Solar collectors with horizontal evacuated tubes are widely used to alleviate the energy crisis. This paper describes a numerical and theoretical study on a horizontal single-ended evacuated tube. A range of validated numerical simulations were conducted to investigate the fluid flow and heat transfer within the tube. The results showed that the secondary flow had a significant influence on the natural circulation flow rate and the temperature distribution within the tube. A theoretical model accounting for the secondary flow was developed to estimate the circulation flow rate and axial temperature distribution. Based on the numerical results, correlations of the circulation flow rate and heat transfer were proposed. The circulation flow rate and heat transfer were correlated to the solar input, tank temperature and tube aspect ratio. Compared to an existing model, the developed model showed a better accuracy in predicting the circulation flow rate and temperature distribution. (C) 2019 Published by Elsevier Ltd.