Model Test And Numerical Simulation Of The Effect Of Karst Groundwater Seepage On The Heat Transfer Performance Of Vertical Ground Heat Exchanger

Ground source heat pump(GSHP)is a kind of energy saving technology which can exchange heat with constant temperature ground by circulating water in buried heat exchanger,Use geothermal energy for cooling in summer and heating in winter.In the Ground source heat pump system composition,Buried pipe heat exchanger as the main component,It is the key to determine the energy efficiency of Ground source heat pump,Groundwater is closely related to the heat transfer efficiency of buried heat exchangers,Therefore,the research on the influence of groundwater seepage on buried heat exchanger is one of the key points of Ground source heat pump technology research.In addition,there are some special soils in China,such as red clay,mainly distributed in Guangxi,And this kind of special soil often has special physical and mechanical properties,In order to better promote the soil-source heat pump technology throughout the country,it is necessary to combine the soil-source heat pump technology with this kind of special soil,Therefore,the research on the heat transfer performance of buried heat exchanger in special soil is one of the key points in the future research of soil-source heat pump technology.In this paper,the local geology and hydrology of Guilin,Guangxi are based on Darcy’s law and heat transfer theory of groundwater flow,A test platform for groundwater seepage and buried pipe heat transfer is developed,The heat transfer efficiency of three-dimensional soil under the coupling action of temperature field and seepage field was discussed by using the circulating water and seepage velocity as variables in the buried heat exchanger.The experimental results show that:(1)The self-developed three-dimensional soil heat-permeability transfer test bed has the advantages of good integrity,high test accuracy,simple operation and so on,It can effectively simulate the heat transfer efficiency of three-dimensional soil in karst area under the coupling action of temperature field and seepage field.(2)Under the condition of saturated soil without seepage,The isotherm of the soil in the saturated region is approximately circular,That is,the temperature in the soil is approximately symmetrical with respect to the heat source,The temperature gradient is greatest near the heat source,the soil temperature decreases as it moves away from the heat source;(3)In case of saturated seepage,The distribution of isotherms in the soil in the seepage zone is no longer circular,It is offset in the direction of seepage,That is,the temperature gradient downstream is greater than the upstream,As the flow velocity increases,The area of the isothermal curve is decreasing.Based on the finite element method of multi-physics simulation software COMSOL Multiphysics,the three-dimensional heat and permeability coupling model of buried pipes was built,The influence of heating seepage velocity in winter and cooling seepage water temperature in summer is simulated and analyzed,The temperature field of three dimensional buried pipe coupling model system running for120 h was simulated and analyzed,The main conclusions are as follows:(1)Simulation of the influence of heat flow velocity in winter,Saturated without seepage,The temperature field in the percolation layer is distributed in a circle with the buried heat exchanger as the center,In the case of seepage velocity,The temperature field is offset in the direction of seepage,The low temperature range near the buried heat exchanger decreases with the increase of seepage velocity,The temperature in the upstream region is obviously higher than that in the downstream region;(2)At the same seepage velocity,Seepage water temperature has a significant effect on the heat transfer of buried heat exchanger,When the water temperature of seepage water is lower than the initial soil temperature,The temperature field near the heat source and in the downstream area of seepage in the soil increases sharply at first and then decreases slowly and finally becomes stable,The temperature field in the upstream far away from the heat source decreases sharply and tends to be stable;When the water temperature of seepage water is higher than the initial soil temperature,The temperature field inside the soil increases sharply first and then tends to be stable;Seepage water temperature influences the heat transfer of buried heat exchanger by changing soil temperature distribution.