In this experimental investigation of the heat transfer characteristics of turbulent air flow in a three-pass square channel, the test section consisted of three straight square channels jointed by two 180° turns, modeling the internal cooling passages of gas turbine airfoils. Naphthalene-coated ribs were attached to the top and the bottom walls of the naphthalene-coated, three-pass channel. The rib height-to-hydraulic diameter ratio was 0.063; the rib pitch-to-height ratio was 10; the rib angles were 90° and 60°. For α = 60°, both the crossed ribs (on two opposite walls of the cooling channel) and the parallel ribs (on two opposite walls of the cooling channel) were investigated. The combined effects of the two sharp 180° turns and the rib orientations on the distributions of the local mass transfer coefficient in the entire three-pass channel were determined.

The rib angle, the rib orientation, and the sharp 180° turn significantly affect the local mass transfer distributions. The combined effects of these parameters can increase or decrease the mass transfer coefficients after the sharp 180° turns. The angled ribs, in general, provide higher mass transfer coefficients than the transverse ribs; the parallel ribs give higher mass transfer than the crossed ribs.

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