A discussion of internal flow characteristics and performance of super long gravity heat pipes for deep geothermal energy extraction

Abstract

Super long gravity heat pipe (SLGHP) provides a novel way for harvesting thermal energy from a deep geothermal reservoir. Previous research has demonstrated the feasibility of this innovative technique, most studies have focused on the overall performance of the heat extraction system, whereas detailed investigations into the internal flow characteristics remains ambiguous. This hampers the application and further advancement of SLGHPs. In this study, a preliminary analysis of the internal flow resistance characteristics is carried out using of the homogeneous model for two SLGHPs, a regular one and a modified one with an inner coaxial pipe. It indicates that the regular SLGHP has higher flow resistance under the same thermal condition of the reservoir. Specifically, the regular SLGHP experiences approximately 1.75 times larger flow resistance than the modified SLGHP (average temperature of the evaporation section and condenser is 95.6 °C and 55 °C, respectively, with water as the working fluid). While on the other hand, the heat transfer capability of the modified SLGHP was improved 29.73%∼77.77% at various condensation temperature compared with the regular SLGHP. The dominant factor contributing to pressure drop in the regular SLGHP is the frictional loss resulting from the counter-current flow of rising vapor and falling liquid film. Conversely, the SLGHP with a coaxial pipe effectively separates the vapor and condensate flows, reducing frictional resistance and improving heat transfer performance. It is speculated that explosive nucleate pool boiling may occur in the liquid pool at the bottom of the SLGHP, enhancing heat transfer from the liquid bulk to the upward-flowing vapor.