Capture Envelopes of Rectangular Hoods in Cross Drafts

Abstract

The suction fields of the rectangular hoods of various aspect ratios varying from 0.1 to 10 that are subject to the influence of cross drafts were experimentally 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 draft, the suction field presents a characteristic capture envelope, which is described by a dividing streamline. The characteristics of the capture envelope 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 from those with aspect ratios greater than one. If areas of the hood openings had the same values, the hydraulic-diameter normalized characteristic length scales of the capture zone of the square hood were as same as those of the circular hood. When the diameter of a circular hood was equal to the width of a square hood, the physical dimensions of the capture zones created by these two hoods coincided with each other.

Development and Characterization of a Wake-Controlled Exterior Hood

“…Huang et al (20,21) have obtained semiempirical formula of normalized dividing streamline and characteristic length scales in the flow field. They also found that the centerline distance of the capture-envelope boundary obtained from the streamline method (20) almost coincides with that obtained from the 50%-capture-efficiency method, (11) so that the velocity measurements are closely correlated to the concentration sampling results.…”