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Two-phase loop thermosyphon (TPLT) shows an oscillation phenomenon since the complex phase-change heat transfer and flow behaviors, decreases the system reliability in practice. A combined three-dimension CFD simulating/experimental investigation was carried out to investigate the phase-change flow inside TPLT. Results of the 3D-CFD model show good agreement with the experiments, with a maximum deviation of 2.86% and 3.98% for temperature distribution and pressure, respectively. In the vertical evaporator heating mode, the filling ratio of 23.3% produced the lowest total thermal resistance of 0.28–0.22K/W since the dominant heat transfer mechanism of phase-change in the loop. With the increasing FR, from 48.0 to 84.1%, the dominant heat transfer mechanism might be transformed to single-phase convection. Thermal performance with the horizontal evaporator heating mode is better than that with the vertical evaporator heating mode since the good flowability at high FR = 84.1%. In the horizontal evaporator heating mode, a bidirectional flow was observed in the loop with the horizontal evaporator even at low FR = 23.3%, which caused pressure fluctuation and reduced thermal performance. But the flow stability could increase by 90% with the auxiliary of a porous media in the horizontal evaporator, indicating that the porous media had a positive effect on avoiding bidirectional flow.

A novel ellipse-concave plate heat pipe (ECPHP) receiver using sodium as the working fluid was developed for using on the solar thermochemical reactor. The start-up and heat transfer performance of the ECPHP under different cross-section heat fluxes were experimentally investigated. By introducing the temperature difference index parameters, the start-up regime of the ECPHP was specified as the melting stage, vapor flow transfer stage, and the effective start-up stage, respectively. The effective start-up time decreased to 21.7 min when the cross-section heating flux increase to 279.3 kW/m2, the temperature of evaporator at effective start-up was about 680–770 °C. In addition, the ECPHP demonstrated good heat transfer performance in the quasi-steady stage. The temperature difference on the condensing section was less than 93.8 °C, the thermal resistance and the heat transfer coefficient were in the range of 0.054–0.083 °C/W and 670.4–1032.3W/(m2·°C), respectively. Moreover, the temperature image on the evaporator illustrated that the ellipse-concave design of the absorber could effectively relieve the hot spot on the heating surface, which would reduce the thermal stress, thereout enhancing the heat transfer stability of the receiver when applied to concentrate the solar energy.

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.