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Show fitted with orifices of various sizes and shapes. At lowamplitude modulation, the atomizer behaved similar to other electro-mechanical droplet array generators (Dressler, 1991). Under these conditions, the fluid jet becomes unstable when the disturbance wavelength is sufficiently long, and a stream of uniformly sized droplets is generated. On the other hand, when driven with high-amplitude modulations, these atomizers produce sprays with wide droplet size distributions~ however, the Sauter mean diameter of these distributions is much smaller as compared to the jet diameter (Takahashi et aI., 1994). Takahashi et al. (1995) have characterized the spray produced by a pressure-swirl atomizer driven by a high-amplitude velocity modulator. In this case, the velocity of the fuel stream is perturbed and forms a hollow-cone liquid sheet. The authors propose that, with high-amplitude modulation, results from different segments of the fuel stream having different velocities. Thus, segments of the liquid sheet collide, forming ligaments and ultimately droplets. This process (termed "velocitymodulation atomization"), unlike conventional pressure-swirl atomization, is independent of natural flow instabilities~ it results in more uniform atomization with fewer clusters of droplets. The authors also report that it is possible to affect the characteristics of the spray by changing the modulation frequency. When the piezoelectric crystals are driven at the resonant frequency of the driver, the atomizer produces a narrow spray with greater droplet velocities than the spray without velocity modulation. The voltage appl ied to the driver yielded less influence on the atomization process over the range of voltages studied than the driving frequency. Overall, the addition of modulation reduced mean droplet diameters between 8 and 18%. With high-amplitude modulation, it was also possible to maintain atomization at lower fuel pressures (or lower fuel flow rates) than without velocity modulation, thus increasing the tum-down ratio of the atomizer (Chung et aI., 1996). These results with high-amplitude, velocity-modulated atomizers suggest potential appl ications of velocity modulated atomization in industrial systems. In particular, if the dispersion of liquid droplets can be controlled, it may be possible to produce sprays with desirable characteristics for combustion systems. For example, by controlling particle trajectories it may be possible to affect fuel/air mixing characteristics. In addition, by reducing the droplet sizes, the evaporation of fuel is enhanced. These characteristics may, in turn, affect the combustion chemistry and the emission products formed during combustion. While the effects of velocity-modulated atomization on spray characteristics have been stud ied, it is unclear how these characteristics affect the combustion process. The purpose of this study is to characterize sprays produced by a velocitymodulated, pressure-jet nozzle and to relate the spray features to the combust ion process. Measurements of droplet size and velocity, and gas-phase chemical compositions are presented at two different atomizer driving frequencies. These results are compared to the results obtained for a base case without velocity modulation. The effects of velocity modulation on both the spray characteristics and the chemical composition of the combustion products are investigated in this study. Experimental Apparatus and Procedure The velocity-modulated, pressure-jet atomizer was operated in a spray combustion facility described by Presser et al. (1993). The facility, shown schematically in Fig. 1, includes an industrial-type swirl burner that allows examination of the resulting flow field under a variety of operating conditions. The combustion air is introduced through a 100 mm outer diameter annulus that surrounds the centrally located fuel nozzle. A movable-vane cascade is used to impart the desired swirl intensity to the combustion air stream . The burner is mounted on a three-dimensional, stepper-motor-driven traverse that permits independent movement of the burner relative to the fixed optics and sampling probe (and hence changes the probe volume location) . The velocity-modulated atomizer used in this study is shown in Fig. 2. The atomizer consists of a nominal 1 gph (commercially available) pressure-jet nozzle that is mounted to a piezoelectric driver (Dressler, 1991). The driver assembly consists of a piston driven by a series of four piezoelectric crystals. The piezoelectric crystals and the piston are connected by a centrally positioned, hollow bolt. The fuel flows through this bolt and into the pump formed by the piston and the surrounding housing. Axial movement of the piston, initiated by an electrical signal at the desired frequency to the piezoelectric crystals, imparts a velocity perturbation to the fluid flowing from the nozzle in a manner analogous to a modulated pump. Further design details of the piezoelectric-driven modulator are given by Dressler (1991) and Takahashi et al. (1995). A function generator was used to produce a sinusoidal voltage input signal with specified frequency to the piezoelectric driver. The voltage signal was amplified, and a phase-matching transformer was used to further increase the applied voltage. The applied voltage was measured using an oscilloscope. For the experiments carried out in this study, the flow rate of the combustion air was 0 .042 kg/s, and of the fuel (kerosene) was 0.0011 kg/so The swirl number of the combustion air, calculated using the method of Gupta et al. (1984), was 0.3. All of the system operating conditions remain unchanged except for the driver, which was operated under three conditions. The base case was defined by operating the atomizer without any power to the piezoelectric driver. Measurements were also carried out at 9.0 kHz and 11.8 kHz driving frequencies using 1000 V peak-topeak voltage applied to the piezoelectric crystals. These driving frequencies were selected because of the dramatic changes in the observed spray structure, apparently due to changes to the piezoelectric transducer response . The lower frequency produced the most narrow spray structure, while the higher frequency produced a somewhat wider spray . The structure of the unmodulated base case spray, by comparison to the two modulated cases, was significantly wider. A two-component phase Doppler interferometer (PDI) was used to measure the distributions of droplet size and radial and axial velocity components. Mean properties such as Sauter mean diameter, number density, and volume flux, were then statistically derived from the individual droplet data. Ten thousand val idated droplet measurements were made at each sampling location, and the optimum data rates were determined as discussed by Presser et al. (1994) . An off-axis light collection |
