Premixed swirl combustion modes emerging for a burner tube with converging entrance

The fluid dynamics underlying swirl combustion was experimentally investigated by observing the subsequent behavior of a conical premixed flame which was formed at the nozzle exit when the air flow rate was increased stepwise for each fixed fuel flow rate. In the experiment, the swirling flow of a methane-air mixture was produced by a vane swirler, resulting in a nearly unit swirl number at the nozzle exit for any net gas flow rate. Another characteristic feature of this experiment is that the nozzle with a diameter of 21.5 mm and a length of 100 mm was connected by a tapered connector to a 38 mm diameter duct into which the swirling flow flowed. Hence, the converging connector produces a large axial velocity peak near the nozzle entrance by the contraction of the vortex core, and creates a barrier for flame flashback. Thus, together with the occurrence of turbulent transition, the presence of the converging connector results in a variety of premixed flame behaviors depending on the nozzle flow velocity, stoichiometric ratio and turbulence intensity. The stable combustion mode was either a lifted flame or flame holding in the nozzle tube. Their mode map was experimentally determined on the plots of nozzle flow velocity as a function of fuel flow rate. The flame behaviors in seven distinguished regimes were characterized on the basis of fluid dynamics considerations and supplemental cold flow experiment results. The results are useful for the development of flame control techniques. (C) 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.