A modified piloted burner for stabilizing turbulent flames of inhomogeneous mixtures

Abstract

A modification of the well-known jet piloted burner is introduced to enable the stabilization of partially premixed flames with varying degrees of inhomogeneity in mixture fraction or equivalence ratio. A second tube is added within the pilot annulus which now surrounds two concentric pipes, one carrying fuel and the other air. The central pipe can also be recessed within the annulus upstream of the burner’s exit plane. Two flow configurations are tested: FJ which refers to fuel issuing from the central pipe while air issues from the annulus, and FA where the reverse is true. A key feature of the FJ configuration is that when the central tube is slightly recessed, the fuel partially premixes with air from the annulus inducing inhomogeneity, the extent of which depends on the recession distance.It is found that flame stability is significantly improved due to this inhomogeneity such that, for intermediate recession distances in the range 50–100mm, and for the same air/fuel ratio, the blow-off limits for the FJ cases are more than 50% higher than those of the FA counterparts where fuel is injected in the annulus. Detailed stability limits for both the FJ and FA configurations are presented here along with measurements of velocity and mixing fields at the jet exit plane. Rayleigh scattering is used to image mixture fraction in non-reacting jets while measurements of velocity and turbulence fields are made using standard Laser Doppler Velocimetry. It is shown that, at intermediate recession distances, significant differences in the mean and rms fluctuations of the velocity and mixture fraction profiles exist between the FA and FJ cases. An indicator of stratification, extracted from the mixture fraction images at the exit plane of non-reacting jets, confirms that a high degree of inhomogeneity correlates well with improved flame stability.