摘要： Boiling and condensation play an important role in heat transfer in evacuated tube solar collectors (ETC), especially for low flow rates. Another factor that influences the efficiency of an ETC is deterioration of vacuum inside the ETC. The present study investigates numerically the effect of deteriorating vacuum during single and two phase flows inside an ETC to analyze its thermal performance. A steady-state one-dimensional model is developed which can simulate the steady-state thermal performance of the Evacuated tube solar collector (ETC) with single and two-phase flows under saturation conditions considering effects of vacuum deterioration to simulate a more real-life situation of solar collectors. The rise in pressure inside the ETC has a significant effect on the suppression of boiling and condensation due to rise in overall heat loss coefficient. The radiative, convective and conductive heat loss coefficients from the collector are studied for gradual pressure rise inside the ETC. The two phase model also indicates that if boiling starts inside the ETC in a particular inlet flow condition, consideration of two phase flow along with the effect of vacuum deterioration shows significant deviation of efficiency and temperature profile from those of single phase flow models and predicts more accurate results. Inducing of two-phase heat transfer in ETC, with all the fluid condensing again within the collector tube, will result in improved efficiency for that range of mass flow rates. It also helps in maintaining uniform temperature and equal heat gain throughout the collector absorber plate. The efficiency of the collector for higher inlet temperatures is seen to be effected more significantly by vacuum deterioration than at lower inlet temperatures in single phase flows. However, in two phase flows for a fixed saturation temperature of the fluid, vacuum deterioration has negligible effect on the efficiency at higher inlet temperatures (Tfi ≥ 85 °C).