The Study On Coupling Of Characteristics Of Soil Temperature And Moisture Field Of Air-to-Earth Heat Exchanger

In the greenhouse, the shallow surface layer with earth-to-air heat exchanger system is as heat retainer,playing the role of cold source and heat source. The air is transported to the shallow surface and exchange heat, derived by the temperature difference between the soil and air. Then the air is transported to greenhouse,to lower indoor air temperature at daytime or sunny days and increase air temperature at night or cloudy day. By this way, we can save energy in daytime or sunny and use it in night or cloudy day. Until now, most scholars regard the heat exchange as heat conduction process, when establishing heat transfer models, and ignore the effects of moisture migration process. It results that the earth-to-air heat exchanger designing can not match the actual operation, and investment of the earth-to-air heat exchanger is increasing obviously. Eventually, the popularization and applications are limited. This paper studies the heat and moisture transfer rule of soil around the earth-to-air heat exchanger. In this paper, the coupling of soil temperature and moisture for the earth-to-air heat exchanger has been studied furtherly, based on the original line heat source model of buried heat exchanger tube. The three dimensional mathematical model of heat and moisture couping of soil around the earth-to-air heat exchanger is established to study heat and moisture transfer rule of soil, making the model much more scientific.Firstly, this paper introduces the soil basic feature and thermal parameters and analyzes thermal parameters and moisture content of the soil, and illustrates the mechanism of heat and moisture transfer in porous medium and the model of coupled heat and moisture transfer. Three dimensional mathematical model and physical of coupling temperature and moisture of soil are established in this paper, based on the conservation of energy and mass.For the mathematical model, equations are discreted by control volume method, and soil coupled heat and moisture iterative solution software is compiled with Visual Basic 6.0 language. The initial conditions of the simulation of soil temperature and soil moisture content is as following: the initial soil temperature is 14.1℃, the initial soil volumetric moisture content is 0.35m3/m3, unit pipe heat is 40W/m, earth-to-air heat exchanger is running 8h for heat rejection. The results of numerical calculation show that, in the length direction, soil temperature decreases along the tube more and more slowly, and tends to be stable in the end. And the soil volumetric moisture content increases along the tube and reaches peak at 5.5m of tube, and then gradually lowering to the stable value. In the radial direction, soil temperature and soil volumetric moisture content decrease at the direction of radius, and tends to be stable in the end. But soil volumetric moisture content is on the opposite. With the earth-to-air heat exchanger running, soil moisture diffusion is continuously moving from inner to outer, and soil temperature increases, with soil volumetric moisture content decreasing.Then this paper explores four initial calculation parameters effecting on the soil temperature and moisture around the earth-to-air heat exchanger, including the soil thermal parameters, the initial soil temperature, the initial soil volumetric moisture content and unit pipe heat. The results indict that: the thermal conductivity of sand is larger than clay, while the thermal capacity is smaller than clays and the heat spread in the sand much fastly than clay. Besides, the thermal power of sand is stronger than clay and water holding capacity is poorer than clay. The lower initial soil temperature is, the higher temperature gradient of soil temperature field is and heat transfer is faster. Derived by the temperature gradient, soil moisture migration is more intensely. The initial soil volumetric moisture content and the heat capacity of the soil increase, while thermal expansion coefficient decreases, which results soil temperature spreads slowly. However, the stronger the moisture transfer power is, the intensely soil moisture migration is. It can increase the heat of unit lenth of pipe. Eventually, with the soil temperature gradient increasing and soil temperature spreading fastly, the temperature difference is larger and soil moisture migration is more intensely.Finally, these preliminary research work are summaried, and some expectations and suggestions of experimental investigations are proposed to further study of this research group.