Flame movement mechanisms and characteristics of gas fired cyclone combustors

Abstract

Cyclone combustors are finding application in several fields, ranging from burning waste vegetable refuse, low calorific value gases, MHD applications and as highly flexible modulatable combustors. The combustor, described in this paper, is based on design experience with coal fired cyclone burners and is run on a premixed supply of natural gas and air at varying mixture ratios. Three principal modes of combustion and associated flame movement mechanisms are described. Mode I occurs at rich mixture ratios, combustion taking place partially inside the combustor, but for the most part downstream of the exit. Mode II occurs at near stoichiometric mixture ratios, the flame being located primarily in the inlet/outlet mainifold of the combustor but extending inside the cyclone chamber. Mode III occurs at weak mixture ratios, the flame is of annular form and located at 0.75 of the cyclone chamber diameter. The flame movement mechanism between Mode I and II is caused by differences in heat release rates due to the degree of flammability of the air/fuel mixture. The distinct flame movement mechanisms between Mode II and III is caused by (a) a reduction in entrainment from the descending wall flow to the exhaust flow, (b) a reduction in the annular width of the wall flow, (c) a change in angular and axial momentum flux distributions inside the combustor. The effect of combustion is seen to significantly reduce the local dynamic swirl number throughout the combustor, due to flow acceleration, and is predicted by a simple expression using spatially averaged mean densities.