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Show Jet precession, the rotational motion of a jet about an axis which intersects the axis of its own centerline, has been shown to generate a wide range of mixing conditions (Schneider et al., 1995); these, in turn, are found to correspond to quite different flame types, depending predominantly upon the dimensionless frequency of precession. This dimensionless parameter is the Strouhal number of precession, (1) where f is the frequency of precession, de the exit diameter, and Ue the mean exit velocity (Schneider et al., 1992, 1994). In the lower Strouhal number flow regime, Stp < 0(0.01), the cold-flow mixing characteristics are somewhat analogous to that of a fully pulsed jet (Schneider et al.,1992). The time scale of the precession is large in comparison with the time scales associated with the movement of entrained ambient fluid, so that there are no significant asymmetries in the pressure field surrounding the local jet. Under these conditions, the dominant effect of the precessional motion of the jet is to superimpose an additional shear stress onto those existing in a stationary jet This precessional motion is illustrated in Fig. 1. In general, the additional shear scales with the product of the rotation rate and the sine of the deflection angle,~. As will be shown below, in the low-Stp regime, the flames are observed to be shorter and much less luminous than either conventional jet flames or those precessing in the higher Stp regime. They also have the appearance of being highly sheared and clearly do not generate the same large-scale mixing behavior which occurs at higher Stp. By contrast, for Strouhal numbers in the range, Stp ~ 0(0.01), the time scale associated with the precessing motion of the jet is comparable with time scales associated with the motion of · entrained fluid within the region of space through which the jet precesses. Under these conditions, a low-pressure zone is generated in the region between the jet and the nozzle axis, which also precesses with the jet Consistent with such a flow, large streamline curvature is observed, along with the rapid decay of mean velocity and the establishment of coherent motions whose scales are significantly larger than those inherent in a free turbulent jet (Nathan et al., 1992; Schneider et al., 1994). These modified mixing characteristics are found to influence both the radiation characteristics and the NOx emissions significantly for both open and confmed flames (Nathan et al., 1993; Nathan & Luxton, 1993). Measurements in industrial applications have demonstrated a reduction in NOx emissions of the order of 50-75% and a simultaneous increase in flame luminosity (Manias & Nathan, 1993, 1994; Nathan et al., 1992). The primary aim of the present research is to investigate the effects of precession 3 |
