OCR Text |
Show addition, the axial location of the widest point of the flame was observed to be much closer to the nozzle tip for the lowest Stp flames in comparison to the higher Stp flames. The largest scale of mixing within a stationary jet flame is its cross-stream dimension, which can be approximated by the visually detennined flame width, WCv. Mungal et ale (1991) have used the volume rendering technique to show the importance of such large structures in the dynamical behavior of jet flames by means of visual images. Newbold et al. (1994) have used that technique to investigate the large-scale flame structures in the high-Stp regime of precessing jet flames. That work demonstrates that, in comparison to non-precessing jet flames, the structures are larger and fewer in number along the length of the flame, consistent with smaller LcvIW tv ratio measured here. Such data constitute strong evidence that the dominant mixing scale is comparably larger than in conventional jet flames and suggests that the characteristic flame stretch is lower for the high-Stp regime flames. The mixing characteristic for the low-Stp flames, however, are decidedly different Cold flow data that are available for low-Stp jets (Schneider et al., 1992) suggest that such flows do not possess coherent motion at the scale of the flame width. The observations that the low-Stp flames are the shortest, appear to be highly sheared, and possess different emissions characteristic (presented below), suggest that the flame stretch is likely to be higher, and the characteristic residence times shorter, than for an equivalent conventional turbulent jet flame, contrary to the high-Stp precessing jet flames. Global residence times and radiant fractions Global residence times were calculated from the visually determined flame dimensions following the method described by Turns and Myhr (1991) and are presented in Fig. 6 for methane and propane flames. For both fuels and nozzle diameters, residence times, tG, increase quite rapidly with Strouhal number. For the methane flames with the 10-mm diameter nozzle, residence times peak in the high-Slp regime and then fall again with increasing Stp. Radiant fractions are shown in Fig. 7. The general trend for most of the results is that the radiant fraction increases rather rapidly with Strouhal number, which is consistent with both the change in visual appearance of the flame from low to high luminosity and the increased residence times with increasing Stp. The highest velocity (de = 10 mm) CR4 data, however, fall slightly with Stp, and the other de = 10 mm radiant fraction data for CR4 drop at the highest Stp. Radiant 8 |