Control of soot luminosity and soot emission in counter-flow flames by partial premixing

摘要

This paper pertains to the control of soot and gas radiation as well as soot particulate emissions from partially premixed methane oxygen counter-flow flames. Experiments were conducted in a unique low strain rate counter-flow flame burner. flame temperature profile and laser light extinction measurements of soot volume fraction along the stagnation streamline were carried out for progressive addition of oxygen to the methane fuel. Flames in both the 'merged flame' and the 'interdependent-flame' regimes of the double flames were considered. For progressive partial premixing the peak soot loading and the thickness of the soot zone first decreased and then increased. A novel configuration of partially premixed flames was experimentally observed with soot formation and oxidation between spatially separated premixed and nonpremixed reaction zones on the fuel side of the stagnation plane. Such flames have the potential of enhanced soot radiation with simultaneous control of soot emissions form flamelet combustion. Detailed computational modeling of the flames using Sandia opposed flow flame code, OPPDIIF, with modified energy conservation equation to include an optically thin model for soot and gas radiation was used to assess the effect of partial premixing on soot and gas radiation. Soot radiation first decreased and then increased following the trend for peak soot loading as oxygen was progressively added into the fuel. Although gas radiation from the flame was enhanced due to broadening of profiles of temperature and products of combustion, radiative fraction for gas radiation (radiation per unit energy release) showed a systematic decrease as more and more energy is released in the premixed reaction zone for progressive partial premixing. The net radiative fraction, combining contributions of gas and soot radiation, first decreased and then increased with progressive partial premixing of the fuel side.