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Show system was used, with optics positioned at 30° from the forward scattering angle. Additional information on the PDI arrangement used here is presented elsewhere (Presser et aI., 1994). Results were repeatable within a 5% margin for mean droplet size, and axial velocity. Measurements were taken from the flames at axial positions of z = 10 mm, 15 mm, 20 mm, 25 mm, 35 mm, 40 mm, 45 mm, 50 mm, and 60 mm from the nozzle exit. Downstream of z = 60 mm data rates were too low to provide statistically significant results. Data were taken at radial increments of 1.27 mm from the spray centerline at z = 10 mm. At other axial positions, the radial increments were 2.54 mm. Gas-phase samples were extracted from the flames with a stainless steel, water-cooled sampling probe. The gas samples were quenched in the probe by sudden expansion along with external water cooling. The probe has a diameter of 1.27 cm, a length of 84 cm, and a hemispherical tip. The sampling probe was oriented perpendicular to the longitudinal axis of the flame . Effects of probe orientation were not investigated. The gas samples were conditioned prior to introduction to the gas analysis instrumentation. Fi Iters and a membrane gas dryer were used to remove particulates and water vapor, while a heated sample line (maintained at 93°C) prevented condensation of gases . A vacuum pump, located downstream of the instrumentation, was used to extract samples from the flames and draw the sample through the sampling train and instrumentation. The gas sampling probe was periodically purged using compressed air to remove the accumulated soot and condensed liquids within the probe. A Fourier Transfonn Infrared (FfIR) Spectrometer with a continuous flow gas cell (10 cm path length; KBr windows) was used to obtain absorption spectra of the gas samples extracted from the spray flames . The arrangement penn its continuous passage of combustion gas samples through the gas cell. The absorption spectra were used to determine chemical species composition within the spray flames. A continuous, chemiluminesent analyzer was used to measure NOx concentrations in the gas sample. Radial profiles of the gas species concentrations were taken at axial positions of z = 150 mm, 200 mm, 300 mm, 400 mm, and 700 mm. RESULTS The effect of fluid velocity modulation in a pressure-jet on droplet atomization, transport, combustion, and combustion product concentrations are described in the following section. The paramount interest is to elucidate how changes in velocity modulation affects the spray structure and how the spray characteristics subsequently influence the concentration levels of the combustion products. Observed Flame Features Velocity modulation is observed to change significantly the appearance of both the spray and the resulting flame . As noted previously, the spray is found to become narrower for increasing velocity modulation, with the frequency of 9 .0 kHz producing the narrowest spray. The flame plume also becomes narrower for the velocity-modulated sprays. The flame length was almost twice as long as the unmodulated flame (i .e., 800 mm versus 450 mm, respectively). In addition , the flame front of the unmodulated flame was positioned at the face of the nozzle, while the standoff distance for the two velocity-modulated cases was approximately 20 mm to 50 mm. In the unmodulated case, unburned droplets were observed to escape from the edge of the spray flame. Spray Characteristics The visually observed spray features are consistent with results obtained from the phase-Doppler measurements. For example, the fuel volume flux profiles shown in Fig. 3 provide an indication of how the fuel mass is dispersed within a spray. Volume flux was calculated from measured droplet size, velocity, and number density. With no velocity modulation, the spray is not only wider, but most of the fuel volume is located within a narrow region defined the spray boundary (location of maximum fuel volume flux . For the 11.8 kHz and 9 .0 kHz cases, the spray boundary shifts progressively closer to the spray centerline The variation of droplet Sauter mean diameter (D32) with radial position is presented in Fig. 4 for three selected axial positions (z = 10 mm, 25 mm, and 40 mm) for the base case (i .e., without velocity modulation), and with velocity modulation at 9 .0 kHz and 11.8 kHz. As is characteristic of pressure-jet nozzles, the droplet mean diameters are smallest near the centerline and increase with radial distance from the nozzle. For r < 5 mm at z = 10 mm, droplet mean sizes in the unmodulated base case are generally smaller than those of the modulated cases. The 9.0 kHz modulation frequency produces the largest mean droplet size in this region. The maximum mean droplet size at z = 10 mm, on the other hand, is largest (D32 approximately 54 f.1m) near the spray boundary with no velocity modulation. By comparison, the maximum mean droplet sizes in the 9.0 kHz and 11.8 kHz cases are 39 f.1m and 41 f.1m, respectively. In addition, the droplets produced by the 9.0 kHz modulation frequency (in contrast to 11 .8 kHz) at z = 10 mm are confined to a more narrow region. The location of the maximum mean droplet size corresponds with the spray boundary. The maximum mean size occurs at about r = 5 mm for the 9 .0 kHz case. This position is shifted radially outward to about r = 7.5 mm for the unmodulated and 1l .8 kHz cases . The overall trends at other axial positions are similar to those observed at z = 10 mm. In particular, the unmodulated flame is the widest. In general, the Sauter mean diameter increases with increasing distance downstream of the nozzle exit for the unmodulated case. This result is expected because of the preferential vaporization of the smaller droplets as compared to the larger size droplets, thus shifting the size distribution toward larger droplets and increasing the mean size (Presser et aI., 1993). Downstream of z = 10 mm, the absence of data for the base case near the center of the spray is indicative of the low data rate. As shown in Fig. 5, the baseline case consistently provides the largest droplet mean size at the spray boundary, followed by the 1l.8 kHz and 9.0 kHz cases. Recall also that the spray boundary for the base case is displaced radially outward as compared to the unmodulated cases. |