Experimental investigation of superlong two-phase closed thermosyphons for geothermal utilization

Abstract

A superlong two-phase closed thermosyphon (TPCT) with 48 m in length and 40.3 mm in inside diameter is constructed and tested to examine the heat transfer limit and thermal characteristics of large scale TPCT for wellbore heating based on geothermal energy. The heat transfer limit of the superlong thermosyphon is measured to be about 20 kW, and a heat transfer rate of 15 kW could be achieved with the temperature gradient along the thermosyphon less than 1.5 °C/100 m. Before the TPCT reaches its heat transfer limit, the temperature and heat transfer rate of the TPCT increase with the increase of heating power. However, further increasing heating power will not influence the temperature nor the heat transfer rate of TPCT after entering the thermally blocked state. Meanwhile, the temperature gradient along the TPCT gradually increases with the heat transfer rate of TPCT and gets its maximum value at the heat transfer limit. With the evaporator maintained at a high temperature, the cooling conditions mainly affect the temperature of TPCT, while the heat transfer rate almost unchanged. The dynamic responses of TPCT temperatures and heat transfer rates with increasing and decreasing heating power show hysteresis at high filling ratios, which is interpreted by the depletion of working fluid inventory in the evaporator due to the entrainment of returning liquid film. It is concluded that the heat transfer limit is induced by flooding, which leads to intermittent dryout of the evaporator and oscillation operation state of the thermosyphon. The present experimental results provided valuable guidelines for the design and development of large scale TPCT, which can be made from coiled tubing and employed in wellbore heating based on geothermal energy.