Experimental study for the enhancement of thermal efficiency and development of Nusselt number correlation for the roughened collector of solar air heater

Abstract The solar air heater's thermal efficiency is relatively poor owing to the flat collector surface. This article's primary objective is to increase the collectors' thermal efficiency of rectangular ducts of solar air heater by adopting a novel V-shaped twisted rib element with staggering orientation. Experimentations are performed for various flow Reynolds numbers ranging from 3k-21k, roughness pitch-to-rib height ratio ranging from 7-11, and staggering distance to rib height ratio between 2-6. Dispersion of Nusselt number over the collector surface is achieved through liquid crystal thermography technique. Among the varied rib and flow constraints, it is observed that a maximum thermal performance enhancement index of 2.69 is observed, with the optimum value of the roughness parameter at a rib pitch-to-height ratio of 9 and a staggering distance-to-height ratio of 4. Mathematical correlation has also been developed using a regression model to estimate the Nusselt number in terms of non-dimensional roughness parameters. The percentage deviation between the Nusselt number attained from established relationships and the investigational results are found to be giving very satisfactory outcomes. The thermal efficiency of the smooth surface is recognized at 42.64% which increases for the roughened surface of twisted V-ribs to 73.63%. Hence employing twisted V-ribs as an artificial roughness element no doubt increases the Nusselt number, thermohydraulic performance enhancement index, and thermal efficiency, but it also exerts less frictional power of solar air heater.