Flow visualization and local mass transfer studies for turbulent flow in a wind tunnel with chamfered ribs

Abstract

The paper presents results of flow visualization and mass transfer studies for fully developed turbulent flow of air in a square section wind tunnel with repeated chamfered rib roughness on the bottom of the tunnel (rib head chamfer angles ϕ of -15°, 0°, and 15°; relative roughness pitchp/e = 3, 5, 7.5, and 10). Direct video recordings of flow patterns were made using a simple technique of particles visualization. For the positively chamfered closely spaced ribs (p/e <- 5) vigorous vortex shedding has been seen compared to the square or negatively chamfered ribs, which is found to be a function of the Reynolds number. For the widely spaced ribs, the study shows flow separation at the ribs and reattachment in the inter-rib region. Local mass transfer studies, based on the variation in colour of cobaltous chloride solution impregnated paper due to evaporation of water, showed a significant improvement in mass transfer rate in the recirculating region in the wake of ribs with the change in the chamfer angle from -15° to 15°. The positively chamfered 15° ribs are found to be better than square section ribs atp/e <- 7.5. The performance of negatively chamfered ribs is found to be poor compared to other ribs irrespective of the relative roughness pitch.