Document DMqK7wM0xn7LbdVggNwm826ed

664 CHAPTER 31 1953 Guide Vt = velocity of primary air. Vi " velocity of induced air (for practical use, Vi = 0). Ti ,= velocity of the mixture. . If the velocity of induced air is zero, Equation 1 changes to MtVi = '(Mi + Mi) V, or. Vi Mi + Mt V, ~ Mi (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, ~ Qi ~ Qi - (3) where : Qi = volume of primary air, cubic feet per minute. Qj = volume of secondary air, cubic feet per minute. Qi = 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 space.1 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 having a small aspect ratio. For the more usual problems of the conventional room with outlets near the ceiling, there are 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 to 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 function of aspect ratio.1 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 B of Fig. 1. At AirT)istribution . >) f 5 =-boo a distance of 8 ft from the outlet, the entrainment of the slot is 8.1 as-com pared,with. 8$ for;the square,;.or. an increase cf about 17 percent. . , . . r Curve (7 shows, the further increase in entrainment obtained by using: an' aspect ratio job 48. v Abiincrease of. 40: percent is obtained over the.M.in/x Fig. 1. Typical Relation of Entrainment Ratio to Distance from Outlet for 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, pattern; is'about the same as for a single opening of equal area. Spac ing the openings farther apart gives ah increase in entrainment as shown on curves-P.and E,of Fig. 1. It will.be noted that 2 openings 24 in, x i.h1located..very, .close together will obtain an entrainment which is about the . same,as .obtained.with one 24 in. x 5 in. opening. ( .However, if the slots are spaced 6| in. apart there is a marked increase in entrainment.