A numerical study of the effect of radial confinement on the characteristics of laminar co-flow methane–oxygen diffusion flames

Abstract

A numerical investigation of the characteristics of laminar co-flow methane–oxygen diffusion flames has been carried out. The temperature and nitric oxide (NO) distributions in unconfined and partly confined flames are studied in detail. Radial confinements of different diameters and with a length of 150 times the fuel jet diameter have been considered to allow atmospheric nitrogen entry only from the top. A numerical model with a 43-step chemical kinetics mechanism and an optically thin radiation sub-model is employed to carry out simulations. The numerical model has been validated using the experimental data available in the literature. The effect of oxygen flowrate on temperature distributions is studied thoroughly. Confined flame extents are compared with the corresponding unconfined flame extents with the help of OH contours. The effect of confinement diameter on temperature and NO distributions is analysed in detail. At low oxygen flowrates, the extents of confined flames are higher than those of an unconfined flame. At a higher oxygen flowrate, the extent of unconfined flame becomes higher. The confined flames are in general hotter than the unconfined flames. However, at the highest oxygen flowrate and for an intermediate confinement diameter, the flame has the lowest maximum temperature. The amount of NO produced in confined flames is higher than the unconfined flames, due to air entrainment from the top of the confining tube, which increases the residence time for nitrogen transport and its oxidation. At the highest oxygen flowrate considered, numerical predictions show that for a given confinement length, there is an optimum confinement diameter which results in a minimum net production of NO among all the flames.