number is used to judge the influence of groundwater on the ground heat exchanger,and the thickness of aquifer is taken as the characteristic length.The pairwise number relationship between the increase of heat transfer by groundwater to the ground heat exchanger and the Peclet number was obtained.Based on the function relationship between the increase of heat exchange and the Peclet number,the thermal conductivity coefficient in the aquifer is modified,and then a new model suitable for fast calculation of the temperature distribution of rock-soil body around the ground heat exchanger is obtained.Using the new model,the influence of long-term operation of ground source heat pump on soil under different cold/heat load ratios is analyzed.According to the variation of soil temperature,it can be concluded that when the load ratio is 1.2,the soil can offset the temperature change caused by the load imbalance through its own recovery ability.