Influence of channel orientation on heat transfer in a two-pass smooth and ribbed rectangular channel (AR=2:1) under large rotation numbers

Experiments were conducted in a rotating two-pass cooling channel with an aspect ratio of 2:1 (D_h=16.9 mm). Results for two surface conditions are presented: smooth and one ribbed configuration. For the ribbed channel, the leading and trailing walls are roughened with ribs (P/e=10, e/D _h=0.094) and are placed at an angle (α=45°) to the mainstream flow. For each surface condition, two angles of rotation (β=90°, 135°) were studied. For each angle of rotation, five Reynolds numbers (Re=10K-40K) were considered. At each Reynolds number, five rotational speeds (Ω=0-400 rpm) were considered. The maximum rotation number and buoyancy parameter reached were 0.45 and 0.85, respectively. Results showed that rotation effects are minimal in ribbed channels, at both angles of rotation, due to the strong interaction of rib and Coriolis induced vortices. In the smooth case, the channel orientation proved to be important and a beneficial heat transfer increase on the leading surface in the first pass (radially outward flow) was observed at high rotation numbers. The correlations developed in this study for predicting heat transfer enhancement due to rotation using the buoyancy parameter showed markedly good agreement with experimental data (+/-10%). Finally, heat transfer under rotating conditions on the tip cap showed to be quite dependent on channel orientation. The maximum tip cap Nu/Nus ratio observed was 2.8.