- Travel grouping of particles/droplets in an oscillating flow is described. The tendency for non- grouping behavior is characterized by a new non-dimensional parameter designated as NG. A characteristic time for grouping is also presented. The prediction of grouping tendency and the characteristic time are incorporated in a spray evaporation model for estimating the effect of grouping on the overall vapor production of a multi-sized spray. It is shown that the relatively small droplets tend to form groups and are thus expected to undergo a decrease in their evaporation rate. Larger droplets tend not to group, thus their rate of evaporation is maximized. Therefore, such travel grouping brings the evaporation rates of different droplet size ranges closer together, implying that an optimized fuel spray injected into a combustion system is not necessarily characterized by the smallest Sauter mean diameter (SMD). It is also shown that once a grouping or a non-grouping mode of behavior has been established, evaporation has only a minor effect on this behavior during droplet lifetime. Thus, the initial operating conditions, which are incorporated in the initial value of NG, can predict the extent of grouping stability.