Droplet Size Effects on NO_x Formation in a One-Dimensional Monodisperse Spray Combustion System

A one-dimensional monodisperse aerosol spray combustion facility is described and experimental results of post flame NO/NO_x emissions are presented. Four different hydrocarbon fuels were studied: isopropanol, methanol, n-heptane, and n-octane. The results indicate an optimum droplet size in the range of 48–58 microns for minimizing NO/NO_x production for all of the test fuels. This NO_x behavior is associated with droplet interactions and the transition from diffusive type of spray burning to that of a prevaporized and premixed case. Decreasing the droplet size results in a trend of increasing droplet interactions, which suppresses temperatures and reduces NO_x. This trend continues until prevaporization effects begin to dominate and the system tends towards the premixed limit. The occurrence of the minimun NO_x point at different droplet diameters for the different fuels appears to be governed by the extent of prevaporization of the fuel in the spray, and is consistent with theoretical calculations based on each fuel’s physical properties.