Experimental investigation of the conduit slope impact on mixed convection in solar collectors

ABSTRACT

An experimental investigation on the mixed convection heat transfer of air flowing in circular conduits of a solar collector is conducted. The test rig consists of three different copper pipes with a length of 70 cm. The pipes are heated at constant heat flux. The effect of the pipe inclination angle (θ = 0°, 60°, and 90°), length-to-diameter ratio (L/D = 13.8, 18.4, and 36.7), and heat rate (Q = 5, 11, 19.6, 30, 40.5, and 57.8 W) is studied in the present study. The influences of Richardson number (Ri), inclination angle, and length-to-diameter ratio on the average Nusselt number are examined as well. The results show that the Nusselt number (Nu) is improved by 41.5% as the inclination angle is changed from 0° to 90° at L/D = 13.8. Further, as the length-to-diameter ratio was reduced from 36.7 to 13.8 at 90°, up to 54.9% enhancement in heat transfer is found. Also, it is seen that by increasing the heat rate from 5 W to 57.8 W at 90° and L/D = 13.8, the exchange in heat is improved by 44%. Three empirical correlations were proposed for each studied pipe inclination angle. A good agreement was found between the measured Nu and the calculated Nu from the proposed correlations.

KEYWORDS:

• Mixed convection
• uniform heat flux
• inclined pipe
• experimental
• solar collector

Disclosure statement

The authors confirm that there are no relevant financial or non-financial competing interests to report.

Additional information

Notes on contributors

Ahmed T. Al-Sammarraie

Dr. Ahmed T. Al-Sammarraie is an assistant professor in the Mechanical Engineering Department at Tikrit University, Tikrit, Iraq. He is also a researcher affiliated with the University of California, Riverside, USA.

Manar Al-Jethelah

Dr. Manar Al-Jethelah is an assistant professor in the Mechanical Engineering Department at Tikrit University, Tikrit, Iraq.

Thamir K. Ibrahim

Dr. Thamir K. Ibrahim is a professor in the Mechanical Engineering Department at Tikrit University, Tikrit, Iraq.

Mohammad Reza Salimpour

Dr. Mohammad Reza Salimpour is a professor in the Department of Mechanical Engineering at Isfahan University of Technology, Isfahan, Iran.

Firas B. Ismail Alnaimi

Dr. Firas B. Ismail Alnaimi is the Head of Smart Power Generation Research Centre, Institute of Power Engineering (IPE), Universiti Tenaga Nasional (UNITEN), Kajang, Selangor, Malaysia.