Spray-swirl interaction and early pyrolysis of kerosene and light oils spray flames produced by simplex pressure atomizer

A comparative study of spray flame structures obtained with four light fuel oils having different viscosity and aromatic content has been carried out on pressure atomized sprays in isothermal and combustion conditions. The application of LLS and LDV techniques on isothermal sprays coupled with the chemical analysis of condensed hydrocarbons and soot, collected into the corresponding flames, has shown the strong effect of the spray-swirl interaction on the flame structure. The controlling parameter on the flame structure for fixed swirl number and pressure atomization, when the chemical nature of the fuel is changed, has been found to be the fuel viscosity. The high viscosity fuel, generating a compact spray, not affected by the swirled air, burns as in a plug flow reactor undergoing just a slow evaporation along the flame axis. Soot and pyrolysis products are formed downstream in the flame. On the opposite the low viscosity fuel is dispersed radially in the outer regions of the spray; the recirculation of the air flow on the flame axis allows a rapid mixing that, in burning conditions, generates a flame similar to a partially stirred reactor. In these conditions soot and pyrolysis products formation occurs already in the first region of the flame in spite of the low aromaticity of the fuel. PAH and polar aromatic compounds have shown to be the best signature for the degree of pyrolysis and oxidation reaction which the fuel undergoes.