LAUSR

Abstract The steady thermal performance of ammonia thermosyphons at shallow geothermal temperature, with the feature of low heat flux, are experimentally investigated in this work. In addition, a CFD (Computational Fluid Dynamic) modelling considering the phase-change heat transfer in the thermosyphon is established to reproduce the inside heat transfer behaviors and explore the corresponding relevance to thermal performance of these thermosyphons. The experimental results show that the influence of flow rate of cooling water qcooling on the total thermal resistance is significant, but the heat transfer power Qpractical is mainly dependent on the temperature of cooling water Tcooling. The maximum Qpractical for the thermosyphon of 30% filling ratio under testing conditions is about 38 W, which is obtained with the heating length at 0.6 m. Both the increase and decrease of le can reduce Qpractical and raise Rtotal. For filling ratio at 20%, the heat transfer power almost halves comparing to 30% filling ratio. The simulation results reveal that dropwise condensation is the main heat transfer mechanism at the condenser of these thermosyphons, and a better thermal performance is expected when the steady boiling liquid pool is nearly identical to the evaporator length for these thermosyphons operating at shallow geothermal temperature. This work provides a deep insight into the inner heat transfer behaviors of ammonia thermosyphons at low heat flux and can be used to facilitate the performance optimization of thermosyphons in shallow geothermal energy utilization.