地热井下换热器传热机理研究

Downhole heat exchanger (DHE) is an apparatus for extracting heat by suspending looped pipes in a geothermal well. It gradually has been used in the geothermal resource utilization in recent years. Since it does not need pumping hot water out of the aquifers, just gaining heat from the well, it can eliminate the problem of surface disposal and no impact on the subsurface environment comparing with ground water heat pump system. The convection promoter pipes together installed in the geothermal well with DHE can serve to enhance the outside heat transfer of DHE, i.e. heat output can be increased. A promoter pipe is simply a pipe that is open at both ends and placed in a well with the looped pipes of downhole heat exchanger. Some foreign scholars have pointed out that the convection promoter pipes can enhance heat exchanger, but the supportive or effective experimental data has not been available. There are few punished domestic research papers about the promoter pipes. The numerical simulations of promoter pipes in DHE are not available. The objective of this paper is for understanding the mechanism of heat transfer enhancement about convection promoter pipes through experiments. In addition, the natural convection in an open-ended square cavity partially filled with porous media is numerically simulated. In the experiment, a DHE simulation system is established in the lab according tothe DHE systems in a practical design, in which we can summarize the role of convective promoter pipes in DHE heat transfer enhancement, and analyze the experimental error. A comparison test is carried out for the DHE systems of with and with out convection promoter pipe installation. The experimental results show that the promoter pipes do improve the heat output or the heat transfer performance. The best length of the pipes is obtained from the three kinds of promoter pipes with different length according to the experimental data. In addition, it is found that the higher temperature difference between the inlet water of DHE and the water in the thermal　aquifer, the higher temperature increment between the inlet and outlet of DHE, or the more heat output. In this case, a larger heat transfer coefficient outside the DHE tube can be obtained. However, the thermal power output or the heat transfer performance does not necessarily increase with the circulation flow rate through the DHE. Therefore, an aquifer with a large geothermal gradient is favorable to DHE　installation, but to increase circulation flowrate through DHE is at a price of　increasing electric power consumption due to pumping. Therefore, a comprehensive consideration is necessary in order that both economy and technology are feasible in a real application.

对有较高渗透率地层内的井下换热器传热机理进行理论分析和实验研究，主要研究渗水 层内多孔介质结构；井下花管、对流增速管、换热器等几何参数；换热器运行参数（流量、 入口温度） 等对地下井管内自然对流、 以及周围渗透层内自然对流强度的影响。 研究井管内、外自然对流的耦合关系，建立较为准确的描述井下换热器传热机理的物理模型和数学解析表达式。在此基础上，提出强化自然对流强度的有效模式，分析采用对流增速管的有效性，结合地面供热模式，提出不同地热热储层条件下井下换热器的优化设计及运行模式。为开发不抽取地下水、环境友好、高效传热的节能供热设备提供理论设计依据。对我国合理开采利用地热资源、提高清洁可再生能源的利用率有重要意义。

为了合理开采利用地热资源、提高清洁可再生能源的利用率，本项目建立了模拟高渗透率地层内的井下换热器传热过程的实验台，对地热井下换热器的传热性能进行了深入研究。主要包括渗透层内多孔介质结构、井下花管、对流增速管、换热器等几何参数以及换热器运行参数（流量、入口温度） 等对地下井管内自然对流、以及周围渗透层内自然对流强度的影响。研究了井管内、外自然对流的耦合关系，建立了较为准确的描述井下换热器传热机理的物理模型和实验关联式。在此基础上，提出了强化自然对流强度的有效模式，分析了采用对流增速管的有效性，并结合地面供热模式，提出不同地热热储层条件下井下换热器的优化设计及运行的主要影响因素。为开发不抽取地下水、环境友好、高效传热的节能供热设备提供理论设计依据。