Jet-Stirred Combustor Behavior Near Blowout: Observations and Implications

The behavior of jet-stirred combustors near blowout has been examined. Such units nominally emulate perfectly stirred reactors (PSR). Previous experimental data on instantaneous temperatures and stable species from a toroidal design have suggested non-PSR behavior as overall extinction is approached. Detailed mechanistic modeling in this work shows that such combustors experience global blowout at significantly lower mass flow rates than predicted by a PSR model. It is suggested that, as overall blowout is approached, only a fraction of the physical reactor volume is under active combustion as localized extinction occurs while apparent global stability still exists. This hypothesis is used to successfully predict observed global extinction points for C₂H₄/air and CO/H₂/ air in a toroidal unit, and CH₄/air in a spherical unit. In order to judge whether a stirred combustor is experiencing significant departures from PSR behavior, a test is offered for carbon-containing fuels burned under fuel-lean conditions. It compares the measured CO concentration with the predicted CO concentration obtained from a PSR computer simulation based on an elementary CO/H₂reaction mechanism and a modified feed composition. In this feed, the actual fuel is replaced with a stoichiometrically equivalent amount of CO and H₂0, and the O₂feed is reduced accordingly. If localized instability is occuring, the measured CO level will exceed the predicted CO level. This test has been confirmed for fuel-lean C₂H₄oxidation in air in a toroidal jet-stirred combustor.