| The effect of the Droplet Size Distribution (DSD) in sprays and spray combustion has been studied experimentally by developing techniques in which the DSD can be changed as an independent parameter. By mixing the sprays from two plain-jet airblast atomizers, uniform resultant sprays could be synthesized which had the same Sauter mean diameter (SMD), and liquid-air ratio, but possessing different DSDs. Spray Quality has traditionally been attributed to some mean droplet size within the spray, such as the SMD, which is assumed to correlate with spray behavior. However, even among sprays which possess the same mean droplet size, considerable differences in spray behavior, such as evaporation and burning rates, may arise because of differences in their respective DSDs.; To examine the effect of the DSD on spray combustion, three fuel sprays were produced which had the same initial SMD and air-fuel ratio, but different DSDs. These sprays were then ignited and burned as they issued from a tube concentrically-located within a confined, co-flowing annular airstream. Variations in spray/flame structure resulting from changes in the DSD were determined by measuring droplet sizes and velocities, and gas temperature and composition within the reacting spray field. The results show that more complete combustion occurs in sprays possessing narrower DSDs, even with the SMD and fuel-air ratio held constant. Reductions in the burning efficiency of sprays with wider DSDs is attributed primarily to their greater relative populations of large fuel droplets, and also to droplet size-segregating effects occurring at the spray periphery which influence fuel-air mixing rates. The results establish the DSD as an important indicator of atomization quality and an independent characteristic of the spray. Furthermore, using only the SMD as an indicator of the atomization quality, or any mean droplet size in general, is insufficient to completely characterize spray and spray combustion processes. |
