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Show cases with increasing Uinf (3.0, 5.0, and 7.0 mls) for a 37 degree swirler with a blockage of 65% in the inner tube. The measurements here were taken without the use of an exit tube (R. = ° cm). As expected, the U profiles for the three mean velocities have the same shape, even duplicating the deficit at Rh. Calculating Sv shows some variance with Dinf, but no consistent trend is observed. Operation of Burner with Vane-swirler Of course, the most important issue is how does the vane-swirler perform in WSB. This was evaluated by determining the flame stabilization limits of burners with exit tube R. = 7 cm, fitted with a 37° vane-swirler and different screen blockages. The flow supply and monitoring system for the burner is identical to that described in Yegian and Cheng2 . Firing rate and ~ are determined by two turbine meters measuring separate natural gas and air flow rates, and the WSB was operated open to the atmosphere. Figure 6 show the stable operating range for the four different screen blockages; 60%, 65%, 70%, and 75%. To obtain the conditions at blowoff and flashback, Uinf was held constant while the ~ was varied. This resulted in the upper dotted lines which represent the flashback limit for each screen, and the lower solid line that denotes the blowoff limit. The regions between the two limits are where we found stable operation. For each of the foUr screens, the blowofflimits remain rather independent OfUinf . There is a general lowering of ~ at blowoff with increasing blockage. At blockage of 60%, blowoff occurs at ~ ~ 0.70, while it lowers to ~ ~ 0.55 for a screen blockage of75%. As expected, flashback limits increased with increasing Uinf as the flow velocity becomes significantly higher than the flame speed. For all cases, the data trends indicate that the flashback limit would be beyond ~ = 1.0 around Uinf~ 3.5 - 3.75 mls. As the maximum natural-gas/air flame speed occurs at ~ ~ 1.0, these results suggest flashback should not be a problem for operating conditions with Uinf> 3.75 mls. Thus, the vane-swirler offers a wide operating range from stoichiometric through the lean regime and down to the blowoff limit. Although our results show that screens with high blockage have a broader operating range, we also observed that for blockages> 70%, the flame begins to change into an inverted cone where the apex may be situated within the exit tube. To produce the flat, 9 |
