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Show whose lengths are of the order of twice the length of the blue cone, can be seen to flicker intermittently through the cone. By contrast, at high Strouhal numbers, Stp ~ 0.015, the flame is yellow and highly luminous, except for a small region near to the base of the flame that is blue (Fig. 4). While the initial spread of the flame is large, the spreading angle decreases with axial distance from the nozzle so that flame looks more like a broad conventional jet flame than an inverted cone. Such observations are also consistent with cold flow measurements, which have found that a low pressure zone and a reverse flow zone exists immediately downstream from the exit of the nozzle that act to limit the spread of the jet (Schneider et al., 1992, 1995). In addition to being longer than its low-Strouhal number counterpart, the high-Strouhal number flame appears to be dominated by turbulence of a much greater scale. The high-Stp flame has the appearance of wrinkled laminar flamelets, suggesting turbulence whose scale is much larger than the flame thickness. In contrast, the low-Stp flame has the appearance of a highly sheared reaction zone. Independent measurements of flames generated by the fluid-mechanical nozzle, which operates in the high-Stp regime, for both open (Newbold et al., 1994) and confined (Nathan et al., 1992) conditions, have shown that these flame are dominated by turbulence of a larger scale than exists in conventional jet flames. The visual observations, evidenced from Figs. 3 and 4 and as discussed above, show clear differences between the low- and high-Strouhal number flames. The results presented later in this • paper demonstrate that these visual trends correspond with clear differences in the emission characteristics of the flames. The transition between the two regimes, however, is gradual, and exists approximately within the range 0.005 < Stp < 0.015, for deflection angles between 30° and 60°. This range of transitional flows is comparabl~ t9 .~at. determin~d in c?ld flow e?Cperiments (Schneider et al., 1995). Flame Dimensions A precessing jet flame is both shorter and broader than a non-precessing jet flame. Figure 5 presents nondimensional visual flame lengths, LCvl de, for methane flames, and shows that unconfined precessing flames are typically 20-60% shorter than conventional jet flames. Figure 5 also shows that length-to-maximum-width ratios of precessing jet flames, LcvIW Cv, are about 400/0 smaller than their conventional counterparts. Propane flames (not shown) are also much shorter and relatively wider than comparable simple jet flames. We see also that lowest Strouhal number flames (Stp = 0.01) are significantly shorter than those at the higher Strouhal numbers. In 7 |