A theoretical model of natural circulation flow and heat transfer within horizontal evacuated tube considering the secondary flow

• Published in: Renewable energy Vol. 147; pp. 630 - 638
• Main Authors: Li, Jiarong, Li, Xiangdong, Wang, Yong, Tu, Jiyuan
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.03.2020
• Subjects: Heat transfer; Horizontal evacuated tube; Natural circulation flow; Secondary flow; Natural circulation flow; Horizontal evacuated tube; Secondary flow; Heat transfer
• ISSN: 0960-1481; 1879-0682
• DOI: 10.1016/j.renene.2019.08.135

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. •The circulation flow rate decreases from the tube opening to the sealed end.•The temperature of the circulation flow changes slightly along the tube axial.•An improved theoretical model based on the secondary flow was proposed.•The model performs better in predicting flow rate and temperature distribution.