In the conventional downhole heat transfer mode, using the two-phase closed thermosyphon (TPCT) can greatly improve the heat extraction capability from geothermal reservoirs. In this paper, a set of laboratory-scale performance testing device is built for TPCTs with internal tubes and charged with CO2-nanofluids mixed working fluid. Based on the conditions of geothermal reservoir and heating season, experimental investigations are carried out by various heating temperature (50~70 ℃), heating water flow rate (1.0~3.0m3 /h), cooling temperature (5~15 ℃) and cooling water flow rate (1.0~3.0m3 /h). The results show that the increase of heating temperature plays a dominant part in heat transfer performance, but not always be positively correlated due to the increase in temperature difference between the evaporator section and the condenser section. From the perspective of their own structure, outer fins have the most obvious improvement on their performance and another method of reducing the aspect ratio takes second place.
Keywords geothermal energy, two-phase closed thermosyphon (TPCT), CO2-nanofluids, heat transfer performance