Nonluminous diffusion flame of diluted acetylene in oxygen-enriched air

Abstract

A soot-reducing mechanism of fuel dilution and oxygen enrichment in laminar diffusion flames is suggested. Analysis using the Burke-Schumann theory for the shape of overventilated diffusion flames has shown that there is a critical ratio of stoichiometric coefficients of the fuel and the oxidizer under which the gas flows from the fuel side to the oxidizer side throughout the flame. When this condition is satisfied, the soot growth region vanishes. A similar result is also found in a numerical simulation for diffusion flames that do not satisfy the Burke-Schumann assumption of uniform flow field. KIVA code is used for that purpose. The theoretically predicted direction of gas-flow across the flame sheet is verified in an experiment in a coaxial-flow diffusion flame. Soot cloud and velocity fields are visualized through a laser sheet method in the experiment. The fuel is a mixture of acetylene and nitrogen. The oxidizer is a mixture of oxygen and nitrogen. The compositions of the reactants are controlled so that the adiabatic flame temperature is kept constant to avoid the effect of temperature change. Experimental results show substantial reduction of scattered light intensity by fuel dilution and oxygen enrichment. When a sufficient amount of nitrogen is added to the fuel, nonluminous blue flames are obtained. At higher oxygen concentrations, blue flames are obtained at higher flame temperature region. When oxygen concentration in the oxidizer is 70 vol. %, blue flames are obtained up to 2250 K. The critical condition of the reactants for nonluminous flames agrees with the theoretical prediction when the oxidizer is ordinary air. In oxygen-enriched conditions, the fuel must be diluted more than theoretically predicted.