THE EFFECT OF FLOW BOUNDARY-CONDITIONS ON THE STABILITY OF QUARL-STABILIZED FLAMES

Experiments have been performed to quantify the influence of how bound ary conditions on the flammability limits of quarl-stabilised flames a nd on distributions of local temperatures and concentrations of unburn ed hydrocarbon and oxides of nitrogen. The variables include the shape s of the profiles of axial and tangential velocity at entry to the dif fusing quarl, premixedness of the central fuel stream and Frequency an d the amplitude of oscillations imposed on the air flowing in the annu lus. The geometry of the burner is similar to that used in furnaces an d the heat release around 50 kW for stoichiometric proportions of Fuel and air. The flammability range was found to increase with swirl numb er and to decrease with increase in the proportion of mass flow in the central region of the quarl. The addition of small quantities of air to the central flow of methane led to partial stabilisation of the fla me on the fuel tube so that the lean flammability was decreased for al l swirl numbers. The amplitude of imposed oscillations was enhanced by cavity resonance and their effect depended on consequent velocity flu ctuations; in general, oscillations increased the range of overall equ ivalence ratios for which stable combustion was possible, allowed oper ation of the burner without swirl, and improved rich flammability limi ts, although the lean limit occurred at higher equivalence ratios. The local measurements of temperature and species concentrations quantifi ed the extent to which oscillations shortened the flames, provided exp lanations for the changes in flammability limits and showed that oscil lations did not reduce NOx emissions.