Plenary Speakers

    In domestic refrigerators using R600a, the refrigerant has been observed to exist in a two-phase state between the condenser outlet and the expansion device inlet, even when its temperature is below the corresponding saturation temperature. This behavior deviates from conventional design assumptions, in which vapor compression refrigeration systems are expected to deliver a fully subcooled liquid at the condenser outlet. To investigate this phenomenon, a series of experiments was conducted to examine the thermodynamic state of R600a at the condenser outlet. A transparent horizontal tube with an inner diameter of 4.3 mm was installed to enable direct visualization of the refrigerant flow. The coexisting phases were separated, and the temperatures of each phase were measured individually. Visual observations, along with temperature and pressure measurements, revealed that R600a exists in a non-equilibrium two-phase state, where subcooled vapor and subcooled liquid coexist at temperatures significantly below the saturation temperature.

    To characterize this non-equilibrium condition, a set of equations was developed to estimate the specific enthalpy of the refrigerant. The proposed method showed good agreement with experimentally measured values, demonstrating its validity for describing the thermodynamic properties of R600a under such conditions.

    Additional experiments were conducted to quantify the impact of this non-equilibrium subcooled two-phase state on system performance. Compared to the conventional equilibrium subcooled state, both the condenser heat rejection and the coefficient of performance (COP) decreased when the refrigerant was in a non-equilibrium two-phase condition. These results indicate that the presence of subcooled vapor has a detrimental effect on system efficiency and should be considered in performance analyses of refrigeration systems using R600a. Furthermore, molecular dynamics (MD) simulations were performed to investigate the influence of residual air mixed with R600a. Trace amounts of air may remain in the system prior to refrigerant charging. The simulation results showed that even a small fraction of residual air can significantly reduce the saturation temperature of the mixture. This effect is considered a plausible explanation for the unexpected thermodynamic behavior of R600a observed in practical refrigeration systems.