Multiphase flow and heat transfer characteristics of an extra-long gravity-assisted heat pipe: An experimental study

Abstract

The exploitation of deep geothermal energy has created a great demand for super-long distance heat transportation technology. As one of the most highly-efficient heat transfer devices, the super-long gravity-assisted heat pipe is promising to achieve this heat transportation. The performance of the super-long heat pipe is related to the multiphase flow status inside, the details of which are yet unknown. In this paper, a systematic experimental study on the multiphase flow regimes and the performance of an extra-long heat pipe was conducted. The experimental heat pipe is 40 m in length, 7 mm in diameter, which mimics the major geometric characteristic of the super-long heat pipes that can be practically used for geothermal heat extraction. Thermal performance and surface temperature of the extra-long heat pipe using water, ethanol or acetone as working fluid were measured and analyzed. Particular focus was placed on correlating the heat transfer performance of the heat pipe with the multiphase flow regime inside it. It was found that the extra-long heat pipe works in the following four different flow regimes successively with the increase in inflow heat: (1) nucleating pool boiling regime; (2) geyser boiling regime; (3) evaporating film boiling regime; and (4) overheated boiling regime. The thermal performances of extra-long heat pipe are significantly affected by the flow regimes, while the transition between these flow regimes is influenced by the properties of working fluid, the fluid fill height, and the heat load. It is found that the heat pipe working in the evaporation film boiling regime shows the best thermal performance and the heat pipe using water as working fluid is more suitable for relatively high heat load use. The reachable maximum axial heat fluxes for the water, ethanol, and acetone heat pipes are found to be about 6.5 × 106, 2.21 × 106, 2.08 × 106 W·m−2, respectively. These characteristics of heat transfer and multiphase flow provide very useful hints for the design and development of super-long geothermal heat pipes.