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Show species present in the tlames include methane, ethylene, and acetylene. These data, similar to the CO concentration measurements, show that intermediate species concentrations at a particular axial position are generally higher in the velocitymodulated cases as compared to the unmodul3ted case due to the longer region of reacting constituents. Concentration of NOx is also an important consideration in many industrial applications. The variation of NOx concentration with spatial location is presented in Fig. 14. NOx concentrations are highest near the spray centerline and decrease toward the edge of the tlame. At z = 400 mm, NOx concentrations in the velocity-modulated cases are similar, but greater than those for the unmodulated case. However, NOx concentrations for the modulated cases decrease significantly between z = 400 m and z = 700 mm. This as axial position increase for each case, the NOx levels decrease in a similar manner until fully diluted by the surrounding air. DISCUSSION To explore the significance of velocity modulation on the combustion process, one would like to relate changes in the spray structure to the observed tlame features and measured species concentrations. In this way it is possible to determine how velocity modulation of the fuel stream affects 1) tlame structure and stability, (2) local fuel/air mixing and stoichiometry, 3) spatial distribution of chemical species, and 4) ultimate emission of unburned fuel and gaseous products of combustion. For the base case without velocity modulation, the spray tlame exhibited features typical of hollow-cone, pressure-jet atomizers. Specifically, the majority of fuel is concentrated near the spray boundary, and a much lower concentration of fuel is present near the spray centerline. The values of droplet mean diameter and velocity are also smaller at the centerline and increase toward the spray boundary. Droplet recirculation (presence of negative axial velocities) is evident from the axial velocity distributions even though the mean values are always positive. The short standoff distance of the tlame suggests that high temperature combustion gases are also recirculated near the nozzle. Regarding CO and C02 concentrations, the highest concentrations of CO and CO2 are formed at z < 200 mm in this central region of the tlame and rapidly decrease as CO oxidizes to C02. The velocity-modulated cases, in contrast, did not show any evidence of droplet recirculation near the centerline. In these cases the droplets in this region have sufficient momenta that reduces the probability of droplet entrainment into the gaseous recirculation zone and increases the tlame standoff distance. The high velocities near the centerl ine are consistent with results presented by Takahashi et al. (1995). These authors attribute the high droplet velocities in this region to the pumping action of the piezoelectric driver. Higher velocities and reduced mixing with the combustion air in the velocity-modulated cases cause the evaporation and combustion processes to occur over a longer distance relative to the unmodulated case. Thus, the combustion process is initiated further downstream, and CO concentrations are higher at a given sampling location as compared to the unmodulated case. It is also important to note that the high-velocity droplets in the modulated cases are directed toward the center of the spray, while droplets in the unmodulated spray have higher velocities toward the periphery of the spray. This feature is expected to increase the residence time within the tlame as opposed to following a ballistic trajectory through the tlame sheet and into the surrounding environment with little vaporization (Presser et aI., 1993). Emission of unburned liquid fuel from the tlame, practically, can lead to reduced combustor performance and increased emission in industrial reactors like rotary kilns and thermal oxidizers. Thus, while the evaporation and combustion processes without velocity modulation require less axial distance to achieve the same CO/C02 ratio, it also appears that less liquid fuel is being oxidized in this tlame. In the velocity-modulated cases, droplets are directed along the spray centerline and, have a lower probability of leaving the tlame unprocessed for the swirl number examined in this study. The distribution of fuel droplets can, of course, be controlled further by changing the tlow aerodynamics of the tlame through changes in swirl and distribution of combustion air. Therefore, the longer tlame lengths in the modulated cases are attributed to higher droplet velocities and to reduced droplet dispersion. One may argue that the modulated sprays decrease fuel/air mixing, but, on the other hand, increase the total amount of fuel burned in the tlame. SUMMARY Velocity modulation has a dramatic effect on spray tlame structure. It produces a narrow cone spray with smaller, highervelocity droplets concentrated closer to the spray centerline. The effects of velocity modulation are enhanced at 9.0 kHz relative to 11 .8 kHz. This may be due to a greater modulation amplitude at 9.0 kHz produce by the piezoelectric response at this frequency. The tlame structure thus becomes longer with an increased standoff distance. These results suggest possible benefits from the use of velocity-modulated atomizers. The control of droplet trajectories may provide a means to optimize fuel/air mixing, reduce the amount of unburned liquid leaving the spray tlames, and hence improve overall combustion efficiency and reduce emissions. ACKNOWLEDGMENTS The authors would like to acknowledge C. Connon for performing preliminary experimental work which was useful in planning and executing the current work. The postdoctoral fellowship support from the National Research Council for one of the authors (CAC) is gratefully acknowledged. The capable technical support of B. Shomaker is also appreciated. |