2004 Volume 26 Issue 2 Pages 107-120
Several types of downhole heat exchangers, for example thermosyphon type heat pipe, concentric tube thermosyphon, downhole coaxial heat exchanger (DCHE), U-tube downhole heat exchanger, as well as others, have been proposed in order to extract heat directly from shallow geothermal resources. Among these downhole heat exchangers, U-tube downhole heat exchanger is widely used not only in Japan but also around the world, because of its cost performance and easy construction. A numerical simulation has been performed to investigate the heat extraction characteristics of a single U-tube downhole heat exchanger. First, the present computer simulation program for the heat extraction by a single U-tube downhole heat exchanger has been made and the computed results by this program were checked by both laboratory experiments and previous field research. After inspecting the effectiveness of the present computer program, a numerical simulation has been conducted for the real scale model with this program. The results showed that increasing both the flow rate of the working fluid of heat extraction and the diameter of the U-tube downhole heat exchanger increases the heat extraction rate under the prescribed numerical conditions in this study. The effect of the tube-material on the heat extraction rate was also shown, and the results suggest that the use of a resin tube, such as polyethylene, is a possible alternative, even though resin is inferior to a metal in heat transfer performance.
