Study On Heat Transfer Mechanism Of Vertical Buried Tube Heat Exchanger Under Conditions Of Unsaturated/saturated Layered Soil

Geothermal energy is a kind of renewable energy with huge reserves.At present,extraction by ground source heat pump is its main utilization method,and the key to improving the extraction efficiency of ground source heat pump lies in the perfect design of buried tube heat exchanger.Therefore,the heat transfer performance between the buried tube heat exchanger and the surrounding soil has become the main research object.In the current research,most of the buried tube heat exchanger models are based on pure heat conduction theory,some consider the effect of seepage on heat conduction,and some consider the effect of soil layering on heat conduction.In practical situations,most of the vertically buried tube heat exchangers cross saturated soil and unsaturated soil at the same time.The former is heat-seepage coupled heat transfer,and the latter is heat-moisture coupled heat transfer.If it is not considered separately,it will have a greater impact on the design of the underground heat exchanger and increase its initial investment.But in the existing research,there is no research on such problems.Therefore,it is necessary to analyze the heat transfer characteristics of the vertically buried tube heat exchanger and establish a heat transfer model that simultaneously crosses over-saturated/saturated soils for the heat transfer evaluation of the buried tube heat exchanger.This article focuses on experimental research,supplemented by theoretical analysis.Firstly,an experimental platform was set up,and 18 different working conditions were operated.During the operation,the temperature and humidity parameters of the soil and the temperature parameters of the working medium were monitored.With the help of experimental data,the characteristics of soil temperature and humidity changes in the saturated soil layer without seepage or seepage and the factors affecting the heat exchange efficiency of the U-shaped buried tube heat exchanger were analyzed under the conditions of unsaturated/saturated layered soil.The main conclusions obtained in this study are as follows:1.When there is no seepage in the saturated soil layer,the heat spreads evenly in the soil around the U-shaped pipe,and when there is seepage in the saturated soil layer,the seepage effect on The heat conduction strengthening strength in the seepage direction exceeds 6.5%,which has little effect on the radial heat transfer of the adjacent non-seepage layer;Second,the seepage effect has an enhanced effect on the heat transfer capacity of the buried pipe section in the seepage layer,and the strength is first It gradually becomes smaller and then tends to be stable.The heat transfer capacity of the buried pipe section in the adjacent unsaturated soil layer is first suppressed and then strengthened.3.The overall heat transfer capacity of the U-shaped buried pipe gradually increases with the running time.The heat transfer capacity of each pipe segment does not decrease gradually.The heat transfer capacity of the pipe segments in the adjacent unsaturated soil layer shows a change state that first increases,then stabilizes and then decreases.In addition,in this paper,the heat and moisture coupled heat transfer model in the unsaturated soil layer,the heat and penetration coupled heat transfer model in the saturated soil layer and the heat transfer model in the buried pipe are established;a simulation study is carried out on the established experimental conditions,The data monitored in the experimental conditions and the results of the simulation calculations imported into the mathematical model were compared and verified in terms of temperature change and humidity change.Finally,the mathematical model built has good reliability.