TECHNICAL FEATURE FIGURE 6 Contours of temperature at the central plane with the heat source on the table show that when the cooling capacity of the discharge jet reaches four times the cooling load the downward discharge air can overcome the resistance of the upward buoyant plume. a) 200 cfm (94.4 L/s) b) 400 cfm (188.8 L/s) c) 600 cfm (283.2 (L/s) d) 800 cfm (377.6 L/s) Variations in the Centerline Velocity Unidirectional flow should ideally maintain consistent downward motion without any significant entrainment from the surrounding air. There are no direct measures to quantify the directionality of the unidirectional flows and entrainment from the surrounding air. However, variations in the magnitude of centerline velocity can indirectly help in estimating these parameters. Acceleration in the centerline velocity indicates entrainment from the surrounding air whereas the deceleration indicates lateral spread of the downward air jet and loss of air mass to the surrounding. Figure 10 shows variation of normalized velocity-a ratio of centerline velocity at a specific distance along the vertical centerline to the respective discharge velocity. This variation is plotted against the nondimensional vertical distance-a ratio of a vertical distance for the supply diffuser along the centerline to the height of the supply diffuser from the top surface of the table. Thus, a non-dimensional height of 0.0 is located 30 ASHRAE JOURNAL ashrae.org J U N E 2 0 19 at the supply diffuser where the value of non-dimensional centerline velocity is 1 and the non-dimensional height of 1.0 is located at the surface of the table where the value of non-dimensional centerline velocity is 0. The extent of acceleration in the centerline velocity depends on the thermal gradients in the space. Under isothermal conditions, however, as shown in Figure 10a, a slight increase in the centerline velocity indicates a small entrainment from the surrounding air. This can be attributed mainly to the momentum of the fast moving downward jet. Also as shown in this figure the centerline velocity starts decelerating at about 55% of the downward distance indicating loss of unidirectional motion, which is evident by the airflow patterns and velocity distribution as shown in Figures 2 and 3. Under non-isothermal conditions when the sensible heat sources are present at the table level, the centerline velocity shows consistent decline as the air moves downward indicating that under such situations thehttps://www.ashrae.org/