Document K66JRvnZarZa01K3L7J4VMDe6

!|l U' m CHAPTER 40 1949 Guide Vi = velooity of primary air. : V, = velocity of induced air (for practical use, Vi = 0). Ki = velocity of the mixture. If the velocity of induced air is zero, Equation 1 changes to: M,Vx = (Mi + MJV, or. Vi Mi+M," V,~ Mi . ~ T (2) Since in many applications the densities of primary and room air are about equal,, air volumes may be substituted for mass and Equation 2 becomes: Vi Qi + Qi Qi V, " Q, = Q, " r ,. (3) where ' Qi =Volume of primary air, cubic feet per minute. Qi = volume of secondary air, cubic feet per minute. Q, = volume of mixture of primary air and induced air, cubic feet per minute, r = induction ratio. ; Jet Pattern From Round or Rectangular Openings in a Large Room The relation between the shape of the discharge of a jet and the shape of the conventional outlet has long been the subject of research. It has been proved to be incorrect to assume that the jet retains the outlet shape when it discharges into a free open space1.. Air streams from rectangular outlets having low aspect ratios develop a symmetrical or cone shape within a few diameters from the outlet face. From there on, the jet continues to expand at a fairly, constant rate. Beyond 20 diameters there is very little difference between round and rectangular jets. The assumption can be made that the apex of the cone is in the same position for any;jet haying a small aspect ratio. .For the more usual problems of the conventional:room with outlets near the ceiling, there ate insufficient experimental; data to justify a definite statement.on the effect of aspect ratio. If the round or rectangular opening is divided into a number of orifices having straight sides, the performance of the air stream will be similar to that of a. plain opening. . Velocity Across Jets Results of many tests1 indicate that the ratio of centerline velocity U> average, velocity is.about. 3,.irrespective of. outlet size, shape or.initial velocity. This statement is true for stream cross-sections located beyond 10 diameters from the outlet, and.is fairly accurate for distances up; to 50 diameters. Experimental data are lacking for distances beyond 50 diam eters. Effect of Aspect Ratio on Entrainment In slotted outlets, the air entrainment of the primary jet is a funotionof aspect ratio1. This effect is most pronounced when large changes in the ratio are made. A comparison between a slot' of aspect ratio 24 and a square opening of the same area is given in curves A and Bof Fig. 1. At Air: .Distribution .787. a distance of 8 ft from the outlet, the entrainment of the slot is 8.1 as com-' pared with 6.9for.the square, or an increase of about 17,per cent. . Curve C shows the further increase in entrainment obtained by .using an aspect ratio of 48. An increase of 40 per cent is obtained Over the 24 in. x Fig. 1. Typical Relation of. Entbainment Ratio to Distance fbom Outlet fob Slotted Outlets. (Based on 800 fpm Outlet Velocity.) 1 in. slot. This.indicates that long narrow slots produce air streams that give high induction of secondary air. Parallel Slots ; The use of several slots in .parallel to vary the rate of air entrainment depends mainly;on the distance between the slots. If close together, the air.pattem is about the same as for a single opening of equal area. . Spac ing the openings farther apart gives an increase in entrainment as shown on curves D and E of Fig. 1. It will be noted that 2 openings 24 in. x.J in. located very close ^together will obtain an entrainment which is about the same as obtained with one 24 in. x J in. opening. However, if the slots are spaced 6 J in. apart there is a marked increase in entrainment.