Influence of Fuel-Air Mixing on Flame Dynamics of Premixed Swirl Burners

Abstract

The influence of fuel-air mixing on the flame dynamics of premixed swirl flames is investigated comparing flame transfer functions determined for perfectly premixed (PP) and technically premixed (TP) operation. In PP operation fuel and air are mixed far upstream of the burner so no equivalence ratio fluctuations appear during thermo-acoustic oscillation. In TP operation the fuel is injected in the swirler slots so equivalence ratio fluctuations occur. The employed swirl burner is a modular system that consists of a swirler and a mixing tube with three different lengths. It was investigated in an atmospheric single burner test rig equipped for flame transfer matrix measurements. Flame transfer function data are presented for both the PP and the TP operation for a variation of power at fixed equivalence ratio and a variation of equivalence ratio for constant power. The unforced flame shapes corresponding to these operation points were acquired and analyzed for scaling parameters of the flame response. It was found that a basic frequency scaling can be achieved for both operation modes using the nominal burner velocity and the flame stand-off distance. A detailed comparison of the PP and TP flame transfer functions is performed for the three different mixing tubes at one operation point. Finally the difference between the PP and the TP flame transfer function is presented and discussed. It is shown that the influence of equivalence ratio fluctuations exhibits a generalized delay time behavior.