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A modified CFD model was developed to investigate the heat and mass transfer performance of a two-phase closed thermosiphon (TPCT). In this model, the phase-change temperature of the working fluid was considered to be dependent on the local pressure. Meanwhile, an auto-adjust and control strategy was established for the condensation mass transfer time relaxation parameter, which could balance the phase-change pressure to the working pressure. The modified phase-change model was verified by experiments and then used to investigate the heat and mass transfer behaviors of the TPCT under different heat flux of 12.31–15.95 kW/m2. The results indicated that the maximum relative errors of wall temperature and working pressure of the TPCT were 0.25–0.48% and 0.14–0.46%, respectively. The wall temperature gradually decreases from the bottom of evaporator to adiabatic section, and then increases from the bottom to the top of the condenser due to the temperature difference between the inlet and outlet of the cooling water. Also, as the heat flux increase, the overall thermal resistance reduces from 0.060 to 0.055 K/W. These results indicate that the proposed model can be used to predict the heat and mass transfer of the TPCT.

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.

A novel metal “endoscopic” two-phase closed thermosyphon (METPCT) which could be used to intuitively investigate the phase-change heat transfer and two-phase flow characteristics were developed. The visualized experimental set-up was established by using a rigid borescope and a high-speed camera. The phase-change heat transfer characteristics of the internal working fluid inside the METPCT under various cooling water temperatures (10, 15, 20 °C) and input power (100 ~ 900 W) have been investigated. Different heat and mass transfer regimes, e.g. start-up, geyser boiling and steady-state, were analyzed. During the start-up, the heat transfer in the evaporator is mainly heat conduction, natural convection and surface evaporation. Meanwhile, the start-up time decreases as the increases with cooling water temperature. The geyser boiling includes the incubation and expulsion stage, and the wall temperature response shows slight fluctuation and sharp fluctuation during the expulsion stage. In the steady-state, from bottom to top of the liquid pool in the evaporator, the two-phase flow pattern evolves from bubbly flow to slug flow, churn flow and then dispersed flow, and its transition position is closer to the bottom with the increase of input power. In addition, the heat transfer modes at the condenser are the combination of droplet condensation, discontinuous film condensation, and uniform film condensation. This work provides a new visualized experimental method to investigate the phase-change and two-phase flow properties in a TPCT, which helps to understand the heat and mass transfer mechanism of the TPCT.