Turbulent heat and fluid flow in a passage disturbed by detached perforated ribs of different heights

The spatially periodic turbulent heat transfer and friction in a rectangular passage of width-to-height ratio of 4:1 with perforated rectangular ribs detached from one wall have been studied using laser holographic interferometry, smoke-streak flow visualization, and pressure probing for rib-to-duct hydraulic diameter ratios (H/De) of 0.081, 0.106, and 0.162 and Reynolds number (Re) range of 5 x 10(3)-5 x 10(4). The rib pitch-to-height ratio and the detached distance-to-rib-height ratio are 10 and 0.38, respectively. The laser-Doppler velocimetry measurements were performed to illustrate the local Nusselt number distribution. In contrast to the attached solid-type ribs which provide a monotonic decrease of thermal performance at a constant pumping power (<(Nu)over bar>(p)/<(Nu)over bar>(s)*) with increasing rib height, the detached perforated-type ribs give a best <(Nu)over bar>(p)/<(Nu)over bar>(s)* at a moderate rib height H/De = 0.106 for the lower Re range and a <(Nu)over bar>(p)/<(Nu)over bar>(s)* independent of H/De for the higher Re range. In addition, the thermal performance of the detached solid-type ribs is found to be superior to that of the detached perforated-type ribs and the physical reason for the difference is addressed. (C) 1998 Elsevier Science Ltd. All rights reserved.