The influence of duct geometry on unsteady premixed flames

Abstract

Measurements of the frequency spectrum and intensity of pressure fluctuations have been obtained in the vicinity of confined, bluff-body stabilized premixed flames as a function of equivalence ratio. Methane and air were premixed in a swirl register (swirl was subsequently removed by honeycomb) which provided a closed upstream end to the various duct geometries. The ducts included a straight pipe with a baffle, a sudden contraction, a smooth contraction, and a sudden expansion with and without a central baffle. The results show significant differences in the flammability limits and in the range of equivalence ratios over which smooth combustion can be achieved. Predominant frequencies are usually associated with quarter-waves based on the length of the upstream column of cold gas but can also be associated with higher harmonics. Attempts to modify the intensity of oscillations by using a central pilot flame, with and without a baffle, so as to extend the length over which heat release takes place were unsuccessful but the smooth contraction between the swirl register and flame stabilizer removed them.