The Effect of Flow Boundary Conditions on the Stability of Quarl-Stabilised Flames

Abstract

Experiments have been performed to quantify the influence of flow boundary conditions on the flammability limits of quarl-stabilised flames and on distributions of local temperatures and concentrations of unburned hydrocarbon and oxides of nitrogen. The variables include the shapes of the profiles of axial and tangential velocity at entry to the diffusing quarl, premixedness of the central fuel stream and frequency and the amplitude of oscillations imposed on the air flowing in the annulus. The geometry of the burner is similar to that used in furnaces and the heat release around 50 kW for stoichiometric proportions of fuel and air.

The flammability range was found to increase with swirl number and to decrease with increase in the proportion of mass flow in the central region of the quar!. The addition of small quantities of air to the central flow of methane led to partial stabilisation of the flame on the fuel tube so that the lean flammability was decreased for all swirl numbers. The amplitude of imposed oscillations was enhanced by cavity resonance and their effect depended on consequent velocity fluctuations; in general, oscillations increased the range of overall equivalence ratios for which stable combustion was possible, allowed operation of the burner without swirl, and improved rich flammability limits, although the lean limit occurred at higher equivalence ratios. The local measurements of temperature and species concentrations quantified the extent to which oscillations shortened the flames, provided explanations for the changes in flammability limits and showed that oscillations did not reduce NO_x emissions.

Keywords:

• Quarl burners
• swirl
• stability
• combustion