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Show motions associated with the low-pressure core are not established, and the rates of shear in the flame front, and hence NOx production, can be expected to depend on both Slp and cpo A second observation concerning the NOx emissions from the Cf4 flames is the distinct effect of nozzle diameter. From Fig. 10, we see that both the NOx emission magnitudes and trends -are quite different for the 3-mm and 10-mm nozzles. Furthermore, for de = 3 mm and Stp > 0.008, the emissions data exhibited a bistable behavior (i.e., the data fall randomly on either of two lines). The bistable behavior is attributed to the intermittent presence or absence of the swirl generated by the external boundary layer, rather than to the inherent experimental uncertainties. The intermittent appearance of the swirling flamelet could be observed visually for tests with the 3- mm nozzle, while this effect was not observed with the larger nozzles, where the momentum from the jet was much larger in comparison to that from the external boundary layer. The resolution of the influence of the external boundary layer is the subject of an ongoing investigation. We see also in Fig. 10 that the methane flames generally emitted less NOx than their conventional jet flame counterparts. Maximum reductions of approximately 25% are seen for both the 3-mm and 10-mm nozzle flames. It is interesting to note that, in the present investigation of unconfmed flames, the effects of jet precession on NOx emissions are less significant than in the confmed flames reported by Manias and Nathan (1993, 1994) and Nathan et ale (1992), where reductions in NOx of up to 75% were observed when precessing-jet burners were used in confmed-flow configurations typical of industrial practice, e.g., a furnace. It is well documented that the emission indices of free jet flames, where the quantity of air available is unrestricted and the flames radiate to cold surroundings, are relatively insensitive to fuel-jet initial conditions. For example, Lovett and Turns (1993) showed that pulsing of the fuel jet, while dramatically affecting the visual character of the jet flame, had little or no effect on NOx emission indices. Thus, it seems clear that confining the flow and restricting the total air available play important roles in the NOx production of precessing jet flames. Future efforts are planned to elucidate these roles. Liftoff Height Another effect of jet precession is to greatly reduce the liftoff height, h, of the flame. Measurements using visual and photographic techniques show that h is typically less than one nozzle diameter for all of the precessing flames investigated. Even though the magnitude of the fluctuations in h is of the same order as its mean value, comparisons can be made with simple jet flames investigated elsewhere. If the exit strain rate is defined conventionally as uJde, the liftoff 12 |