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.

构建并测试了长48 m、内径40.3 mm的超长两相闭式热虹吸管(TPCT)，研究了基于地热能的大型TPCT井筒加热的传热极限和热特性。实测超长热虹吸管的传热极限约为20 kW，在沿热虹吸管的温度梯度小于1.5 °C/100 m的情况下，可实现15 kW的传热率。在TPCT达到其传热极限之前，TPCT的温度和传热率随着加热功率的增加而增加。然而，进一步增加加热功率不会影响TPCT进入热阻态后的温度和传热率。同时，沿 TPCT 的温度梯度随着 TPCT 的传热速率逐渐增大，并在传热极限处达到最大值。蒸发器保持在高温下，冷却条件主要影响TPCT的温度，而传热率几乎不变。TPCT 温度和传热率的动态响应随着加热功率的增加和减少而显示出滞后现象，这在高填充率下表现为由于回流液膜的夹带而导致蒸发器中工作流体存量的消耗。得出传热极限是由溢流引起的，导致蒸发器间歇性干涸和热虹吸管振荡运行状态。