respectively, which are selected as the length scales to define Re, Nu and f. To control the rib floor at the identical geometries for present three two-pass test channels, the distance between the tip of divider and the top wall of 180 bend is kept at one channel width for all three test channels; whereas the variation of channel aspect ratio is achieved by adjusting the channel height. As a result, the rib-height (e) to channel-height (H) ratios and the channel hydraulic diameters for present test channels are respectively increased and decreased as AR increases. Each of present twin-pass ribbed channels consists of the square/rectangular sectioned inlet and outlet ribbed legs connected by a 180 bend, Fig. 1(a). The nominal width of the undulant divider for all three test channels is 20 mm. Six pairs of in-line 45 ribs over the two opposite ribbed endwalls along the inlet and outlet legs are subject to the basically uniform heat fluxes. These in-line square sectioned ribs are parallel with a regular pitch (P) of 10 rib heights (e), giving P/e ratio of 10 for all present test channels. The origin of present x–y coordinate system locates at the entry corner of the inlet leg; whereas another set of channel-wise (S-wise) coordinate system locates its origin at x/W = 0.5 and y = 0 along the centerline of the ribbed endwall, Fig. 1(b). Regions IL, TR and OL respectively cover the areas of Inlet Leg, Turning Region and Outlet Leg. While the same attack angle of
45 is selected for both the skewed wall waves and ribs over the channel sidewalls and endwalls, the sidewall waves and the