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Show parameters and jet size on the NOx and CO emissions from free, precessing jet flames and compare the characteristics of such flames with their simple jet flame counterparts. EXPERIMENTAL METHODS Precessing Jet Nozzles A precessing jet flow can be generated either by an axisymmetric nozzle which utilizes a natural fluid flow instability, as shown in Fig. 1 (Nathan, 1988; Hill et al., 1992; Nathan et al., 1995), or by a motor-driven mechanical nozzle (Fig. 2). While the simplicity and utility of the self-excited fluid mechanical nozzle makes it more relevant to industrial application, the greater complexity of its flow, relative to the mechanical nozzle, makes rigorous investigation much more difficult. For example, the jet which leaves the fluid-mechanical nozzle is non-circular, nonuniform and exhibits wide variability in its characteristic exit angle and precession frequency. By contrast, all of the characteristic parameters in the mechanically-rotated nozzle are well known and controllable. A simplified schematic of the flowfield within the fluid mechanical nozzle is shown in Fig. 1. The fuel passing through the sudden expansion reattaches asymmetrically to the inside of the cavity wall and exits the cavity at a large angle (typically 60° from the nozzle axis) due to strong local pressure gradients. The entire flowfield, including the point of reattachment and the deflected exiting jet, precesses azimuthally at a characteristic Strouhal number of 5.10-3 based on the height of the step at the sudden expansion and the mean velocity at the minimum section of the expansion (Nathan & Luxton, 1993; Nathan et al., 1995). It is inevitable that a swirling boundary layer be generated on the external surface of the mechanical nozzle, the effect of which will be superimposed on the effect of jet precession. It was found, however, that the flame which prevails when the swirling boundary layer becomes significant is easily recognizable as a long, yellow, spiraling flame whose base is attached to the surface of the nozzle. The spiraling flame may either be superimposed on the precessing flame or, in extreme circumstances, dominate it entirely. Such flames were easily avoided by appropriate choice of operating conditions when using the 10 mm-diameter nozzle tips. The influence of the external boundary layer, however, is difficult to avoid entirely with flames from the small (de = 3 mm) nozzles. The stationary sleeve shown in Fig. 2, designed to minimize the boundary-layer effect, was not used in the present investigation. 4 |