Research On Heat And Moisture Migration Of Red Clay And Its Influence On Heat Transfer Performance Of Ground Source Heat Pump System In Karst Region

The red clay is a kind of problematic soil, which is extensively distributed in Guangxi. As it is of unique mineral compositions and micro-pore structures, the thermohphysical properties of red clay are more complex than that of the ordinary clays. At the same time, the seepage of ground-water in Karst has significant influence on the heat transfer process of red clay. Therefore, deep research on the heat and moisture migration of red clay and its influence on heat transfer performance of ground source heat pump(GSHP) system has important practical significance to effective use of geothermal energy in Karst region. Focusing on the core problem about heat and moisture migration of red clay in Karst region, this paper is emphasized on three key scientific issues. They are the thermophysical characteristics of red clay, heat and moisture migration of soil in heat transfer process, influence of geological structure and meteorological fluctuation on heat transfer. With laboratory test on thermophysical characteristics, model test of one dimensional heat and moiture migration and in-situ monitoring test of ground source heat pump, the paper is comprehensively revealed the heat and moisture migration of red clay in Karst region. On the basis, numerical analysis on heat transfer performace of ground heat exchanger is presented. To be specific, the main work is as following:1. Through a comprehensive analysis on characteristics of Karst landform and formation and distribution of red clay in Karst region, this paper put emphasis on typical red clays in 4 sites of Guangxi, including Guilin, Liuzhou, Laibin and Wuming. Then, some means such as field sampling, profile feature description, laboratory soil test and microcosmic-test are carried out to have a deep analysis on geological genesis and mineral composition of red clays. Furthermore, a new quantitative analysis method, which can accurately calculate the mineral composition of red clay in Guangxi, is put forward. And the mineral compositions and contents of 4 red clays are determined finally with the method, which can provide an evidence for revealing the special thermophysical properties of red clay and establishing reasonable prediction models of thermal conductivity.2. Combined with soil geological genesis, clay mineral composition and composite medium theory, the thermophysical characteristics test of the above four kinds red clay is carried out to get the change law of the thermophysical characteristis of red clays. The result shows that the thermal conductivity andvolumetric specific heat capacity both increase with the increase of the volumetric moisture content when keeping the dry density as a constant, and there is a critical point on the variation curve; when the moisture content is smaller than the critical value, the thermal conductivity increases fast; when the moisture content is larger than the critical value, the thermal conductivity increases slowly. Further study shows that critical point is corresponding to the optimum moisture content in compaction test and it is different from the general clay due to the special geological genesis of red clay. Finally, the composite medium theory is used to simulate the relationship between the thermal conductivity of unsaturated red clay and the volume fractions of soil tri-phase, and the reasonable models under different soil-water interaction mechanism are determined.3. The theory about heat and moisture migration mechanism is systematically analyzed, and the theoretical model of one dimensional heat and moiture migration is established by improving the water flux equation. Then, the test-bed is built successfully to research on the one dimensional heat and moiture migration in a soil column; with model test and numerical simulation, the research on heat and moisture migration effect of soil under the common influece of temperature gradient and humidity gradient is made. The reseach result effectively verifies the interaction of mositure migration and heat transfer in soil, and it is further proved that the established theoretical model is correct and reasonable.4. Based on practical project, the in-situ observation station of rock and soil mass is established to real-time monitor the operation conditions of GSHP system and the temperaure variation, moisture migration of soils during heat transfer process. Then, through GSHP running test udner different working conditions, this paper reveals the heat and moisture migration effect of soil during the running process of GSHP system and its influnce law on the operation characteristic of GSHP system. The result shows that:Firstly, interval running modes of heat pump unit is beneficial to the restoration of the soil temperature field, and the performance coefficent (COP) of unit increases with the ration between running time and stopping time decreasing. Secondly, the soil temperature is affected by two factors such as the heat exchanger of buried pipes and atmosphere changes, but the influence range and degree of the two factors are different. Thirdly, the change range of soil temperature is decreased with the distance from the buried pipe increasing, and the heat effect radius of vertical buried pipe is around 2.0m, while the heat effect radius of horizontal buried pipe is around 1.0m. Fourth, there are significant effects for the infiltration of surface water and the change of underground water level due to rainfall on soil moisture field, but the heat exchange of buried pipes has little influence.5. According to the theory about energy balance and mass conservation, the numerical models of heat exchanger considering the effect of heat and moisture migration is established. On this basis, the heat exchange process of soil layers of Karst region in Guilin are numerically simulated to analyze the influence of meteorological fluctuation and seepage of ground-water on heat transfer effect of buried pipes, and obtain the heat exchanger laws of soils with the infuence of seasonal variation of grongd-water in Karst region. The simulation results show that:(l)As the depth of horizontal buried pipes is 1.0-2.0m from the soil surface, the heat exchang effect is significantly affected by the meteorological fluctuation and the atmospheric temperature, solar radiation, evaporation are the main influence factors. Based on measured meteorological datas, it is found with numerical simulation that the heat exchange effect can inprove about 5.6% by the shallow layer seepage due to rainfall for red clay. By analysis of heat exchange effect of pipes with different buried depth and spacing, it is found the optium arrangement mode of horizontal buried pipes in Karst region, and the buride depth is 2.5m and the spacing is 2.0m. (2) For the typical Karst layers in Guilin, the heat exchange effect of soils with the seepage is improved 2.69～3.69 times than that of soils without seepage when the seepage velocity changes from 0.01m/d～1.0m/d. Then, according to the influence characteristic of ground-water seepage on heat exchange effect of buried pipes, this paper puts forward the reasonable arrangement mode of vertical buried pipes and it can provide theoretical reference for the engineering deaign.