Application And Research On Portable Device For In-Situ Testing Strata Thermophysical Properties

With the rapid development of world economy and rapid population growth, social demand for energy is also growing faster, while the earth traditional fossil fuel energy reserves are limited and non-renewable, the fossil fuel energy will soon exhausted.. In recent years, The whole world is committed to low-power, non-polluting and renewable energy projects of new technology research and its application. Ground-coupled heat pump systems are quickly attracted by the advent of the eyes of the world, due to their characteristics in saving energy and no pollution to the environment, and they have become a hot research topic.In the ground-source heat pump system heat exchanger length of the design process, selection of soil thermal parameters directly determine the accuracy and economic effect of ground source heat pump system, it is the most important part in the design process. Based on the research of the domestic and abroad application of thermal properties of the soil testing system, the thesis developes a portable thermal properties in situ testing equipment of strata thermophysical properties. The equipment is simple in structure, convenient in transportation, it can accurate testing strata thermophysical properties. At the same time, the equipment can adapt to different input power's needs of the constant heat flux test. In particular, the design of the structure can be split to make transportation more convenient, so the equipment will not be affected by the site, and it can adapt to a variety of adverse working conditions. The testing equipment using line source model or column source model, combined with matlab software to calculation of the average rock and soil around the borehole thermal parameters. Contrast the data fitting out by line source model and column source model, the results show that:The value of specific heat capacity of the soil has a greater influence of column source model, so in the areas where The value of specific heat capacity of the soil is unknown, test results should adopt the line source mode to calculate if the pipe diameter is small enough, and test time is long enough.The thesis analyzes the influence of strata thermophysical properties by test time, test power and test flow, the results show that:test accuracy is increased with test time. For the project which has a less demanding of test accuracy(10%),50h can meet the needs of the test accuracy; for the project which has a higher demanding of test accuracy(5%), text time should be 80h to meet the needs of the test accuracy; when test time is more than 80h, the value of specific heat capacity of the soil has a small influence with test time, more than 80h's test time is not recommend in the actual engineering tests. The text also shows that the average thermal conductivity formation is increased with test power, drilling thermal resistance is decreased with test power. When test power is unchanged, the average thermal conductivity formation and drilling thermal resistance have a small influence with test time, they are almost unchanged. So text power and test time should be selected according to testing standards and practical needs in the actual engineering tests to make the design of heat exchanger more reasonable. The thesis also established two-dimensional vertical heat exchanger numerical heat transfer model by FLUENT software to analyzes the influence of vertical heat exchanger heat transfer by mass flow rate of fluid in the pipe, inlet temperature of fluid in the pipe, the value of the temperature difference between inlet temperature of fluid in the pipe and the original temperature of the soil around the ground, thermal conductivity of the soil around the ground, thermal conductivity of backfill materials, thermal conductivity of U-shaped tube, the results show that:thermal conductivity of the soil around the ground and thermal conductivity of backfill materials have a greater influence of vertical heat exchanger heat transfer. Usually, thermal conductivity of the soil around the ground is unchanged. So we should pay more attention to control the quality of backfill in the actual engineering tests to get better heat transfer effect. The thermal conductivity of U-tube U-shaped tube has little effect on vertical heat exchanger heat transfer, actual project site is not necessary to use too expensive pipe which has a high thermal conductivity to improve heat transfer.