Study On Heat And Moisture Transfer In Soils Around Buried Tube During Heat Absorption And Release Processes

The ground source heat pump system,as the main method for green and efficient geothermal energy development,has developed rapidly in recent years,the problem of soil heat imbalance has become increasingly prominent,due to the unbalanced building cooling and heating load exceeding the heat recovery capacity of the soil around ground heat exchanger.In addition to the forced heat and moisture recovery of the soil around buried tube,by external intervention means such as the structure and operation strategy of the ground source heat pump system,the heat and mass transfer in the soil,as a randomly distributed porous medium,should also be paid attention to.Such as the differences in heat and moisture migration in the soil around the buried pipe during heat absorption and release processes under different temperature and humidity gradients,the soil temperature distribution with time and space in the upstream and downstream groundwater seepage area,and the development rules of soil thermal disturbance radius.The objective of this work is to explore the influence of soil heat and moisture migration direction on the heat transfer and moisture migration during heat absorption and release processes,as well as the characteristics of soil heat transfer under groundwater seepage condition.It is expected to provide guidance for the long-term and efficient operation of ground source heat pump system and the optimal layout of buried tubes in groundwater seepage area,and provide a reference for the development and utilization of shallow geothermal energy.Based on the theory analysis for soil heat and moisture transfer,due to the complex randomness of soil internal structure,it is difficult to accurately describe the heat and moisture transfer affected by soil internal structure during heat absorption and release processes,therefore,this paper adopts the experimental research method to observe and discuss the complex heat and moisture migration law during soil heat absorption and release processes in the non-seepage area.For the soil in saturated seepage zone,since its internal pores are filled with liquid water,and there is no microscopic moisture migration and phase transition process,numerical simulation method is used to analyze the heat transfer characteristics of the soil in saturated seepage zone.An experimental setup for soil heat and moisture migration was established to carry out experimental research on heat and moisture migration in the soil around heat exchange tube under non-seepage condition,during the process of soil heat absorption and release.Firstly,for the soil with regular structure,the temperature distribution with time and the heat and moisture migration during the process of heat absorption and release driven by the temperature and humidity gradient were studied,the effect of the heat and moisture migration direction on heat transfer and moisture migration during the process of heat absorption and release was explored,the influence rules of temperature and humidity driving potential on the soil heat and moisture migration were analyzed,and through the regression analysis of the experimental data,an empirical formula suitable for predicting the temperature distribution in the soil with regular structure was established.The research shows that,for the regular structure soil,the temperature variation during heat absorption and release processes is symmetrical,and the influence of the heat and moisture migration direction on the heat transfer could be ignored.The soil temperature variation has a linear relationship with the excess temperature,has no obvious correlation with the initial moisture content,has a good logarithmic relationship with heat transfer time,radial distance and longitudinal distance,and the temperature disturbance in radial direction is much greater than that in longitudinal direction.Affected by the inherent structural characteristics of soil,the moisture migration only shows a nonlinear upward trend with the increase of excess temperature and initial moisture content,and the occurrence of soil moisture migration is determined by both excess temperature and initial moisture content.Secondly,the experimental study on heat and moisture migration during the process of heat absorption and release was carried out for the soil with irregular structure,focusing on the influence of the main structural parameters of the soil on heat and moisture migration during the process of heat absorption and release,and exploring the differences in temperature distribution for the soil with irregular structure during the process of heat absorption and release,caused by the heat and moisture migration direction,and the temperature and humidity conditions on which it is based.The research shows that,temperature variation of the soil with irregular structure also increases linearly with excess temperature,and has no obvious correlation with the initial moisture content,and the moisture migration also shows a nonlinear upward trend with the excess temperature and initial moisture content.However,the heat and moisture migration direction would have a obvious impact on heat transfer under a higher temperature gradient,and the temperature distribution deviation and moisture migration difference during the process of heat absorption and release both increase with the porosity,the maximum temperature distribution deviation reaches 15.7%,the maximum moisture migration difference reaches 31.7%,and the soil temperature variation and moisture migration during heat absorption process are all greater than those during heat release process.Soil moisture migration increases with the porosity,and the effect of porosity parameter on moisture migration during heat absorption process is greater than that during heat release process.The initial moisture content is another factor that determines whether there is an obvious temperature distribution deviation during the process of soil heat absorption and release,and the higher the soil porosity,the lower the initial moisture content required for that the soil temperature distribution during heat absorption and release processes shows obvious deviation.Finally,for the soil around the buried pipe in saturated seepage zone,a three-dimensional finite element numerical model for fluid-solid coupling heat transfer was established,based on the governing equation for heat transfer and fluid flow in saturated porous media.Analyzing the influence of soil permeability coefficient and excess temperature on the upstream and downstream soil temperature distribution,exploring the development rules of the soil thermal disturbance radius in upstream and downstream seepage direction and perpendicular to the seepage direction,and analyzing the fitting relationship between the soil temperature variation,the thermal disturbance radius and the main influencing factors.Research shows that,groundwater seepage can promote the stability of soil temperature distribution,soil permeability coefficient is the main internal factor determines the soil seepage velocity and the distribution shape of the temperature isoline extending along the seepage direction,but can't change the final value of soil temperature,excess temperature is the decisive factor for soil temperature change,and there is also a linear relationship between soil temperature change and excess temperature under seepage condition.The temperature peak of the upstream and downstream soil will appear under higher seepage velocity,and increases with the excess temperature,its occurrence time lags with the increase of excess temperature or distance from the heat exchange tube wall.The downstream thermal influencing radius has a linear relationship with water head driving potential and a logarithmic relationship with excess temperature,the thermal influencing radius perpendicular to the seepage direction has a logarithmic relationship with excess temperature,but it hardly changes significantly with water head driving potential.The upstream thermal influencing radius has a logarithmic relationship with excess temperature under the condition with no seepage,while first decreases and then tends to be flat with the increase of water head driving potential under the condition with seepage,and hardly changes with excess temperature.The upstream thermal influencing radius has a second-order nonlinear function relationship with water head driving potential and excess temperature under seepage condition,after reducing the soil permeability coefficient.