The Effect of Placing Vortex Generators Above Ribs in Ribbed Ducts on the Flow, Flow Temperature, and Heat Transfer Behavior

Flow temperature and laser-Doppler velocity measurements are presented to understand better the flow and heat transfer behavior resulting from the placement of vortex generators above the first and fifth ribs in a ribbed duct air flow (Re = 3450) and as a benchmark for comparing model predictions. Because the generator–rib spacing, at the smallest spacing (s/h = 0.25), was too small for the flow to pass through it, the generator–rib pair functioned as a single element with an increased effective height, comprising the rib height, the generator–rib space, and the generator proper, which resulted in a downstream shift in the flow reattachment and peak Nusselt number locations. At s/h = 0.75, the flow expanded as it passed through the generator–rib space. Dictated by the upstream flow conditions, the expansion resulted in a vertical downward flow behind the first generator–rib configuration and no vertical downward flow behind the fifth. The vertical downward flow compensated for generator-induced reductions in the near-wall streamwise velocities behind the first generator–rib configuration, resulting in increased local Nusselt numbers, whereas the generator-induced reductions in the streamwise velocities in the absence of the vertical downward flow gave way to reduced Nusselt numbers behind the fifth generator–rib configuration.