Document KJa9QyGV9YaKMVxbebN6Ex93Q

648 CHAPTER 30 1952 Guide. V, = velocity of primary air. Vt = velocity of induced air (for practical use, Vi -- 0). Vj = velocity of the mixture. If the velocity of induced air is zero, Equation 1 changes to MiVi = (Mi + MJV, .' or, V, M, + M1 V, ~ M, ~ r ' (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 -f Qi ^ Qi V,~ Qi ~ 0, (3) where Q, = volume of primary air, cubic feet per minute. Qi -- 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 nr 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 nutlet 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 AiriDiStribution 649 a distance of 8 ft from the outlet, the entrainment of the dot is 8.1 as conP parecj,'with 6-9, for.jlhe. square, ;or,an.increase, of about 17; percent,; 'al purye,(7 shows .the further-increase,in entrainment obtained by using iap aspect rati0[Of(48.; .An increase/of^ percent is obtained over the; 24->iri-::x 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 an increase in entrainment as shown on ciiryes 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; isAbout the same as obtimed with brie 24 in. x 5 in. opening. However, if the slots are' spaced 6 in. apart there is a marked increase in entrainment.