The internal cooling passages ofthe turbine blades should be designed to yield the desired heat transfer augmentation with a minimum friction factor. The combined effect of rib pitch, rib geometry, and rib alignment on the friction factor and pressure distributions on rib surface for fully developed flow in a rectangular duct with two opposite ribbed-walls were determined for Reynolds numbers (based on the duct hydraulic diameter) ranging from 12,000 to 65,000. The duct aspect ratio is 4: 1 and the rib pitch to height ratios were 10, 20, and 40;· the rib height to duct hydraulic diameter ratio and duct height are kept at 0.081 and 0.125, respectively. Pressure taps are distributed over the bottom~ribbed wall and the rib itself to calculate the friction factor and static pressure coefficient distributions on the rib surface, respectively. ANSYS FLOTRAN CFD computer package was applied to predict the flow separation, recirculation, reattachment, and wake regions to support the experimental results from the flow visualization and the duct measurements. A comparison between the theoretical and experimental results and also with the previous results was investigated. Experimental correlations for the average static pressure coefficient on the rib surface and the friction factor are developed in terms of Reynolds number (Re) and rib pitch to height ratio (p/e) for fully developed flow in ribbed-channel with different rib shapes. The experimental correlations of the average static pressure coefficient on rib surface with different rib shapes are developed in terms of Reynolds number and rib pitch to height ratio. Also the friction factor correlations for different rib shapes are developed.
Cp =al .Re^a2 .(p/e)^a3 (1)
f = b1.Re^b2 .(p / e)^b3 (2)
The constant coefficients al , a2 , a3, bI, b2, and h3 are listed in table 2. The maximum relative errors in equation (1) are ± 6 %, ± 9 %, and ±4 % for trapezoidal, square and inverted trapezoidal ribs, respectively, and in equation (2) are ± 9 %, and ± 12 % for
trapezoidal and inverted trapezoidal ribs, respectively.
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