Counterflow diffusion flame with polydisperse sprays

Abstract

An experimental study was performed on counterflow diffusion flames, in which polydisperse n-heptane sprays carried by a nitrogen stream from the lower duct, burn in oxidizer streams from the upper duct. The droplet-size distributions were varied with the nitrogen flow rate for atomization. The burning behaviors of the spray were observed and the oxygen concentration at extinction was determined for different droplet-size distributions. The flat blue diffusion flame was observed between two ducts, which was supported by vaporization of small droplets. Relatively large droplets penetrated the flat blue flame and continued to burn on the oxidizer side with an envelope flame surrounding each single droplet. The burning behaviors for different droplet-size distributions were discussed based on the Stokes number, St0, and the ratio, ψ, of the vaporization time to the characteristic flow time. The extinction limit was also found to be affected by the droplet-size distribution. The oxygen concentration at extinction had a minimum value for a specific volume mean droplet diameter irrespective of the fuel flow rate. This dependence was explained as a result of a competitive effect of St0 and ψ. The extinction limit of an n-octane spray was also examined to verify the effect of St0 and ψ on the extinction limit of the spray.