FLOW-FIELD OF A DIFFUSION FLAME ATTACHED TO A THICK-WALLED INJECTOR BETWEEN 2 COFLOWING REACTANT STREAMS

The how held of a diffusion flame attached to a thick-rim injector bet ween two coflowing streams of fuel and oxidiser is analysed in the Bur ke-Schumann limit of infinitely fast reaction rate. The length of the recirculation region immediately behind the injector and the velocity of the recirculating fluid are proportional to the shear stresses of t he reactant streams on the wall of the injector for a range of rim thi cknesses, and the structure of the flow in the wake depends then on th ree main non-dimensional parameters, measuring the gas thermal expansi on due to the chemical heat release, the air-to-fuel stoichiometric ra tio of the reaction, and the air-to-fuel ratio of wall shear stresses. The recirculation region shortens with increasing heat release, and t he position of the flame in this region depends on the other two param eters. An asymptotic analysis is carried out for very exothermic react ions, showing that the region of high temperature around the flame is confined by neatly defined boundaries and the hot fluid moves like a h igh-velocity jet under a favourable self-induced pressure gradient. Th e immediate wake is surrounded by a triple-deck region where the inter acting flow leads to an adverse pressure gradient and a reduced shear stress upstream of the injector rim for sufficiently exothermic reacti ons. Separation of the boundary layers on the wall of the injector, ho wever, seems to be postponed to very large values of the gas thermal e xpansion.