Research Of Enhancing Heat Transfer In Downhole Heat Exchanger By Convective Promoter Pipe

Downhole heat exchanger (DHE) is an apparatus for extracting heat by suspending looped pipes in a geo thermal well. It gradually ha s been u sed in the geothermal resource utilization in rece nt 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 d isposal 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 t he promoter pipes. The numerical simulations of promoter pipes in DHE are not available. The objective of this paper is for understanding the mechanism o f heat transfer enhancement about convection promoter pipes through experiments. In addition, the natural convection in an o pen-ended square cavity part ially filled with po rous media is numerically simulated.In the experiment, a DHE simulation system is established in the lab according to the 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 without 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 obta ined from the three kinds of promoter pipes with different length according to the experimental d ata. In addition, it is found that the higher temperature difference between the inlet water of DHE and the wat er 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 t he circulation flow rate through t he DHE. Therefore, an aq uifer 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 t o pumping. Therefore, a comprehensive consideration is necessary in order that both economy and technology are feasible in a real application.As a theoretical investigation for the DHE in a open perorated well in an aquifer, this article establishes a model of an open-ended square cavity partially filled w ith porous media. This model is solved by using LBM and the treatment of interface and open b oundary condition whi ch is introduced in deta il. The co mputation code is validated in both mesh number dependence and accurac y by comparing the results with those of previ ous work in porous media. The relationship between flow field, temperature field, local Nu distribution on the left wall of the cavity and Ra,Da,φ,the width ratio of pure fluid region and porous region has been obtained and analyzed. A correlation of the averaged Nu on the left wall of the cavity with Ra,Da,φis proposed. Some heat transfer characteristics inside the aquifer are discussed in detail.Finally, based on the current experimental and theoretical inadequacies, recommendations for improvement are proposed and the follow-up study of t he subject project is suggested.