Variable Property and Temperature Ratio Effects on Nusselt Numbers in a Rectangular Channel With 45 Deg Angled Rib Turbulators

Abstract

Measured local and spatially-averaged Nusselt numbers and friction factors (all time-averaged) 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 Toi/Tw 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 +45 deg and −45 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 globally-averaged rib Nusselt numbers to baseline, constant property Nusselt numbers, Nu̿/Nuo,cp, increase from 2.69 to 3.10 as the temperature ratio Toi/Tw decreases from 0.95 to 0.66 (provided Reynolds number ReH is approximately constant). Friction factor ratios f/fo,cp then decrease as Toi/Tw decreases over this same range of values. In each case, a correlation equation is given which matches the measured global variations. Such global 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.