Theoretical and experimental study of the heat transfer inside a horizontal evacuated tube

• The upper temperature distribution along the tube length is linear.
• The mass flow rate increases with increasing heat absorbed by the evacuated tube.
• The mass flow rate in the evacuated tube increases in the “following wind mode”.
• The mass flow rate in the evacuated tube decreases in the “upwind mode”.
• A moderate mass flow rate and a small flow speed are preferred in the header.

The objective of this study is to investigate the impact of the water flow conditions in the header on heat transfer inside a horizontal evacuated tube. A mathematical model based on the dynamic flow and on the resistance of the inner flow field is developed to investigate the relationship between the water flow volume and solar radiation, the temperature of the evacuated tube inlet and the flow inside the solar header. To demonstrate the accuracy of the model, various experiments were performed, and the results of the model were verified; moreover, the results indicated that the water flow inside the horizontal evacuated tube is due to the natural convection produced by gravity. The characteristics of the water flow inside the horizontal evacuated tube were studied by varying the temperature at different inlets of the evacuated tube and the diameter of the evacuated tube. This study also considers the influence on the flow field of the direction that the water flows into the header inside the horizontal evacuated tube. These results demonstrated that under the “following wind mode” condition, the flow field inside the header can accelerate the water volume inside the evacuated tube, whereas under the “upwind mode” condition, the flow field is weakened.