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Experimental results, measured on and above a dimpled test surface placed on one wall of a channel, are given for Reynolds numbers from 1250 to 61,500 and ratios of air inlet stagnation temperature to surface temperature ranging from 0.68 to 0.94. These include flow visualizations, surveys of time-averaged total pressure and streamwise velocity, and spatially resolved local Nusselt numbers, which are measured using infrared thermography, used in conjunction with energy balances, thermocouples, and in situ calibration procedures. The ratio of channel height to dimple print diameter is 0.5. Flow visualizations show vortical fluid and vortex pairs shed from the dimples, including a large upwash region and packets of fluid emanating from the central regions of each dimple, as well as vortex pairs and vortical fluid that form near dimple diagonals. These vortex structures augment local Nusselt numbers near the downstream rims of each dimple, both slightly within each depression, and especially on the flat surface just downstream of each dimple. Such augmentations are spread over larger surface areas and become more pronounced as the ratio of inlet stagnation temperature to local surface temperature decreases. As a result, local and spatially averaged heat transfer augmentations become larger as this temperature ratio decreases. This is due to the actions of vortical fluid in advecting cool fluid from the central parts of the channel to regions close to the hotter dimpled surface.

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Measured local and spatially-averaged Nusselt numbers and friction factors (all timeaveraged) are presented which show the effects of temperature ratio and variable properties in a rectangular channel with rib turbulators, and an aspect ratio of 4. The ratio of air inlet stagnation temperature to local surface temperature T oi /T w varies from 0.66 to 0.95, and Reynolds numbers based on channel height range from 10,000 to 83,700. The square cross-section ribs are placed on two opposite surfaces, and are oriented at angles of ϩ45 deg and Ϫ45 deg, respectively, with respect to the bulk flow direction. The ratio of rib height to channel hydraulic diameter is 0.078, the rib pitch-to-height ratio is 10, and the ribs block 25 percent of the channel cross-sectional area. Ratios of globallyaveraged rib Nusselt numbers to baseline, constant property Nusselt numbers, Nu ញ /Nu o,cp , increase from 2.69 to 3.10 as the temperature ratio T oi /T w decreases from 0.95 to 0.66 (provided Reynolds number Re H is approximately constant). Friction factor ratios f / f o,cp then decrease as T oi /T w decreases over this same range of values. In each case, a correlation equation is given which matches the measured global variations. Suchglobal changes are a result of local Nusselt number ratio increases with temperature ratio, which are especially pronounced on the flat surfaces just upstream and just downstream of individual ribs. Thermal performance parameters are also given, which are somewhat lower in the ribbed channel than in channels with dimples and/or protrusions mostly because of higher rib form drag and friction factors.