Influence of stepped cylindrical turbulence generators on the thermal enhancement factor of a flat plate solar air heater

This study presents the numerical analysis on the enhancement of thermal performance of a flat plate solar air heater provided with stepped cylindrical turbulators attached below the absorber plate for assessing performance parameters such as Thermal Enhancement Factor and Thermohydraulic Performance Parameter for Reynolds numbers ranging from 3000 to 24,000. The study reveals that the cylindrical stepped turbulators led to the flow becoming highly turbulent. This flow behavior with the flow separation around the stepped generators has a beneficial aspect with regards to the performance. An attempt has been made in this paper to explain the complex flow behavior as observed from the analysis. For core diameter varying from 3 mm to 7 mm in steps of 1 mm and the relative roughness pitch ratio is varied as 11.11, 16.67, 22.22, and 27.78. The number of steps on the turbulator is varied from 1 to 3 and the number of rows of turbulators is varied from 1 to 3. The maximum Nusselt number of 76.41 is obtained for a core diameter of 7 mm corresponding to a three-row configuration at a flow Reynolds number of 24,000. It is found from the study that the thermal enhancement factor increases with the increment in number of rows and also increases with decrease in core diameter for Reynolds number range beyond 12,000. With the increasing roughness pitch the thermal enhancement factor decreases for a given configuration, while for the increase in number of steps there is a remarkable improvement in the thermal enhancement factor. A maximum thermal enhancement factor of 1.14 is achieved corresponding to a Reynolds number of 15,000. A maximum thermohydraulic performance parameter of 1.49 is achieved corresponding to a Reynolds number of 18,000 from the analysis.