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Show EFFECT OF ACOUSTIC ATOMIZATION ON COMBUSTION EMISSIONS Charles A. Cook, S. Rao Charagundla and Cary Presser Chemical Science And Technology Laboratory National Institute Of Standards And Technology Gaithersburg, M D John L. Dressler Fluid Jet Associates Spring Valley, OH Ashwani K. Gupta Department or Mechanical Engineering University of Maryland College Park, M D ABSTRACT Measurements of droplet characteristics and gas-phase chemical composition are reported for spray [lames produced by a velocity-modulated, pressure-jet atomizer. A piezoelectric driver was used to modulate the velocity of kerosene fuel through a commercially available pressure-jet nozzle. Phase Doppler interferometry was used to measure droplet size and velocity in the spray flames. Gas samples were extracted from the spray flames, and concentrations of chemical species were determined using FTIR spectroscopy and a chemiluminescent NOINOx analyzer. Experiments were performed at two di fferent piezoelectric driving frequencies and compared to a base case without velocity modulation. The effect of velocity modulation on both the spray characteristics and chemical product formation were studied. The results show that velocity modulation decreases overall droplet size and increases mean droplet velocity and number density near the spray. These characteristics increase the flame standoff distance and flame length. However, the spray characteristics resulting from velocity modulation decreases the number of droplets escaping the spray flame unburned. INTRODU eTlON Control of the atomization process of liquid fuels is important in many combustion systems because of its effect on system performance and emission levels. Applications, such as aircraft engines, power generation, and chemical waste treatment systems, require effective atomization to be maintained over a wide range of fuel flow rates in order to ensure efficient combustion at all operating conditions. The size, trajectory, and momentum of the liquid droplets significantly affect fu e l vaporization rates, the mixing process between fuel and oxidizer, combustion efficiency, and emission of pollutants. Therefore, new strategies are sought to provide more control over the droplet behavior in [lames . One of the methods used to control liquid atomization is to introduce disturbances in the liquid [lowing through a nozzle or an orifice. It is well established that a stream of liquid will break into droplets if it is subjected to a disturbance with a wavelength greater than the jet circumference (Rayleigh criterion) (Dabora, 1967). Electro-mechanical devices are often used to drive this instability in a repeatable manner so as to generate monosized droplet arrays. For example, Ashgriz and Yao (1987) developed a droplet generator in which a piezoelectric-driven diaphragm was used to produce pressure perturbations in a reservoir that feeds an array of orifice nozzles. In other devices, the nozzle itself is vibrated at a prescribed frequency (Dabora, 1967; Strom, 1969; Schneider and Hendricks, 1964; Switzer, 1991). These devices produce perturbations with relatively small amplitudes, and drive only the Rayleigh instability of the liquid stream. While such methods produce very uniform droplet arrays, the mlntmum droplet size that can be generated is approximately twice the orifice diameter. In contrast, a high-amplitude, velocity-modulated device, such as that described by Dressler (1991), can produce unstable [lows at other frequencies. In this design, an electro-mechanical piston is used to impart large velocity disturbances to the velocity of the fuel stream. This process is referred to "velocity-modulation atomization" (Takahashi et aI., 1995). Depending on design and operating parameters, the magnitude of these perturbations may approach the magnitude of the average fuel velocity (Takahashi et aI., 1995). Using disturbances with such large amplitudes, unstable flows can be produced with wavelengths shorter than those required by the Rayleigh criterion. Thus, the size of droplets produced by high-amplitude, velocity-modulated atomizers is not necessarily limited by the orifice size. Atomization with a larger orifice to accommodate higher [low rates without plugging is therefore possible with high amplitude, velocity-modulated atomizers (Chung et aI., 1996) . Dressler et al. (1991,1993) and Takahashi et al. (1994) have characterized the performance of velocity-modulated atomizers |
