The Investigation On Heat And Mass Transfer Of Moisture Soil Intermittent Store And Exothermic Process

Under the intermittent regenerative and exothermic process of the moisture soil, the reduction of heat transmission underground affects not only ground thermal energy, but also the performance and economical efficiency of geo-energy directly. Controlled intermittent technology, which could restore the rock temperature cyclically, improve the heat transfer rate of geotechnical, control the attenuation of heat transfer underground effectively. Ground-coupled heat transfer characteristics of heat and moisture affects the intermittent cycle control directly, thereby affecting the entire geothermal energy system integration control. Therefore, the research on the mass and heat transfer of soil places an important significance to the problem of heat transfer reduction, control heat release process reasonably, ensure abundant diffusion and heat delivery with sound temperature trend and optimize the operation of geothermal energy system. Beginning with working features, this paper analyses and summarizes the relationships between changing regularity and geo-variation, on which basis, underground temperature variation is further studied, in addition, math models about the intermittent regenerative and exothermic process of moisture soil based on the assumption that soil is saturated and unsaturated are built according to Volume average theory, Darcy rule and the heat and moisture transfer theory of unsaturated porous media. The models are used to predict and analyze the variation of the soil temperature and influencing factors of geothermal as a means looking forward to restraining reduction, and provide a theoretical basis for the operation of geothermal energy systems.Firstly, based on the improvement of the existing geothermal energy test system, the intermittent experiment of a vertical Ground Source Heat Pump (GSHP) system is investigated and the corresponding geo-temperature variations are studied. The performance of the GSHP system under intermittent operation and the comparisons of different intermittent modes are presented in the paper. The parameters of soil backfill material, air temperature and inlet volume flow rate, which affect the GHSP performance, are also investigated. Experimental results suggest that, due to the recovery in ground thermal energy in intermittent time, the heat exchange rate and the operation performance coefficient (COP) of the heat pump increases, and the compressor power decreases in the successive working. But an insufficient soil recovery time leads to a rapid decline of the performance parameters and the soil temperature. The temperature transports faster under large soil thermal conductivity conditions and the soil temperature decreases more quickly and recovers more slowly with larger inlet flow rate and lower weather temperature for different soil thermal diffusivities. Through multiple nonlinear regression analysis, a curve formula can be fitted to predict the soil temperature variations under intermittent operation of the ground source heat pump in winter, Dalian. It can be found that the soil temperature increases at an exponential function with recovery time, soil thermal diffusivity, air temperature and soil initial temperature of recovery.Secondly, the moisture soil was considered as a saturated porous media and the corresponding heat and mass transfer within the saturated porous media during the process of intermittent heat releases were researched. a) Considering the percolation in saturated porous media, a three-dimensional unsteady model was proposed to predict the process of intermittent heat and mass transfer within the saturated porous media under the condition of attenuation heat source through amending the heat boundary condition. This model is validated by experimental results. And then, the model was used to predict the heat and moisture transfer between underground vertical U-shaped buried tubes and the surrounding moisture soil, both the temperature changing regularities of moisture soil with geothermal energy as well as the temperature recover characteristics without geothermal energy were obtained. b) Moreover, the effects of thermal conductivity, porosity, backfill material on the soil recovery process were analyzed. Based on the simulation results, it can be found that the influence of properties of moisture soil was important:the soil temperature recovers more quickly when the ground heat conductivity increases, the soil porosity decreases and the backfill material is cement mortar. c) In addition, the effects of solar radiation, air temperature and wind velocity on soil temperature recover behaviors were studied as well. The results show that the influences were less. Considering surface radiation, the heat exchange within sallow soil can be improved during soil heat release but be reduced during soil heat storage, and this phenomenon can become notable with the increasing of solar absorption coefficient. Under the heating mode, the soil temperature recovers more quickly when the air temperature increases and the wind reduces, while the reversed situation can be found in the cooling mode.Finally, the moisture soil was considered as an unsaturated porous media and the corresponding heat and mass transfer were researched as well. a) A heat and mass transfer model was established to describe the transfer between underground U-shaped tubes and surrounding moisture soil during the process of intermittent heat release, where the moisture soil was taken as the unsaturated porous media with water, water steam and air filled within its porosities. In order to predict the water infiltration and water evaporation within soil, the corresponding heat and mass transfer models needed to be built too. b) Under the condition of three-phase thermal-dynamic equilibrium, the water infiltration and water evaporation are ignored, and the heat and mass process of moisture soil are also simulated. The obtained data are compared with the ones using saturated porous media model and it can be said that the results based on the unsaturated porous media model was more close to the experiment data. c) The infiltration is taken into account in the heat and mass transfer model. The influence of the infiltration and its factors:soil moisture rate, soil saturated hydraulic conductivity on the heat and mass transfer under the intermittent regenerative and exothermic process. The results show that, the soil temperature decreases quickly in the process of exothermic and increases slowly in the recovery process when thinking about infiltration, and with the depth of soil, the effect tapers off. When the soil moisture rate and soil saturated hydraulic conductivity become larger, the soil temperature decreases quickly in the process of exothermic and increases slowly in the recovery process. d) Based on taking into account the evaporation, the effect of surface evaporation, inter evaporation and their factors, such as the rate of soil moisture, soil moisture content, dry soil layer thickness, water vapor diffusion coefficient, and so on on the temperature characteristics are also numerical studied. The results show that evaporation of moisture has a greater impact on the rock temperature underground, the soil temperature falls faster and rises more slowly under the intermittent exothermic process. The difference between them is that the impact of soil surface evaporation process decreases with the depth of the soil, but the effect of soil internal evaporation is the same at different depths in soil. When the soil saturated moisture content, wind velocity and water vapor diffusion coefficient increase, the soil temperature falls faster and rises more slowly under the intermittent exothermic process. The opposite results can be obtained while the soil actual moisture content, dry soil layer thickness and air temperature increase.