| OCR Text |
Show Velocity modulation also affects the dispersion of droplets as shown by the radial profiles of droplet number density in Fig. 6. Maxima in droplet number density correspond approximately to the position of the spray boundary. For the two modulated cases, the maximum droplet number density occurs toward the spray centerline. In contrast, the number density for the unmodulated spray reaches a maximum at larger radial positions. Although the droplet number density for the unmodulated case is lower than that of the modulated cases near the spray edge, the larger droplets in the unmodulated case produce a higher volume flux in this region. So again, velocity modulation directs large numbers of droplets toward the spray centerline. The variation of droplet mean total velocity (defined as the vectorial sum of the axial and radial velocity components) with radial position is presented in Fig. 7 . Without modulation, the droplet velocities at z = 10 mm show features typical of hollowcone nozzles, i.e., the droplet velocity is low at the centerline and increases toward the edge of the spray. At the 11.8 kHz modulating frequency, the results are similar, although velocities near the centerline are greater than those for the unmodulated case. At 9.0 kHz, on the other hand, centerline droplet velocities are significantly greater than the other two cases and decrease toward the edge of the spray. As was the case with the droplet size data, the peak in the velocity profile shifts radially toward the centerline for the velocity-modulated cases. Similar trends in droplet velocities are observed at the other axial locations. As shown in Fig. 8, the droplets near the spray boundary decelerate as they interact with the surrounding gases with increasing axial distance from the nozzle. The fastest droplets for the unmodulated base case, as expected, maintain the higher velocities near the spray boundary. In summary, the effects of velocity modulation are clearly evident in the droplet size, velocity, and dispersion data. Velocity modulation produces a more narrow spray with less radial variation in mean droplet size, number density, and fuel volume flux as compared to the unmodulated case. In addition to The piezoelectric driver appears to impart energy to the fuel stream, resulting in higher initial droplet velocities in the central region of the spray. The effects of velocity modulation are enhanced at 9.0 kHz relative to 11.8 kHz, due to the change in piezoelectric response with applied frequency. Thus, there is a significant increase in the number of droplets are directed from near the spray boundary toward the center of the spray in the velocity-modulated cases relative to the unmodulated base case. This results in an increase in both droplet mean size and velocity near the spray centerline for the modulated cases. In contrast, the highest droplet velocities in the unmodulated case occur near the defined spray boundary, where larger droplets predominate. This change in spray structure with velocity modulation also results in a longer flame plume. It would also be expected that the distribution of chemical species would change spatially in a sim ilar manner. Gas Species Concentrations The effect of velocity modulation on combustion product formation and distribution now presented. FnR absorption spectra measured for samples from within the flames in all three cases showed the presence of carbon dioxide (C02) and carbon monoxide (CO), as well as unburned gaseous fuel and intermediate hydrocarbons including methane (CH4), ethylene (C2H2), and acetylene (C2H4). Concentrations of C02 and CO in the extracted gas samples were quantified from the FllR absorption spectra using calibration gas absorption spectra. The axial variation of CO along the spray centerline are presented in Fig. 9. Concentrations of CO generally decrease with increasing axial distance from the nozzle. This decrease results from oxidation of CO and from mixing of the combustion products with other gases (such as combustion air), and thus dilutes the CO. The variation of CO concentration with radial position is shown in Fig. 10. This figure indicates that carbon monoxide concentrations are lowest for the unmodulated base case, except at z = 200 mm where the values of CO are higher near the centerline. It is important to recognize, however, that at z = 400 mm the sampling location is near to the tip of the unmodulated flame, while approximately half of the velocity-modulated flames extends beyond this sampling location . Carbon monoxide concentrations for the 11.8 kHz case are generally higher than those for the 9.0 kHz case near the centerline, while CO concentrations for the 9.0 kHz case are greater toward the outer edge of the spray. The variation of C02 concentration with axial position is shown in Figure 11. As the axial distance increases, carbon dioxide fractions first increase due to the CO oxidation to C02 and then decrease due to the dilution effects. The variation of C02 concentration with radial position is presented in Fig. 12 for several axial positions. The general trends described for the CO concentrations also apply to C02. That is, except near the centerline, C02 concentrations are higher in the velocitymodulated cases as compared to the unmodulated base case. This is due to the significantly longer flame structure resulting from velocity modulation . For a given axial location of the sampling probe, the combustion products are more fully diluted with entrained combustion air in the unmodulated case. To further illustrate the effects velocity modulation on the flame structure, the ratio of CO volume fraction to C02 volume fraction is shown in Fig. 13. The CO/C02 ratio is an indicator of the extent of chemical reaction . As CO is oxidized to C02, this ratio will decrease. In the complex turbulent flames, as examined in this study, the CO/C02 ratio is also influenced by the amount of recirculated CO and C02 to the sampling point. It is, nonetheless, a convenient measure of the extent of oxidation of the gases present at the sampling location. The CO/C02 ratio decreases most rapidly with increasing axial distance in the unmodulated case. The CO/C02 ratio for the 9.0 and 11.8 kHz cases are generally greater than those of the base case. Thus, using only the CO/C02 ratio as an indicator, combustion appears to progress toward completion faster in the unmodulated base case, consistent with the shorter flame length in this base case. Again note, however, that the z = 400 mm sampling location is near to the tip of the unmodulated flame, while, since the velocity-modulated flames are much longer, the sampling probe is near the center of modulated flames. While CO and C02 are important indicators of the completeness of the combustion process, it is important to consider the production of other chemical species. Other major |
