Investigation on Heat Transfer Characteristics in Super-long Gravity-assisted Heat Pipe for Deep Geothermal Energy Exploitation

Abstract

The exploitation of deep geothermal energy has created a great demand for super-long distance heat transportation technology. Heat pipe is capable of transferring heat very effectively from its high to the low temperature end relying on the working fluid phase transitions. We proposed using super-long gravity-assisted heat pipe to exploit earth-deep geothermal energy. The heat pipe that can be used for earth-deep geothermal energy exploitation differs from its normal size (less than 10 m long) counterparts that have been widely used in various industries due mainly to the extremely large length-to-diameter ratio. This geometric characteristic may significantly affect the multiphase flow and heat transfer inside the heat pipe. In this work, a systematic experimental study on the multiphase flow regimes and the performance of an extra-long heat pipe was conducted. The experimental heat pipe has a length-to-inner diameter ratio (L/D=40 m/7 mm =5714), which mimics the major geometric characteristic of real super-long geothermal heat pipes. Particular focus of the present work was placed on correlating the heat transfer performance of the heat pipe with the multiphase flow regimes inside. The thermal performance of the experimental heat pipe was found to be significantly affected by the depth of liquid pool accumulated at the bottom and can be correlated with four different multiphase flow regimes inside. The transition between these flow regimes is closely related to the properties of working fluid and the fluid fill height. It is noteworthy that the correlation of the thermal performance and the multiphase flow inside extra-long heat pipe is quite different from that for normal size heat pipes, while the influence of above-mentioned effecting factors also shows great diversity. Besides too deep liquid pool, the experimental results suggest as well the liquid dry-out operation must be avoided for real geothermal heat pipes. These results can provide suggestive information for the operation of the geothermal heat pipe and guidance the improving tactics.