Studies of helmholtz resonance in a swirl burner/furnace system

Abstract

This paper describes and analyzes the combustion-excited Helmholtz resonance in a swirl burner/furnace system. Preliminary work showed the importance of variables such as air/fuel ratio, mode of fuel entry, and length of exhaust pipe attached to the exit of the furnace. Conditions were chosen (50% axial, 50% premixed natural gas and air, 600-mm-long exit pipe, air/fuel ratio=1.53) that gave a clean, near sinusoidal pressure signal at the furnace exit. This signal was used as a trigger to enable phase-averaged axial and tangential velocities as well as temperatures to be taken throughout the furnace and into the exit pipe. The results show that a lifted flame core region exists over all of the oscillation cycle. Combustion excitation via the Rayleigh criteria comes via a periodic combustion wave of annular form located next to the furnace wall. The minimum pressure of the resonance causes a large inflow into the furnace with well-known swirltype flow patterns, which extinguishes the annular wall flame. The central flame core boundary is located in a low-velocity region between the forward and reverse flow regions. Conversely, the peak resonant pressure virtually stops the inflow into the furnace from the burner and allows the annular wall flame to propagate backward into the furnace. Complex coherent structures in the tangential radial direction are formed close to the junction of the swirl burner and the furnace. The influence of the precessing vortex core (PVC) in the swirl burner exhaust in setting up the resonance seems to be small; however, it does appear to further excite the resonance to higher amplitudes when its frequency is close to that of the Helmholtz oscillation.