Numerical investigation of heat transfer augmentation of solar air heater with attached and detached trapezoidal ribs

Abstract

The solar air heater has a low heat transfer coefficient due to the development of a laminar sublayer inside the channel of the solar collector. Adding artificial roughness or ribs to the surface of the wall of the absorber plate is an effective and promising method to increase heat transfer by interrupting the laminar sublayer. In this study, a trapezoidal rib was used to enhance the thermal performance of the solar air heater. The characteristics of flow and heat transfer were numerically analyzed using a RNG k-ɛ turbulent model based on the Reynolds number range of 5000, 10000, 15000, 20000, 25000, and 30000. Eight different types of trapezoidal ribs, arranged in shapes that increase in height towards or against the direction of flow, were studied. Four case studies included a solar collector with attached trapezoidal ribs, while another four case studies included a solar collector with detached trapezoidal ribs. The current research involved validation of the results compared to the previously obtained results and showed good acceptable agreement. The results demonstrated that the best thermal performance ratio of the solar collector was achieved when the ribs were attached and detached to the inner surface of a wall, with an increase in the height of the trapezoidal ribs in the opposite direction of the airflow for all ribs. However, these cases resulted in the highest coefficient of friction ratio. On the other hand, the thermal performance was at its lowest level in the cases where the increase in the height of the attached or detached trapezoidal ribs is in the direction of the airflow.

Keywords

• Solar air heater
• Turbulent
• Enhancement heat transfer
• Trapezoidal rib