Experiments are conducted to study the effect of three types of rib-arrays, namely composite, fully-attached, and fully-detached ribs, on friction factors and center-line heat transfer coefficients in rectangular channels. Two opposite walls are roughened by alternate attached and detached in-line rib-arrays for the composite ribs. The Reynolds number (Re) based on channel hydraulic diameter ranges from 12,000 to 70,000; whereas the rib pitch-to-height ratio varies from 10 to 30. The rib-to-channel height ratio (or rib height-to-channel hydraulic diameter ratio), and the ratio of the rib clearance to height are fixed at h/2B = 0.2 (h/De = 0.125), and c/h = 0.5, respectively, with a channel aspect ratio (W/B) of 4.0. It takes a longer distance from the channel inlet to set the local heat transfer coefficient into a periodic constant-value distribution for the composite-ribbed wall due to the more complex turbulent transportation. In the fully developed flows, the composite rib-roughened wall yields the highest heat transfer augmentation, and gives moderate pressure-drop penalty among the three types of ribbed walls. Performance evaluation under the constant pumping-power constraint reveals that the composite-ribbed channel performs best of the three ribbed arrangements. Semi-empirical correlations for friction and heat transfer in composite-ribbed channels are developed to account for rib spacing and Reynolds number for the design of gas turbine blade cooling passages.

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