OCR Text |
Show Ua = (Uinf*R2 - Uc*Rh2)/(R2 - R/) (4) Here U· f is the mean flow velocity deduced from the total flow rate measured by a , In turbine flow meter. Figure 3 shows the U profiles of the six conditions. With Uinfheld constant at 3.0 mis, the results indicate swid numbers ranging from 0.27 < Sv < 0.58 for the screens, Sv = 0.03 for the open case, and Sv = 0.67 for hub case. These U profiles are characterized by a uniform core flow region from -16 mm < r < 16 mm surrounded by the swided region where the flow velocity can be higher or lower than the core region depending on the amount of center blockage. As expected, when there is no screen, restriction in the annulus region forces the bulk of the flow to accelerate through the inner tube, resulting in .Uc much greater than Uinf. At the other extreme, with complete blockage of the inner tube, U c is negative, indicating that a recirculation zone has been generated downstream of the bluff body hub. This is in accord with the criterion that recirculation occurs when S 2: 0.6. For a 60% blockage, acceleration beyond Uinf still occurs as Uc is higher than Uinf = 3.0 mls. Further increases in blockage force more flow into the annulus such that Uc < Uinf· With a blockage of 70%, UiUa reduces to 0.5 and the calculated Sv = 0.48. Increasing the blockage further, to 75%, creates a situation where recirculation is imminent (Sv = 0.57). As this is undesirable for the WSB stabilization mechanism, increasing the blockage further was deemed unnecessary. The results of Figure 3 show that blockage of above 650/0 but-less than 75% would be appropriate for use in WSB. Compared in Figure 4 are isothermal velocity profiles generated by the earlier air-swider and by our new vane-swider with 65% blockage. The divergent flowfield produced by the air-swider under this condition is known to support lean premixed flames. It can be seen that the vane-swider produces essentially the same flowfield as that of the air-swider. The mean tangential velocity profiles show two distinct regions; an inner core where there is no tangential velocity component W and an outer annulus where W is present. U remains faidy constant across both profiles until dropping off toward the end of the flow region. As Sv is dependent on the ratio UiUa, and not sensitive to flow rate, it is not sensitive to U, varying Uinf rate should not affect Sv. Figure 5 shows U profiles for three 8 |