Effect of duct length variation on solar air heater performance for smooth and D-shaped roughened absorber plate

The study aims to examine how duct length affects the performance of a solar air heater (SAH) with a D-shaped ribbed absorber plate, compared to a smooth absorber plate. The optimized D-shaped ribs from previous research investigations are utilized in the present work to explore the absorber plate's length influence on the Nusselt number. The study reveals a slight decrease in the Nusselt number as the length of the absorber plate with D-shaped ribs is increased. The observed behavior is attributed to the diminishing capacity of air to extract heat from the heated surface within the elongated duct. Moreover, the study calculates the pressure drop and thermo–hydraulic performance parameter (THPP) associated with the D-shaped ribs and formulates correlations to establish a quantitative understanding of the relationship between duct length and D-shaped rib performance. The maximum value of THPP was found to be 1.17 within the considered range of duct height. Furthermore, correlations have been derived for the Nusselt number and friction factor in terms of duct length to hydraulic diameter ratio and Reynolds number with maximum deviations of +1.7 and +2.13, respectively. These correlations serve as valuable tool for engineers and researchers seeking to optimize the design of SAHs, enabling them to balance the benefits of D-shaped ribs with the considerations of duct length. This research contributes to the growing body of knowledge of SAH design, offering insights into the trade-offs and intricacies of utilizing D-shaped ribs and adjusting duct length for improved THPP.