An experimental investigation of heat transfer characteristics for steam cooling in a rectangular channel with parallel ribs

An experimental study of heat transfer characteristics in superheated steam cooled rectangular channels with parallel ribs was conducted. The distribution of the heat transfer coefficient on the rib-roughed channel was measured by IR camera. The blockage ratio (e/Dh) of the tested channel is 0.078 and the aspect ratio (W/H) is fixed at 3.0. Influences of the rib pitch-to-height ratio (P/e) and the rib angle on heat transfer for steam cooling were investigated. In this paper, the Reynolds number (Re) for steam ranges from 3070 to 14800, the rib pitch-to-height ratios were 8, 10 and 12, and rib angles were 90°, 75°, 60°, and 45°. Based on results above, we have concluded that: In case of channels with 90° tranverse ribs, for larger rib pitch models (the rib pitch-to-height ratio=10 and 12), areas with low heat transfer coefficient in front of rib is larger and its minimum is lower, while the position of the region with high heat transfer coefficient nearly remains the same, but its maximun of heat transfer coefficient becomes higher. In case of channels with inclined ribs, heat transfer coefficients on the surface decrease along the direction of each rib and show an apparent nonuniformity, consequently the regions with low Nusselt number values closely following each rib expand along the aforementioned direction and that of relative high Nusselt number values vary inversely. For a square channel with 90° ribs at Re=14800, wider spacing rib configurations (the rib pitch-to-height ratio=10 and 12) give an area-averaged heat transfer on the rib-roughened surface about 8.4% and 11.4% more than P/e=8 model, respectively; for inclined parallel ribs with different rib angles at Re= 14800, the area-averaged heat transfer coefficients of 75°, 60° and 45° ribbed surfaces increase by 20.1%, 42.0% and 44.4% in comparison with 90° rib angle model. 45° angle rib-roughened channel leads to a maximal augmentation of the area-averaged heat transfer coefficient in all research objects in this paper.