1645 Twenty-Sixth Symposium (International) on Combustion/The Combustion Institute, 1996/pp. 1645–1651 GAS PROPERTY EFFECTS ON DROPLET ATOMIZATION AND COMBUSTION IN AN AIR-ASSIST ATOMIZER ROBERT AFTEL and ASHWANI K. GUPTA Department of Mechanical Engineering University of Maryland College Park, MD 20742, USA CHARLES COOK and CARY PRESSER National Institute of Standards and Technology Gaithersburg, MD 20899, USA Nitrogen, Ar, and CO 2 were used as the atomizing gas in an “air-assist” fuel nozzle to determine the effect of these gases on droplet size, number density, and velocity in kerosene spray flames using a two- dimensional phase Doppler interferometer. Data were obtained with these atomizing gases and compared to the reference, air-assist case, since air is the most commonly used atomizing fluid. Comparisons were made between the gases on a mass and momentum flux basis. Both burning and nonburning sprays were investigated. The results show that under burning conditions, the presence of O 2 in the air-atomized sprays influences the combustion process, reducing droplet size and increasing droplet velocity, especially in the region near the nozzle exit. Differences in droplet characteristics (size, velocity, and number density) are minimized in the momentum flux controlled cases, indicating the relevance of momentum flux over mass flux to control atomization. In both the nonburning and burning sprays, lighter gases more effectively atomized the fuel in comparison to the denser gases. Ar and CO 2 produced larger, slower moving droplets than the air and N 2 assisted cases under mass flux controlled conditions. The Ar and CO 2 atomized flames were also observed to be significantly more luminous than air-atomized flames. These results suggest that the presence of O 2 in the atomizing gas and gas density have significant effect on the atomization, mixing, and combustion processes, which, in turn, influence droplet lifetimes, flame structure, and emission levels. Introduction Air-assist nozzles are used in propulsion and power generation applications to atomize bulk liquid fuel into a multitude of droplets. Knowledge of the droplet size distribution is important for any com- bustor since this affects pollutant formation and combustor efficiency. Several studies have been car- ried out on air-assisted atomization of liquids [1–4], but little information exists on the advantages of us- ing gases other than air to influence spray flame characteristics. The purpose of this study was to in- vestigate, using phase Doppler interferometry, the effect of N 2 , Ar, and CO 2 on the atomization of ker- osene in an air-assist atomizer. The objective was to determine whether these gases can influence atom- ization as a result of their specific heat or density. Table 1 gives a summary of the properties of the gases used. To compare one gas with another, it was important that either the mass or momentum flux of the gas be held constant. Previous studies have suggested that both mass and momentum flux may affect the drop- let atomization processes [5]. A semiempirical rela- tionship for droplet mean diameter in air-assist at- omizers, derived by Wigg [6], suggests that droplet size is dependent on atomizing gas velocity (relative to the fuel velocity), density, and mass flow rate. These results indicate that as atomizing gas velocity is increased, droplet size decreases. To maintain con- stant mass or momentum flux as gas density is in- creased, the atomizing gas velocity must decrease; therefore, increased droplet sizes are predicted. Both nonburning and burning sprays were inves- tigated while maintaining either the mass or mo- mentum flux of the atomizing gas constant. Air was considered as the baseline case. Since N 2 has a simi- lar density to air, one case was carried out for N 2 representing both the mass- and momentum-con- trolled cases. Ar and CO 2 were examined separately under constant mass and momentum flux. Experimental Apparatus Experiments were carried out using the spray combustion facility described in Ref. 7. The spray burner is mounted on a stepper-motor–driven,