Experiments with Disk Stabilized Kerosene-Fuelled Flames

Abstract

Kerosene-fueled flames with the spray emanating from a commercial atomizer positioned on the axis of stabilizing disks with blockage ratios of 0.74, 0.56 and 0.39 without and with a quarl diffuser are characterised in terms of flammability limits, droplet velocities according to their size, mean droplet sizes,liquid fluxes and local temperatures. Detailed measurements were obtained with a disk blockage ratio of 0.74, for an overall equivalence ratio of 0.26 and a Reynolds number of 53700 based on the area averaged velocity of the air in the annulus upstream of the bluff body and the outer diameter of the pipe. The velocities and liquid fluxes are presented for three 5 µm size classes, namely 10-15, 30-35 and 50-55 µlm, to emphasize the behaviour of small, medium and large droplet sizes in the region of flame stabilization and isothermal-flow results are presented in the absence of the quarl to allow comparison of the near disk flow with and without combustion.

The results show that decrease of the ratio of the momentum of the liquid spray to that of the annulus air flow increases the radial dispersion of droplets within the recirculation region and the flammability limits of the flame and was quantified in terms of a transit time and a centrifuge Stokes number. The fuel vapour produced in the near-burner flow was quantified by the ratio of the radial transit time of the droplets from the burner axis to the reaction zone or of the axial transit time along the recirculation region to their evaporation time. As a consequence, it was possible to estimate the amount of unburned liquid fuel escaping the recirculation zone and the magnitude of wall wetting and to explain the improvement of burner efficiency by the quart, For the blockage ratio of 0.74, without the quart diffuser and for conditions close to flame extinction, some 10% of the total fuel flowrate remained within the recirculation region and was essential to the stabilization of the flame. Combustion efficiency improved substantially with the quarl diffuser so that the liquid content of the flame was reduced by more than 40% in the plane corresponding to that of the quarl exit. The overall equivalence ratio at the lean limits was higher for the larger disk and for the flames with the quarl mainly due to the higher turbulent strain rates and the variations in the equivalence ratio inside the recirculation zone, based on the mass flowrate of the fuel vapour released and the entrained air.

Key words:

• spray flames
• droplet size-velocity correlation
• liquid fuel flux
• flammability limits
• flame length
• phase Doppler velocimeter