Simulation Of Heat Transfer And Development Of In-situ Testing Equipment Of Rock And Soil Thermophysical Properties

Ground-source heat pump technology is an effective way to develop and use low-temperature geothermal resources, in Europe, the United States, Japan and other developed countries. It has long been widely promoted, and has been more and more widely used in domestic fields in recent years. When we design the heat exchange of ground-source heat pump systems or underground power storage, the accuracy of thermal properties parameters of sock and soil is very important. At present, the majority of ground-source heat pump system projects mainly depend on estimating the capacity of heat exchanger pipe and testing the thermal properties parameters of the actual stratum is very rare. This has probably not only led to a smaller scale of system designed, which can not meet the requirement of air-conditioning load, but are also likely to result in a larger scale of system designed, which causes the original investment increased. Therefore, it loses the comparative advantages in use with other energies.To solve this problem, firstly, this thesis introduces the testing method of thermal physical properties of sock and soil and their respective characteristics, and focuses on advantages and working principles of thermal response testing. Subsequently, the thesis describes the production, development and application situation of field testing equipment at home and abroad in detail, as well as the differences between these devices. In addition, the thesis also makes a detailed account of the existing analytic method heat transfer model and numerical method heat transfer model.This thesis develops a field testing equipment of thermal physical properties of sock and soil, using heat pump as cold and heat source to test the thermal properties under different conditions without additional heaters, in which structure is simple and reasonable. We use electric three shunt valve to assist heat pump, ensuring a constant heat imputing and extracting from the ground in order to facilitate use of existing heat transfer model. Devices in the equipment include the circulatory system of fluid path, data acquisition and controlling systems, as well as data processing system. Data acquisition system employs high-precision Pt1000 platinum resistance, electromagnetic flow meters and power sensors to measure temperature, flow of fluid and power of equipment; controlling system can achieve a variety of manual, automatic controls, as well as alarm at abnormal state of function flow, pressure and temperature etc.; In human-computer interaction system, a variety of experimental procedures and conditions can be set, data and associated curve can be fully observed in intuitive interface, in order to adjust timely to experiments and make corresponding decisions. Data-processing system of testing equipment uses the international popular on-line source model and column source model to calculate thermal properties parameters, in which Matlab is used to write procedure for calculation, combined with parameter estimation to solve stratum average thermal conductivity and borehole thermal resistance.By using the testing equipment, a number of heat exchange wells testing laboratories under storing and extracting energy condition have been carried out in Changchun, Beijing and Tianjin and other places. Much of the testing data is acquired. This thesis makes use of line source model, column model and numerical model for calculating the data in energy storage condition, and a conclusion is drawn that the line source model can meet the requirements of calculation accuracy in condition of enough testing time and sufficient borehole depth. The thesis uses variable heat flux model to calculate the data of extracting energy condition, viewing that, to some test of large heat flux, the accuracy of the calculation is higher when we use variable heat flux model. However, due to its large workload of calculation, the conditions of constant heat flow calculation should be met in the experiment. The thesis also verifies the regulating ability of the electrical shunt valve can completely meet accuracy of temperature difference control (±0.1℃) requirements.In addition, the thesis also analyzes the effects of the specific heat of rock and soil, heat flux density, the fitting time on thermal conductivity. Analysis results show that the heat flux density, the initial ground temperature and the radius of borehole are the main factors to impact its calculation. The impact of rock and soil specific heat is very small, which almost can be ignored. This thesis takes calculation results of line source model as an example, revealing that the heat flux increased by 10% in the case of thermal conductivity can increase by 10%, 16% reduction in thermal resistance, borehole depth can be reduced by 4.9% at most; the initial ground temperature is increased by 10%, the borehole thermal resistance lowers up to 15.7%, reducing the borehole depth up to 2.8%; borehole radius increased by 10%, the resistance increased by 9.2% at most, with incensement in borehole depth up to 4.1%. Thus, in the design of ground source heat pump system, it is necessary to choose a backfill materials of good thermal conductivity properties, in order to enhance heat transfer with surrounding soil; During construction, size of drilling hole must be controlled as smaller as possible to reduce the borehole thermal resistance; At the transitional season or in summer in the cold regions of the north, solar energy and other means can be applied to make up for the ground temperature to increase the initial ground temperature. Thereby depth of borehole can be reduced to save the project investment.Study of fitting time has shown that the stabilization time of heat transfer between pipes and soil around is different, therefore, when we process data, it is necessary to pay attention to such changes, and choose the start time and time length in fitting according to the actual test data. According to the testing results, this thesis suggests that about 15 to 20 hours of data be discarded, the fitting time should not be less than 60 hours, and the testing time should be no less than three days.According to the heat load condition of a ground-source heat pump systems engineering, the thesis uses test results of the field testing equipment of thermal physical properties to re-design the system, the original 500 m deep borehole increased by 200 m, which shows the drawbacks of the design estimating value for the heat of per extend meter. As a result, the importance of testing equipment turns out.The thesis also applies ANSYS to model soil temperature field simulation in the process of testing, and makes some comparison and contrast of heat exchange condition of pipe and sounding soil between different types, different working conditions and different heat flux, the radius of thermal effect is determined. This analysis method is also applicable to simulate temperature changes in long-term operating mode of heat pump system. Finally, aiming at experimental problems arising in the course, the thesis puts forward suggestions for the improvement of the equipment and ensures the development direction of the testing equipment. The use of estimate value of heat for per extend meter to design the geothermal system is not very scientific, it tends to leave potential problems to the system, and even causes project failure. Therefore, it is necessary to strengthen the specification management of ground-source heat pump system design and reinforce the popularization and application of the filed testing equipment. The field testing equipment of thermal physical properties of sock and soil can easily determine the average thermal conductivity of rock and soil and borehole thermal resistance, the parameters we got is more in line with engineering practice, which can provide the basis for accurate design of ground source heat pump system and have great economic benefits to promotion. In addition, we also use mobile testing equipment for nation-wide soil thermal physical properties parameters testing, and then, establish a database of geothermal utilization, which facilitates the rational use of geothermal resources in China to reduce energy consumption as well as CO2 emission. Consequently, it will bring about great social benefits, and thus has broad application prospects.