Ignition of Polydisperse Sprays: Importance of D20

Abstract

Ignition of a polydisperse single-component fuel spray is studied. The physical model consists of a mist of air and fuel droplets contained in a tube. The left end of the tube acts as the ignition source. The polydisperse character of the spray is represented by discrete and finite size distribution. Other notable features of the two-phase ignition model are that the temperature in the droplet interior is resolved spatially as well as temporally, that the inter-phase slip is considered, that a one-step reaction scheme with non-unity exponents of fuel and oxygen concentrations is employed, and that the interesting physical phenomenon is resolved on the scale of the spacing between droplets. An Eulerian-Lagrangian hybrid scheme is used to solve the two-phase equations. The one-dimensional unsteady equations are integrated to obtain the ignition time delays and ignition energies. Influence of initial droplet size, size distribution, overall equivalence ratio, fuel type, and the locations of the nearest droplets to the ignition source is examined. The results clearly demonstrate that the Sauter mean diameter is not capable of representing the ignition characteristics of a polydisperse spray. The polydisperse spray ignition can, however, be well correlated to an equivalent monodisperse spray by using a mean diameter based on the total surface area of the spray. This observation is confirmed over a range of fuel volatility, initial droplet sizes, size distribution, and overall equivalence ratios. For a given size distribution, fuel volatility and overall equivalence ratios, the values of initial droplet sizes for the minimum ignition delays are also obtained. The results also indicate a strong dependence of ignition delays upon the location of the nearest droplets to the ignition source, which underscores the statistical character of the spray ignition process