Effect of rib angle orientation on local mass transfer distribution around sharp 180 deg turn with rib-turbulators mounted in entire two-pass channels

Abstract

The local mass transfer distributions around sharp 180 deg turn with rib-turbulators in a relatively short (L/D=4) two-pass, square channel were determined via the naphthalene sublimation technique. The rib height-to-hydraulic diameter ratio (e/D) was 0.05 and the rib pitch-to-height ratio (p/e) was 10. Experiments were conducted for five attack angles (α=90,45,60,−45, and −60 deg), and for three Reynolds numbers (3.0×10⁴, 6.0×10⁴ and 9.0×10⁴). Results show that the rib-roughened wall Sherwood numbers after the turn are higher than those in the turn region, which, in turn, are higher than those before the turn due to the ribs installed in the turn region. While in the previous studies, the ratio Sh/Sh ₀ in the turn region was found to be lower than that in the before turn region, because of the absence of rib in the turn region. The rib angle and rib orientation have significant effect on both the local and average Sherwood number ratios. The average Sherwood number ratios (Sh _m /Sh ₀) for α = 60 deg have the highest values, then comes the case of α = 45 deg, and the Sherwood number ratios (Sh _m /Sh ₀) for α = 90 and −45 deg are the lowest. Correlations for the average rib-roughened wall Sherwood number ratios for the before-turn, in-turn and after-turn segments are provided.

Zusammenfassung

Der lokale Stoffübergang in einem relativ kurzen (L/D = 4) 180^∘-Umlenkkanal mit turbulenzerzeugenden Rippen wurde unter Einsatz der Naphtalin-Sublimationstechnik untersucht. Das Verhältnis von Rippenhöhe zu hydraulischem Durchmesser betrug e/D = 0,05 und das von Rippenabstand zu Rippenhöhe p/e = 10. Die Experimente wurden mit 5 verschiedenen Anstellwinkeln (α=90^∘,45^∘,60^∘,−45^∘,−60^∘) und bei drei Reynolds-Zahlen (3,0·10⁴, 6,0·10⁴, 9,0·10⁴) durchgeführt. Ripppenwinkel und Rippenorientierung zeigten wesentlichen Einfluß auf die lokalen und mitt-leren Sherwood-Zahlen, wobei die höchsten Werte bei α=60^∘ auftraten; dann folgten immer niedrigere für 45^∘,90^∘ und −45^∘. Berechnungsgleichungen für mittlere Sherwood-Zahlen in den 3 Segmenten vor, in und nach der 180^∘– Umlenkung werden mitgeteilt.