Capture envelopes of rectangular hoods in cross drafts

The suction fields of the rectangular hoods of various aspect ratios varyin g from 0.1 to 10 that are subject to the Influence of cross drafts were exp erimentally studied in an apparatus consisting of a hood model/wind tunnel assembly. The velocity field on the symmetry plane was measured with a two- component laser Doppler anemometer. Being under the influence of cross draf t, the suction field presents a characteristic capture envelope, which is d escribed by a dividing streamline. The characteristics of the capture envel ope were found to be determined by the cross-draft to hood-suction velocity ratio R and the hood-opening aspect ratio AR. The flow characteristics of the hoods with aspect ratios less than unity were dramatically different fr om those with aspect ratios greater than one. If areas of the hood openings had the same values, the hydraulic-diameter normalized characteristic leng th scales of the capture zone of the square hood were as same as those of t he circular hood. When the diameter of a circular hood was equal to the wid th of a square hood, the physical dimensions of the capture zones created b y these two hoods coincided with each other.