Document 7dmGgeEv9axwkzZ7j2daExg6

712 CHAPTER 31 1956 Guide? r' Fig. 8. Jet Velocity Patterns fob a Square Outlet Discharging along Wall ob Ceiling (Vo = mo, A = 0.10, Ci = 0.78) Effect of Walls and Ceilings Jets discharging parallel to a wall with one .edge of the outlet coinciding with the wall, take the form of one-half of an axial jet discharging from an outlet twice as large, similar to radial jets from ceiling plaques. Entrain ment takes place practically only along the surface of a half cone and the mavimum velocity remains close to the wall.2 (See Fig 8). ^Values of K and K' are approximately those for a free jet multiplied by V2, that is to say the normal maximum of 7.0 for K' for free jets becomes 9.9 for a sim- X ilar jet adjacent to, and discharged parallel to, a wall, and in Fig. 3, should be used in place of X VT,, When a jet is discharged parallel to, but at some distance from a wall, its expansion in the direction of the wall is reduced and entrained air must be obtained by recirculation from the jet itself instead of from ambient air.3,9 The jet expands normally to within a short distance from the wall, which nearer to the outlet only affects velocity distribution in the outer shell of the jet, but further on also affects centerline velocities. This happens at some distance from a plane where the jet outline becomes parallel to the wall, and where the jet enters its fourth or terminal zone. Few engineering data are available on this important phase of jet expansion.5 Air Distribution 713. Effect of Resistance on Return Path of Jet Air Laboratory experiments on jets usually involve recirculated air with negligible resistance to flow on the return path of the jet air. Experience and experiments in tunnels of small cross-sectional area in mines, where considerable resistance is offered to the return flow of jet air to outlets, show that the expansion of the jet terminates abruptly at a distance that is in dependent of velocity of discharge and but slightly affected by size of out let. Such distances are determined primarily by size and length of return path. In a long 5 ft x 6 ft tunnel, a jet may not travel more than 25 ft -w -15 -JO "30 -40 --SO - -60 - -70 . - -80 - -81000 -150 - -J00 -400 Fig. 9. Entrainment Ratios for Stbaight-Flow Outlets Fig. 10. Nomogram for Center, or Peak, Velocities for Rounded, Rectangular Jet Sources whereas in a relatively short opening, 25 by 60 ft, it may travel more than *50 ft. Few engineering data are available on this phase of jet expansion out it can be of great practical importance.5 Entrainment Ratios Equations for the entrainment of circular jets and of jets from long slots nave been mathematically derived.10 They are: For third zone expansion of circular jets, Qx = 2_ JC_ Q* K' VTo (8)