OCR Text |
Show jet flame strain rates and scales of the turbulent motions were achieved using fuel-jet precession, the rotational motion of the fuel jet about an axis which intersects that of its own centerline. Controlling the jet precession enabled the characteristic scales of turbulence and shear rates to be varied independently from jet diameter, jet velocity (and, hence, heat release), and fuel type. From these experiments, we conclude the following: 1. Two broad regimes of flame type exist for precessing jet flames. For Stp < 0.01, flames are non-luminous and, in comparison with conventional flames, are short and broad. For Stp > 0.03, flames are highly luminous, and of a shape which is between that of a conventional jet flame and the low-Stp flame; the transition between these two regimes is gradual. 2. In general, radiant fractions increase with precession Strouhal number, in correlation with the visual increase in flame luminosity. 3. N02-to-NOx ratios near unity result for flames operating in the low-Stp regime. Concomitantly, CO emisions are relatively high in this regime. These results are attributed to the presence of small quantities of unburned hydrocarbons associated with the very high mixing rates of the low-Stp regime, which promote oxidation of NO to N02 and the formation of CO. At higher Strouhal numbers, both N02-to-NOx ratios and CO emission indices fall and approach the values typical of conventional jet flames. 4. NOx emission indices are influenced by jet precession and found to be functions of Stp. The particular dependence on Stp varies with fuel type (methane or propane), nozzle size (3 mm or 10 mm), and jet deflection angle . . The emission indices are related to the trends of global residence time and radiant fraction associated with these parameters. NOx emission indices are found to be somewhat lower than those of conventional jet flames, with the largest reduction being approximately 25%. 5. Jet precession causes a reduction in liftoff height by a factor of 5 to 10 relative to a conventional jet flame. This result further emphasizes the dramatic effect of jet precession on the mixing characteristics of the flame. |