Effects of rib arrangements and rotation speed on heat transfer in a two-pass duct

The present study investigates heat/mass transfer characteristics in a rotating two-pass duct for smooth and ribbed surfaces. The duct has an aspect ratio (W/H of 1:2) of 0.5 and a hydraulic diameter (A) of 26.67 mm. 70°-angled rib tabulators are attached on the leading and trailing sides of the duct in parallel and cross arrangements. The pitch-to-rib height ratio (p/e) is 7.5 and the rib height-to-hydraulic diameter ratio (e/D_h) is 0.075. The Reynolds number based on the hydraulic diameter is constant at 10, 000 and the rotation number ranges from 0.0 to 0.2. Detailed local heat/mass transfer coefficients are measured using a naphthalene sublimation technique, which is analogous to the two-side heating condition of heat transfer experiment. The results show that the secondary flows generated by the 180°-turn, rib tabulators, and duct rotation affect the wall heat/mass transfer distribution significantly. The curvature of the 180°-turn produces Dean vortices causing high heat/mass transfer in the turning region and in the upstream region of the second-pass. When the duct is roughened with ribs, the disturbed main flow produces recirculation and secondary flows near the ribbed surfaces. Consequently, the heat/mass transfer is enhanced two to three times more. As the duct rotates, the rotation-induced Coriolis force deflects the main flow and results in differences on the heat/mass transfer distribution between the leading and trailing surfaces. Its effects become more dominant as the rotation number increases. Discussions are presented describing how the rib configuration and the rotation speed affect the flow patterns and local heat/mass transfer in the duct.