An investigation of duct aspect ratio effects on heat/mass transfer in a rotating duct with 90° ribs

The effects of duct aspect ratio on heat/mass transfer are investigated. Mass transfer experiments are conducted to obtain detailed local heat/mass transfer coefficients on the leading and trailing surfaces in a rotating two-pass duct with 90°-rib turbulators. The duct has three aspect ratios ( W/H=0.5, 1.0, and 2.0) with a fixed hydraulic diameter (D_h) of 26.7 mm. 90°-rib turbulators are installed on the leading and trailing sides symmetrically. The rib height-to-hydraulic diameter ratio (e/D_h) is 0.056, and the rib height remains constant in all duct cases. The Reynolds number based on the hydraulic diameter is fixed to 10,000 and the rotation number ranges from 0.0 to 0.20. The results show that Sherwood number ratios are 2.5 tunes higher than the fully developed value in a stationary smooth pipe due to the flow reattachment near ribbed surfaces. The overall heat/mass transfer in the first pass is more enhanced in higher aspect ratio duct because the rib height-to-duct height ratio increases, which results in more turbulated and accelerated flow in the midsections of the ribs. Dean vortices augment heat/mass transfer in the turn and in the upstream region of the second pass. The rotation of duct produces heat/mass transfer discrepancy, having higher Sherwood number ratios on the trailing surface in the first pass and on the leading surface in the second pass. However, the effects of turning region and rotation on heat/mass transfer become less significant with the increment of duct aspect ratio.