Interference effects during burning in air for stationary n-heptane, ethyl alcohol, and methyl alcohol droplets

摘要

Experiments have been conducted for the determinationudof the evaporation constant and flame shapes of two andudof five closely spaced droplets burning in air. Droplets ofudapproximately the same and of different diameters wereudused at various distances between the droplet centers.udThe apparent flame shape, which was observed only for n-heptane droplets, changes very little during burning.udThe square of the droplet diameter decreases linearly withudtime for fixed spacing between droplet centers, at leastudwithin the experimental limits of accuracy. In general,udthe average evaporation constant for two droplets, K',udmust be assumed either to vary continuously during burningudor else to be a function of average initial drop diameter,udD^0. The change of K' with time corresponds to the secondudderivative in plots of the square of the diameter vs. time.udThese second derivatives are not defined in our work becauseudof unavoidable scatter of the experimental data. Attempts at understanding the observed results by consideringudpublished theories for single droplets, as well as groupings obtained from dimensional analysis, have beenudunsuccessful. It appears that the diffusion model forudthe heterogeneous burning of single fuel droplets will require serious revision and extension before the burning ofuddroplets arrays and sprays can be understood quantitatively.udFurthermore, the effective value of K' for a sprayudprobably depends not only on the fuel-oxidizer system butudalso on the injection pattern. For this reason additionaludstudies had best be carried out under conditions corresponding to those existing in service models.