CFD simulation of dynamic heat transfer behaviors in super-long thermosyphons for shallow geothermal application

Abstract The use of super-long thermosyphons (TSs) is experimentally proved to allow a higher efficiency in extracting shallow geothermal energy. In this work, transient CFD (Computational Fluid Dynamic) simulation has been carried out for the super-long ammonia geothermal TSs, total length up to 54 m, in order to understand their heat transfer behaviors for thermal performance optimization. The results show that due to the influence of hydrostatic pressure of liquid column, boiling initially starts at the liquid pool surface and then gradually moves downward in start-up. With the initial liquid-column height higher than 10 m, the lower part of liquid pool in TSs would be throughout in quiescent state and make no contribution to heat extraction. At steady state, boiling zone in liquid pool shows a much higher heat transfer efficiency than that of partially-wetted area at the upper part of evaporator. Geothermal TS with 20% filling ratio of ammonia in evaporator, namely the initial liquid column height at 10 m, has the best thermal performance under the simulation test conditions. For super-long geothermal TSs (>50 m) using ammonia as working fluid, it is recommended to fill ammonia as much as possible, but the premise is that the initial height of liquid column must be controlled below 10 m.