Repeated rib roughness elements have been used in advanced turbine cooling designs to enhance the internal heat transfer. Often the ribs are perpendicular to the main flow direction so that they have an angle of attack of 90 deg. The objective of this investigation was to determine the effect of rib angle of attack on the pressure drop and the average heat transfer coefficients in the fully developed turbulent air flow in a square duct with two opposite rib-roughened walls for Reynolds number varied from 7000 to 90,000. The rib height-to-equivalent diameter ratio (e/D) was kept at a constant value of 0.063, the rib pitch-to-height ratio (P/e) was varied from 10 to 20, and the rib angle of attack (α) was varied from 90 to 60 to 45 to 30 deg, respectively. The thermal performance comparison indicated that the increased heat conductance for the rib with an oblique angle to the flow (α = 45–30 deg) was about 10–20 percent higher than the rib with a 90 deg angle to the flow, and the pumping power requirement for the angled rib was about 20–50 percent lower than the transverse rib. Semi-empirical correlations for friction factor and heat transfer coefficients were developed to account for rib spacing and rib angle. The correlations can be used in the design of turbine blade cooling passages.

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