The Research Of The Heat Transfer Characteristics Of Ground Heat Exchanger Based On The Compaction Drilling

The research of Ground Source Heat Pump (GSHP) mainly concentrate on three points:building heat transfer models of Ground Heat Exchanger (GHE), strengthening the heat transfer mechanism and the coupling of GSHP and GHE. These are all concerned with the GHE, which obviously makes it a instrumental problem in study of GSHP.In the establishment of the heat transfer model to research GHE, the heat transfer zone is usually divided into two parts depends on the borehole wall. One is rock-soil region located on the outside of the borehole wall, while the other one is heat exchanger located on its inside region. The effect of the borehole wall is just a geometric boundary, which is treated as the inner wall of the cylinder.The underground space needed for ground heat exchanger is obtained normally by the drilling methods. The borehole wall is regarded as a irregular critical surface of the underground space, which is formed when the stratum is under external force. The boundary conditions of the borehole wall are the status of the stratum density changes and the radial dimension changes of the underground space critical surface. And these statuses directly has an influence on the variation of the stratum thermal physical properties.The effects of the size and density of the borehole wall on the stratum heat transfer quality are studied in this paper, based on the drilling mechanics. The change of borehole wall area boundary conditions is accomplished by compacting drilling. Its scientific significance is that the new type of the heat transfer partition model was obtained. That is, the heat transfer regions are divided into three parts including rock-soil zone, compact zone and heat transfer zone of the buried pipe. The research object of the heat transfer is the compact zone.The theoretical basis of the extruding drilling is that the soil has the properties of compaction and compression. The typical soil samples were chosen in this project, and then the relationships between the heat conductivity coefficient of the soil and its compaction and compression properties are test in the lab. The results show that the heat conductivity coefficient increases with the increase of the compression stresses and the soil densities, while it decreases with increasing of the soil void ratio.The typical sand and clay were selected to test in the drilling experimental platform. The results show that as the depth increase the radial stress gets more and more big, and is associated with the structure of the bit.According to the simulation of the drilling results,5 kinds drill bits of in different diameters and structures are selected, respectively by conventional rotary, compact drilling and secondary compact drilling, to drill 5 wells for experiment. Then, five parameters of geophysical well logging were made excluding borehole diameter, sonic, natural gamma ray, density and resistivity. Realized how the density and the diameter changes with the depth increase under different external force. Results show that the borehole diameter changes better under the compact drilling. Under the condition of secondary compact drilling, the density of borehole wall in plastic formation increases stability, and is in a state of thermal resistance reducing, which is conducive to strengthening heat transfer of ground heat exchanger.Using Fluent software simulated the thermal efficiency of heat exchanger before and after changing the borehole wall boundary conditions.It is concluded that the heat exchanger efficiency is improved by about 21% after compaction and the thermal interference performed well. With increase of borehole diameter, the heat transfer rate decreased and the thermal interference aggravated and with the increase of the thermal conductivity, the heat transfer rate raised and the thermal interference relieved. The improvement of the heat transfer effficiency has a great influence in design and research of the GSHP.