Numerical Study on Effect of Rectangular Shaped Ribs Arranged in Different Patterns on Thermal Performance of a Solar Air Heater Duct

摘要

This work is concerned with a two-dimensional numerical study done to predict the influence of transverse rectangular cross-sectioned ribs on a solar air heater’s convective heat transfer properties. Solar air heater is a useful device that can be utilized to augment the temperature of air by extracting heat from solar energy. It is a rectangular duct consisting of an absorber plate on its top and heat falls only on the top of absorber plate. When ribs/baffles are introduced just beneath the absorber plate, there is a considerable alteration in the thermal performance of air flowing through the rectangular duct. A comparison was made between the results of thin (high aspect ratio) and square ribs arranged in three patterns, namely, single wall arrangement, staggered arrangement and in-line arrangement on two opposite walls. The Nusselt number variation with Reynolds number range 5000-24000 was checked at a fixed rib pitch (p) and height (e) values. Computational fluid dynamics (CFD) simulations were performed using commercially available software ANSYS FLUENT v15.0. The results were compared with the existing experimental ones while performing simulations under similar conditions. Two methods were used to calculate the average Nusselt number in which one method extracted the local Nusselt number at many points and on averaging these, gave the average Nusselt number and the other method resembled the one used in the existing experimental work. The results revealed that, as compared to smooth duct, the introduction of ribs led to a considerable augmentation in heat transfer. Good agreement was found between the existing experimental results and numerical output, when the second method was adopted to calculate the Nusselt number. However the Nusselt number calculated using method 1 yielded values lower than the existing ones. The results revealed that the thin ribs yielded better performance than the squared ones. Out of the three arrangements, the best thermal performance was given by thin inline ribs whose convective heat transfer coefficient was 1.83 times smooth duct’s convective heat transfer coefficient